Preobrazhensky thermotechnical measurements. Basic information about thermotechnical measurements and devices. General information about measuring instruments

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Preobrazhensky V.P. Thermotechnical measurements and devices: A textbook for universities in the specialty "Automation of heat and power processes." - 3rd ed., revised. - Moscow: "Energy", 1978. -704 p.

The book discusses the main methods and means of measurement,used to automate heat and power processes. Oswethe method of measuring temperature, pressure, flow and other quantities. Measurement errors are considered, ways to reduce them.solutions, advantages and disadvantages of individual methods and means ofmeasurements. The material presented in the book is accompanied by examples of calculations. The second edition appeared in 1953. The third edition reworked.

The book is a textbook for the course "Thermotechnical measurementsand devices" for students of higher educational institutions studyingin the specialty "Automation of heat and power processes."

Publishing house "Energy". 1978

Contents of the textbook Thermotechnical measurements and devices

Foreword
Introduction

SECTION ONE. GENERAL INFORMATION ABOUT MEASUREMENTS

Chapter first. General information about measurements
1-1. The concept of measurement, types and methods of measurement
1-2. General information about measuring instruments
1-3. General information about measurement accuracy and measurement uncertainty
1-4. Evaluation and accounting for errors in accurate measurements
1-5. Basic information about the metrological characteristics of measuring instruments
1-6. General information about dynamic characteristics measuring instruments
1-7. Evaluation and accounting for errors in technical measurements

SECTION TWO. TEMPERATURE MEASUREMENT

Chapter two. General information about temperature measurement
2-1. Understanding temperature and temperature scales
2-2. Practical temperature scales

Chapter three. Thermometers based on the expansion and pressure change of the working medium
3-1. Liquid glass thermometers
3-2. Manometric thermometers
3-3. Dilatometric and bimetal thermometers

Chapter Four. Thermoelectric temperature measurement method
4-1. General information
4-2. Fundamentals of the theory of thermoelectric thermometers
4-3. Inclusion of the measuring device in the circuit of a thermoelectric thermometer
4-4. Correction for the temperature of the free ends of a thermoelectric thermometer
4-5. Determination of the thermo-emf of various materials in the study of their thermoelectric properties
4-6. Basic requirements for thermoelectrode materials
4-7. General information about thermoelectric thermometers
4-8. The device of thermoelectric thermometers
4-9. Extension thermocouple wires
4-10. Devices for ensuring the temperature constancy of the free ends of thermoelectric thermometers
4-11. Millivoltmeters
4-12. KT device and circuits for connecting several thermoelectric thermometers to one millivoltmeter
4-13. Thermo-emf measurement with a millivoltmeter
4-14. Compensation method for measuring thermo-emf
4-15. Normal elements
4-16. Portable and laboratory potentiometers
4-17. General information about automatic potentiometers
4-18. Schematic diagrams of automatic potentiometers
4-19. Method for calculating the resistance of resistors in the measuring circuit of automatic potentiometers
4-20. Amplifier Basics
4-21. Understanding Power Conditioners
4-22. The device of automatic potentiometers
4-23. Automatic Rhechordless Potentiometers

Chapter five. Resistance thermometers and measuring instruments for them
5-1. General information
5-2. Basic information about resistance thermometers and the metals used for their manufacture
5-3. The device of platinum and copper resistance thermometers
5-4. Semiconductor resistance thermometers
5-5. Compensation method for measuring the resistance of a thermometer
5-6. Measuring the resistance of a thermometer with a bridge
5-7. Logometers
5-8. General information about automatic balanced axles
5-9. Principal measuring schemes of automatic balanced bridges
5-10. Schematic diagram of an automatic balanced bridge
5-11. The device of automatic balanced bridges
5-12. Automatic compensation devices for working with low-resistance resistance thermometers

Chapter six. Temperature measurement technique by contact methods, measurement errors and methods for their consideration and reduction
6-1. General guidelines
6-2. Methodological errors in measuring gas temperatures due to the influence of heat transfer by radiation
6-3. Methodological errors in measuring the temperature of the medium, due to the removal or supply of heat through the thermal receiver
6-4. Installation of thermal receivers for measuring the temperature of gases, steam and liquids
6-5. Temperature measurement of high velocity gas streams
6-6. Measurement of surface and inside body temperature

Chapter seven. Measurement of the temperature of bodies by their thermal radiation
7-1. General information
7-2. Theoretical foundations of methods for measuring the temperature of bodies by their thermal radiation
7-3. Optical pyrometers
7-4. Photoelectric pyrometers
7-5. Spectral ratio pyrometers
7-6. Total radiation pyrometers

SECTION THREE. MEASURING TRANSMITTERS AND REMOTE DISPLAY TRANSMISSION CIRCUITS

Chapter eight. Measuring transducers and schemes for remote transmission of readings
8-1. General information
8-2. Rheostat transducers and remote transmission circuits
8-3. Measuring strain gauges
8-4. Differential transformer converters and remote transmission circuits
8-5. Ferrodynamic Converters and Remote Transmission Circuits
8-6. Mechanoelectric transmitters
8-7. Transmitters with magnetic compensation
8-8. Electric power converters
8-9. Frequency converters with string vibrator
8-10. Pneumatic power converters
8-11. Pneumatic transmitters
8-12. Electropneumatic and pneumoelectric converters
8-13. Normalizing transducers

SECTION FOUR. PRESSURE AND DIFFERENTIAL PRESSURE MEASUREMENT

Chapter nine. Liquid pressure gauges with visible level
9-1. U-shaped and cup cutlery
9-2. Micromanometers
9-3. Corrections to readings of liquid instruments
9-4. Mercury barometers

Chapter ten. Pressure instruments with elastic sensing elements
10-1. General information and basic properties of elastic sensitive elements
10-2. Elastic sensing elements
10-3. Direct acting pressure gauges
10-4. Electrocontact devices and pressure switches
10-5. Pressure devices with electric and pneumatic converters

Chapter Eleven. Electric pressure devices
11-1. Piezoelectric pressure gauges
11-2. Resistance gauges

Chapter twelve. Differential pressure gauges
12-1. General information
12-2. Bell pressure gauges
12-3. Ring pressure gauges
12-4. Float differential pressure gauges
12-5. Differential pressure gauges with elastic sensitive elements

Chapter thirteen. Basic information about the pressure measurement technique
13-1. General guidelines
13-2. Measurement of near-atmospheric pressure of gaseous media
13-3. Pressure measurement of gases, liquids and steam
13-4. Separators liquid and membrane

SECTION FIVE. FLOW AND QUANTITY MEASUREMENT OF LIQUIDS, GAS, STEAM AND HEAT

Chapter fourteen. Measurement of the flow and quantity of liquids, gas and steam by pressure drop in the orifice
14-1. Fundamentals of the theory and flow equations
14-2. Standard constriction devices
14-3. Flow coefficients and correction factors for them
14-4. Correction factor for medium expansion
14-5. Determining the density of the measured medium
14-6. Basic calculation formulas for consumption
14-7. Guidelines for measuring the flow of liquids, gases and steam with flow meters with a restrictive device
14-8. Flow measurement errors
14-9. Basic information about the method for calculating narrowing devices
14-10. Flow measurement at the inlet to or outlet of a pipeline
14-11. Flow measurement at low Reynolds numbers
14-12. Flow measurement of contaminated liquids and gases
14-13. Flow measurement at supercritical pressure ratio

Chapter fifteen. Measurement of velocities and flow rates of liquids and gases with pressure tubes
15-1. General information about the method of measuring flow rates
15-2. Pressure tube arrangement
15-3. Definition average speed flow and flow

Chapter sixteen. Constant Differential Pressure Flowmeters
16-1. General information
16-2. Fundamentals of the theory of rotameters
16-3. Rotameter device

Chapter seventeen. Tachometric flow meters and quantity counters and electromagnetic flow meters
17-1. Tachometric liquid counters
17-2. Tachometric flowmeters for liquids
17-3. Electromagnetic flowmeters

Chapter eighteen. Measuring the quantity and consumption of heat in district heating systems
18-1. General information
18-2. Basic information about the design of heat meters

SECTION SIX. LEVEL MEASUREMENT OF LIQUIDS AND SOLID BODIES

Chapter nineteen. Liquid level measurement
19-1. General information
19-2. Measuring the water level in the drum of steam generators
19-3. Level measurement of liquids in condensers, heaters and tanks using differential pressure gauges
19-4. Level measurement of liquids with float and displacer level gauges
19-5. Capacitive level transmitters
19-6. Acoustic and ultrasonic level gauges

Chapter Twenty. Level measurement of bulk solids
20-1. General information
20-2. Bulk solids level switches
20-3. Devices for measuring the level of bulk solids

SECTION SEVEN. METHODS AND INSTRUMENTS FOR MEASURING GAS COMPOSITION

Chapter twenty one. Methods and means for measuring the composition of gases
21-1. General information
21-2. Chemical gas analyzers
21-3. Thermal gas analyzers
21-4. Magnetic gas analyzers
21-5. Optical gas analyzers
21-6. Gas chromatographs
21-7. Guidelines for Sampling Gas for Analysis

SECTION EIGHT. METHODS AND TECHNICAL TOOLS FOR QUALITY CONTROL OF WATER, STEAM, CONDENSATE AND SOLUTION CONCENTRATION

Chapter twenty two. Methods and technical means for monitoring the quality of water, steam, condensate and concentration of solutions
22-1. General information
22-2. Measurement of the specific electrical conductivity of aqueous solutions
22-3. Liquid conductometers with degassing and sample enrichment
22-4. Electrodeless conductometric liquid analyzers
22-5. Analyzers for the determination of oxygen dissolved in water
22-6. Analyzers for determination of hydrogen dissolved in water and steam

Applications
Bibliography
Subject index

Download a book Preobrazhensky VP Teplotekhnicheskie izmereniya i priborov [Teplotechnical Measurements and Instruments]. Textbook for universities in the specialty "Automation of heat and power processes". Publishing house "Energy", Moscow, 1978

Introduction

1. Composition term paper

2. Choice of technical means of measurement

3. Explanations for the graphic part

4. Explanations for the calculation part

Literature

INTRODUCTION

The decisive role in solving the problems of ensuring production efficiency, reliability and safety of operation of technological equipment belongs to automated process control systems (APCS). The basic system of any modern automated process control system is an automatic control system that allows obtaining measurement information about the regime parameters of technological processes. The issues of organization of measurements, selection of measuring instruments and measured parameters are closely related to the specifics of technological processes and must be resolved at the design stage of the corresponding technological installations, i.e., a heat power engineer participating in the design of a technological installation must have appropriate knowledge of methods for measuring various physical quantities and the skills to use them.

Future specialists studying in the specialty 140104 "Industrial Heat Power Engineering" receive this knowledge when studying the discipline "Heat Engineering Measurements". The course work, provided for by the work program of this discipline, contributes to the consolidation, deepening and generalization of the knowledge gained by students during the training, and the application of this knowledge to complete solution specific engineering tasks for the development of schemes for thermal control of thermal power plants.

Course work includes the development of a measuring channel for monitoring one of the parameters of a process unit, the choice of measuring instruments, the calculation of a narrowing device or a measuring circuit of a secondary device, depending on the task option.

1. COMPOSITION OF COURSE WORK

Course work on the design of the measuring channel for monitoring the physical parameter of the process consists of an explanatory note and a graphic part.

The text part (explanatory note) of the course work includes the following main sections:

· Introduction;

· Choice of technical means of measurement;

· Calculation of measurement channel error;

· Calculation of the narrowing device (measuring circuit of the secondary device);

The graphic part of the work includes:

· functional diagram of the developed measuring channel;

· drawing of the narrowing device (assembly drawing of the installation of the primary converter on the process equipment).

2. CHOICE OF TECHNICAL INSTRUMENTS OF MEASUREMENT

This part of the explanatory note includes a description of the technological process and the rationale for choosing a method for measuring a given physical parameter. The main design decisions are made on the basis of the analysis of the technological process and the current state and industry regulations.

Specific types of measuring instruments are selected taking into account the characteristics of the technological process and its parameters.

First of all, they take into account such factors as fire and explosion hazard, aggressiveness and toxicity of the environment, the range of information signal transmission, the required accuracy and speed. These factors determine the choice of methods for measuring process parameters required functionality instruments (indication, recording, etc.), measurement ranges, accuracy classes, type of remote transmission, etc.

Devices and transducers should be selected according to reference literature, based on the following considerations:

To control the same parameters of the technological process, it is necessary to use the same type of measuring instruments, mass-produced;

At large numbers the same parameters, it is recommended to use multi-point devices;

The accuracy class of instruments must comply with technological requirements;

To control technological processes with aggressive media, it is necessary to provide for the installation of special devices, and in the case of using devices in their normal version, they must be protected.

The most common types of industrial secondary devices included in state system industrial instruments and automation equipment (GSP) are presented in table 1.

Table 1

PV devices are secondary devices of the "Start" pneumatic system and are used to measure any technological parameters previously converted into pressure compressed air(unified pneumatic signal).

KSP automatic potentiometers, KSM balanced bridges, KSU milliammeters are used to measure and record temperature and other parameters, the change of which can be converted into a change in DC voltage, active resistance, DC current.

KSP-4 potentiometers, depending on the modification, can work either in a set with one or more (if the device is multipoint) thermocouples of standard calibrations, or with one or more DC voltage sources.

KSM-4 balanced bridges work with one or more standard calibration resistance thermometers, and KSU-4 milliammeters with one or more DC signal sources.

Secondary KSD devices work together with primary measuring transducers equipped with differential transformer sensors.

Each type of instrument mentioned above is available in various modifications that differ in size, measurement ranges, number of input signals, availability of auxiliary devices, etc.

When choosing one or another device on the basis of functionality, it is necessary to combine the simplicity and cheapness of the equipment with the requirements for monitoring and regulating this parameter. The most important parameters should be monitored by self-recording instruments, which are more complex and expensive than indicating instruments. The adjustable parameters of the technological process must also be controlled by recorders, which is important for adjusting the settings of the regulators.

When choosing secondary devices for joint work with the same type of sensors of the same calibration and with the same measurement limits, it should be taken into account that the KSP, KSM, KSD devices are produced with the number of points 3,6,12. Multi-point devices have a switch that automatically and alternately connects the sensor to the measuring circuit. The printer, located on the carriage, prints points on the diagram with the serial number of the sensor.

When choosing the type of unified signal of the communication channel from the sensor to the secondary device, the length of the communication channel is taken into account. With a length of up to 300 m, any unified signal can be used if the automated technological process is not fire and explosion hazardous. In case of fire and explosion hazard and a distance of no more than 300 m, it is advisable to use pneumatic automation equipment, for example, devices of the "Start" system. Electrical measuring instruments are characterized by much less delay and surpass pneumatic instruments in terms of accuracy (accuracy class of most pneumatic instruments is 1.0, electrical - 0.5). The use of electrical means simplifies the introduction of computers.

When choosing sensors and secondary devices for joint operation, attention should be paid to matching the output signal of the sensor and the input signal of the secondary device.

For example, with a current output signal of the sensor, the input signal of the secondary device must also be current, and the type of current and the range of its change for the sensor and the secondary device must be the same. If this condition is not met, then you should use the intermediate converters of one unified signal to another available in the GSP (Table 2).

table 2

The most common intermediate GSP converters

The intermediate converter NP-3 is used as a normalizing converter for converting the output signal of a differential transformer converter into a unified current signal.

Converter EPP-63 carry out the transition from the electric branch of the GSP to the pneumatic one.

When choosing sensors and instruments, one should pay attention not only to the accuracy class, but also to the measurement range. It should be remembered that the nominal values of the parameter must be in the last third of the measurement range of the sensor or device. If this condition is not met, the relative measurement error of the parameter will significantly exceed the relative reduced error of the sensor or device. Thus, it is not necessary to select a measurement range with a large margin (it is enough to have an upper measurement limit no more than 25% higher than the nominal value of the parameter).

If the measured medium is chemically active with respect to the material of the sensor or device (for example, spring pressure gauge, hydrostatic level gauge, differential pressure gauge for measuring flow using the variable pressure method), then its protection is carried out using separating vessels or diaphragm seals.

The developed measuring channel is depicted in the figure in the form of a functional diagram, made in accordance with GOST 21.404-85.

The functional diagram shows a part of the technological installation with a primary converter placed on it, an intermediate converter and a measuring device. The selected measuring instruments are entered into the instrument specification. Examples of images of individual measuring channels are shown in Figures 1-5.

201-1 Spring pressure gauge M-….

202-1 Pneumatic primary pressure transducer, measurement limit 0 ... 1.6 MPa, output signal 0.02 ... 0.1 MPa, brand MS-P-2 (bellows pressure gauge with pneumatic outlet);

202-2 Electrocontact pressure gauge with signal lamp EKM-1;

202-3 Signal lamp L-1.

204-1 Primary pressure transducer with a standard current output of 0…5 mA, brand MS-E (or Sapphire-22DI, etc.);

204-2 Milliammeter showing registering for 2 parameters, brand A-542.

301-1 Diaphragm brand DK6-50-II-a/g-2 (chamber diaphragm, pressure P y = 6 atm, diameter D y = 50 mm);

301-2 Differential pressure gauge with pneumatic outlet 0.02 ... 0.1 MPa, brand DS-P1 (for pneumatic circuit) or Sapfir-22DD (for electrical circuit);

302-1 Rotameter RD-P (with pneumatic outlet) or RD-E (with electrical outlet).

To measure fluid flow, primary transducers are installed in the pipeline section, therefore, in the diagram, their designations are depicted as built into the pipeline.

When using restrictive devices, such as diaphragms, the pressure drop across them is measured by differential pressure gauges, so the automation schemes are similar to pressure control schemes.

The functional diagram of thermal control is the basis for drawing up a custom specification of measuring instruments.

The specification for all devices and converters shown in the functional diagram is drawn up in the form of a table. An example specification for a fragment of the temperature control functional diagram is shown in Table 3.

Table 3

Specification form for the functional diagram (Fig. 1).

3. EXPLANATION TO THE GRAPHIC PART

Developed graphic documents:

Sheet 1. Scheme of thermal control.

Sheet 2. Installation drawing. Installation of the primary converter on the process equipment.

Sheet 3. A drawing of a narrowing device or a measuring circuit of a secondary device, depending on the variant of the task.

All drawings are made in the graphic editor AUTOCAD in full compliance with the requirements of ESKD. Drawing formats A4.

4. EXPLANATIONS TO THE CALCULATION PART

4.1 Calculation of the narrowing device

Orifice diaphragms for medium flow measurement can be used without preliminary calibration in pipelines round section with a diameter of at least 50 mm at m=d 2 /D 2 from 0.05 to 0.64 (d is the diameter of the diaphragm opening, D is the inner diameter of the pipeline) in case of a certain length of straight sections before and after the diaphragms. The liquid must fill the entire section, its phase state must not change. The flow rate of the medium can be specified in units of mass G - kg/s or in units of volume Q - m 3 /s. Calculation formulas for determining the flow rate of the medium have the form

where a is the flow rate; ε - correction factor for the expansion of the medium (for gaseous media); F 0 - the area of the flow section of the diaphragm, m 2; r is the density of the medium in front of the diaphragm, kg/m 3 ; P 1 - P 2 \u003d ΔР - pressure drop across the diaphragm, Pa.

The diaphragm must be chosen in such a way that for all values of the expected flow rate of the medium, the flow coefficient α is a constant value. The minimum value of the Re criterion, with further growth of which the discharge coefficient α remains constant, is called the limiting value of the Reynolds criterion.

At a minimum flow rate, the value of Re should be greater than Re prev.

4.2 The order of calculation of the throttle device

1. The following initial values are set:

a) measured medium;

b) medium parameters (pressure, temperature, composition);

c) maximum and minimum expenses environment;

d) allowable pressure loss across the throttle device or pressure drop across the diaphragm.

2. According to the maximum flow rate, the inner diameter of the pipeline is determined by the formula

, m,

where w is the average velocity of the medium in the pipeline, m/sec.

The values of the average flow velocity for the calculation of pipelines are given in table 4.

Table 4

Often the gas flow is set in normal cubic meters per unit of time (for example, m 3 n / s). In this case, to go to mass flow, the volume flow should be multiplied by the gas density under normal conditions ρ n. The values of ρ n for combustible gases and air are given in table 5.

Table 5

Physical parameters of combustible gases and air

According to the calculated value of the diameter, the nearest standard one is selected according to special guidelines, or, in the absence of the latter, it can be taken from table 7. For pipelines with temperatures above 450 ° C, it is possible to take for calculation the inner diameter the same as for pipes with temperatures up to 450 °C.

3. Select a design flow rate that corresponds to the upper limit of the differential pressure gauge of the flow meter. The maximum flow rate can be taken as the calculated one.

4. Determine the value of the Reynolds criterion for the accepted design flow rate (Re pac h) from the expression

where f is the cross-sectional area of the pipeline, m 2.

Table 6

Dynamic coefficient of viscosity, μ 10 7 Pa × s, water and steam

Note. Above the line - water, below the line - steam.

Table 7

Values of standard pipeline diameters

For combustible gas and air, the dynamic viscosity coefficient is given in table 5, for water and steam - in table 6. When determining the numerical values of the dynamic viscosity coefficient, linear interpolation should be used. In the first approximation, we can assume that the dynamic coefficient of viscosity of gases does not depend on pressure, but is determined by only one temperature.

5. Select the maximum design differential pressure

ΔP \u003d P 1 -P 2.

If the allowable pressure loss Р v is set, then approximately ΔР = 2Р v can be taken. The value of ΔР is determined by the type of differential pressure gauge-flowmeter.

6. Determine the diameter of the pipeline at operating temperature t according to the equation

where a 0 - average coefficient of linear thermal expansion of the pipeline material; K t - correction factor for thermal expansion. The K t values are shown in Table 8.

Table 8

Correction factor K t for thermal expansion of the pipeline and diaphragms

7. Determine the aperture bore diameter d in the following sequence:

a) calculate the values of mα from the relations

The value of ε is taken from Table 9 according to the calculated value of ΔΡ/P 1 , assuming m = 0.3 (in the first approximation).

Table 9

Values of the correction factor for the expansion of the medium, ε

b) for the found value mα find the value m.

To find the value of m from the known value mα, a graphical dependence mα = f(m) is plotted for accepted meaning D. To do this, according to Table 10, four corresponding values of m and mα are taken and a graph is plotted mα = f(m). When determining mα, it is necessary to interpolate if the diameter of the pipeline differs from that indicated in the table. It is desirable that out of 4 points, two have a value of mα greater and two less than what happened when calculating by the formula. According to the constructed graph, the numerical value of m is determined. The value of m is recommended to be determined with the number of significant digits corresponding to an error of the order of 0.1%.

Table 10

Dependence of the product mα on m and D

c) determine the preliminary value of the diaphragm bore diameter at a temperature of +20°C from the ratio

8. Determine the pressure loss P v in the diaphragm at the estimated flow rate from the ratio

, Pa.

The value of K, which is a function of m, is taken from Table 11.

9. Check the determination of the diameter of the bore of the diaphragm hole d.

Table 11

It should be borne in mind that the flow rate is determined from the ratio

where α u is the initial flow rate; K 1 - correction factor, which is introduced when the value of Re is less than the limit; K 2 - correction factor for the relative roughness of pipes; K 3 - correction factor for the softness of the leading edge.

a) Calculate the value of α using the formula. To do this, according to the calculated value of m, using table 12, determine α u with an accuracy of at least the third digit (interpolation is used in the interval). Then, according to table 13, the product K 2 ×K 3 is determined (while m and D are known). At the calculated flow rate, Re must be greater than Re before therefore K 1 =1.

b) determine the exact value of ε from the known values of m and ΔΡ/P 1 according to table 9 (with an approximate estimate, m was taken equal to 0.3).

c) determine the mass or volume flow according to the formulas

, m 3 / sec.

Table 12

The values of the initial flow coefficient α u and the limit values of the Reynolds criteria (Re prev)

Table 13

The product of correction factors K 2 ×K 3, for normal apertures

If the obtained flow rate differs from the calculated flow rate within ±0.5%, then the calculation is correct. If the discrepancy does not exceed ±2%, it is allowed to specify the aperture diameter of the diaphragm using the equations

where G (Q) is the estimated flow rate; G*(Q*) - flow rate obtained when checking the bore of the diaphragm.

For discrepancies greater than 2%, the calculation is performed again.

10. Determine the lowest flow rate at which it is not necessary to enter the correction factor K 1 from the expressions

or .

The limit value Re is determined according to table 12 from the calculated value of m.

4.3 Orifice design

To measure the flow rate of the medium, three types of normalized narrowing devices have become widespread: a flow orifice, a flow nozzle and a Venturi nozzle, which have a round hole in the middle. Empirically, for these narrowing devices, the exact values of the flow coefficient α were found, which allows them to be used without preliminary calibration.

Normalized narrowing devices can be used in pipelines with a diameter of at least 50 mm with m values: 0.05-0.64 for diaphragms, 0.05-0.65 for nozzles and 0.05-0.6 for Venturi nozzles.

According to the method of pressure extraction to the differential pressure gauge, flow-measuring diaphragms and nozzles are divided into chamber and tubeless (with point selection, Fig. 1). More advanced of them are camera devices. The inner diameter of the diaphragm body is equal (with a tolerance of +1%) to the pipeline diameter D 20 .

In a chamber diaphragm, pressures are transmitted to the differential pressure gauge by means of two annular equalizing chambers located in its body in front of and behind the disk with a hole, connected to the pipeline cavity by two annular slots or a group of radial holes evenly spaced around the circumference (at least four on each side of the disk). The annular chamber in front of the disk is called the positive one, and behind it is called the negative one. The presence of annular chambers at the diaphragm allows the pressure to be averaged over the circumference of the pipeline, which provides a more accurate measurement of the pressure drop. The area ab of the cross section of the annular chamber must be at least half the area of the annular gap or a group of holes, the area of each of which is equal to 12-16 mm 2 . The thickness h of the inner wall of the annular chamber is taken not less than twice the width of the annular slot.

The differential pressure in the tubeless diaphragm is taken from two separate holes in its body or in the pipeline flanges in front of and behind the disk. In this case, the measured pressure drop is less representative than with annular chambers.

The width from the annular slot and the diameter of a separate opening for pressure tapping in chamber and tubeless diaphragms at m £ 0.45 does not exceed 0.03 D 20 , and at m > 0.45 lies within 0.01-0.02 D 20 . At the same time, the size c should not go beyond 1-10 mm.

The thickness E of the diaphragm disk does not exceed 0.05 D 20 . The hole in it with a diameter of d 20 is the calculated value. On the side of the flow inlet, it has a sharp inlet edge at an angle of 90°, behind which is a cylindrical part of length e, which is 0.005-0.02 D 20 . When the disk thickness is E > 0.02 D 20, the cylindrical part of the hole ends at the outlet of the stream with a conical expansion at an angle φ equal to 30-45°. For m > 0.5, the value of e is approximately equal to 1/3 E.

The accuracy of flow measurement using diaphragms depends on the degree of sharpness of the inlet edge of the hole, which affects the value of the flow coefficient α. The edge should not have roundings, burrs and notches. At d 20< 125 мм она должна быть настолько острой, чтобы луч света не давал от нее отражения.

The permissible displacement of the axis of the opening of the narrowing devices relative to the axis of the pipeline should not exceed 0.5-1 mm.

For the manufacture of the flow part of diaphragms and nozzles, materials are used that are resistant to corrosion and erosion, i.e. stainless steel, and in some cases brass or bronze.

On the rim of the constriction device or on the attached marking plate are usually applied: device type designation and serial number; diameters d 20 and D 20; an arrow indicating the direction of flow; brand of material; signs "+" and "-", respectively, from the side of the inlet and outlet of the flow. In addition, a graduation certificate is attached to the narrowing device, which indicates: the name and design parameters of the medium being measured; values obtained in the calculation of the narrowing device (m, α, ε, d 20, etc.); the formula by which the correctness of the calculation was checked; the main characteristics of the narrowing device and differential pressure gauge.

The following normalized diaphragms are produced: chamber type DK for nominal pressure up to 10 MPa for pipelines with a diameter of 50-500 mm and tubeless type DB for pressure up to 32 MPa for diameters of 50-3000 mm.

On fig. 10 shows a DB type tubeless diaphragm installed between pipeline flanges.

4.4 Calculation of the measuring circuit of the automatic potentiometer

It is recommended to calculate the measuring circuit of an automatic potentiometer in the following sequence. The measuring circuit of the automatic potentiometer is shown in fig. eleven.

Rice. 11. Measuring circuit of automatic potentiometer

The following designations are accepted in the scheme and calculation formulas: R 1 - rheochord; R 2 - reochord shunt, which serves to adjust the resistance of the reochord to standard value R P = 90, 100, 300 ohms; R PR - reduced resistance of the reochord circuit; R 3 - resistor for setting the initial value of the instrument scale; R 5 - resistor for setting the scale range of the device; R 4 and R 6 - trim resistors, R 4 \u003d R 6 \u003d 1 Ohm; R 9 - copper resistor used to compensate for temperature changes in the free ends of the thermocouple; R 8 , R 11 - resistor in the power supply circuit; λ - non-working sections of the reochord, R 8 \u003d 790 Ohm; t = 20 °С; λ= (0.02...0.35); E(t H, t 0) - EMF of the thermocouple at the temperature of the working end t H (beginning of the scale) and the calculated temperature of the free ends t 0 ; E(t K, t 0) - EMF of the thermocouple at the temperature of the working end t K (end of scale) and the calculated temperature of the free ends t 0 ; I 1 - the nominal value of the current in the upper branch of the measuring circuit, I 1 = 3×10 -3 A; I 2 nominal value of the tone strength in the lower branch of the measuring circuit, I 2 = 2×10 -3 A; R - resistance of the measuring circuit of the device, R uc = 1000 Ohm.

The calculation of the measuring circuit is carried out without taking into account the trimming resistors R 4 and R 6 .

Reduced resistance of the reochord circuit

. (2)

Given that , determine the resistance value of the resistor R 5

. (3)

The resistance value of the resistor R 10 must be determined from the condition that the voltage drop across the resistor R 10 is equal to the EMF of a normal element:

. (4)

If the measuring circuit of the device is balanced at the beginning of the scale (point a), then according to the Kirchhoff law we obtain the following equation:

When the measuring circuit is in equilibrium at the end of the scale, we can write the equation

From equations (5) and (6) you can get an expression for determining the resistances R 3 and R 7:

; (7)

. (8)

To determine the resistance of the resistor R 9, it is necessary to write equation (5) for two values of the ambient temperature t H \u003d 0 ° С and t Н \u003d 20 ° С. In this case, we neglect the change in current I 2:

The difference between equations (9) and (10) gives:

Considering that the resistance of the copper resistor R 9 will change with the change in ambient temperature in accordance with the dependence:

, (12)

where α = 4.26×10 -3 K -1 - temperature coefficient of copper resistance.

From equations (11) and (12) we get:

. (13)

In (13) t 1 \u003d 20 ° С, the value

represents the sensitivity in the temperature range 0...20°C. In real conditions, for the temperature range of 0...100°C, it is customary to consider

, (14)

where is the EMF of the thermocouple at a working end temperature of 100 and free ends at 0 °C. The resistance of the resistor R 9 must be considered for graduations XK 68, XA 68, PP 68. For graduations PP 30/6 68, RK and PC, the resistance of the resistor R 9 is assumed to be 5 ohms and is made of manganin.

Let us determine the resistance of the measuring circuit of the device relative to the points c-d:

. (15)

Then, taking into account (15), we obtain

. (16)

Typically, the resistance of the resistor R 8 is taken equal to 790 Ohms, and the resistance of the resistor R 11 is determined from the dependence:

. (17)

The resistance of the trimming resistors R 4 and R 6 is taken equal to 1 ohm, and the resistance of the resistors R 3 and R 5 should be reduced by 0.5 ohms, and the remaining 0.5 ohms are additional. With this in mind, it is necessary to correct the obtained values of the resistances of the resistors R 3 and R 5 .

; (18)

. (19)

The resistance of the resistors of the measuring circuit must be calculated with an accuracy: R 3, R 5, R 9 - ± 0.05 Ohm; R 10, R 7, R 11 - ± 0.5 Ohm.

4.5 Calculation of the measuring circuit of the automatic bridge

The measuring scheme of the automatic bridge is shown in Figure 12.

Fig.12. Measuring scheme of the automatic bridge

In the figure and in the calculation formulas, the following designations are accepted: R 1 - rheochord; R 2 - reochord shunt, which serves to adjust the resistance of the reochord to the standard value R P = 90.100, 300 Ohm; R PR - driven resistance of the reochord circuit; R 3 and R 4 - resistors for setting the initial value of the bridge scale; R 5 and R 6 - resistors for setting the upper value of the instrument scale; R 4 and R 5 - trim resistors, R 4 = R 5 = 4 ohms (ohema calculation is performed if the sliders of the resistors R 4 and R 5 are in the middle position); R 7 , R 9 , R 10 - bridge circuit resistors; R 8 - resistor to limit the current in the power circuit; R l - resistor for adjusting the resistance of the external line; R t - resistance thermometer; ~ 6.3V - power supply voltage; λ - non-working sections of the rheochord, λ= 0.020...0.035.

With a three-wire connection diagram of a resistance thermometer shown in Figure 12, the total resistance of the connecting wire R cn and the trimming resistor R l is

, (20)

where R ext is the resistance of the external circuit of the bridge, Ohm.

The strength of the current I 1 flowing through the resistance thermometer must be selected according to GOST 6651-84 from the range: 0.1; 0.2; 0.5; 1.0; 2.0; 5.0; 10.0; 15.0; 20.0; 50.0 mA. In this case, the change in the resistance of the thermometer at 0°C due to the released heat should not exceed 0.1%. The current strength is indicated in the technical specifications for a particular type of resistance thermometer. In technical measurements, resistance thermometers with a nominal static characteristic NSH 50 P, gr 21, 50 M, gr 23 are usually used, for which the current strength should be taken equal to 5 or 10 mA.

For the given temperature measurement limits t n and t in according to GOST 6651-84, we determine W tv and W t n at W 100 \u003d 1.3910 for platinum and W 100 \u003d 1.4280 for copper thermometers.

Thermometer resistances corresponding to the initial t n and final t in the scale marks are calculated by the formula

(21)

where R 0 is the resistance of the thermometer at 0 ° C, Ohm.

The resistance of the resistor R 7 must be such that a change in the resistance of the thermometer when the temperature changes from t n to t c causes a change in current I 1 by an amount not exceeding 10 ... 20%, i.e.

, (22)

where I 1 min and I 1 max - the current strength in the thermometer circuit with its resistance corresponding to the final R t in and initial R t n marks of the bridge scale, respectively, mA; η - coefficient equal to 0.8...0.9.

The voltage drop between points a and b with the resistance of the thermometer corresponding to the initial and final marks of the bridge scale is:

The solution of equations (22) - (24) allows you to obtain a formula for determining the resistance of the resistor R 7:

The sum of the resistances (R 3 + R 4 / 2) is taken as an average of 5 ohms in the calculation.

In formula (25) R PR is unknown and, since the resistance R 7 is calculated first of the resistors of the bridge circuit, the calculation formula is simplified, considering

. (26)

The resulting value of R 7 is usually rounded up to a multiple of 10 ohms.

To find the value of the resistance of the resistor R 10, we write down the equilibrium condition of the measuring bridge circuit at any point on the scale;

. (27)

After transforming expression (27), we obtain

So that a change in the resistance of the communication line with changes in ambient temperature does not affect the readings of the device, it is necessary to select the resistors of the circuit in such a way that in the last equation the terms containing R l in the left and right parts are equal and reduced:

Since the relative error increases towards the beginning of the scale, it is advisable to achieve full compensation of the temperature error at the initial position of the reochord slider (η = 0). Then

Considering that equal-arm bridges in pairs have the highest sensitivity, equality (29) also satisfies this requirement.

Let us compose the equilibrium equations for the measuring circuit of the bridge for two values of the resistance of the thermometer:

As a result of the joint solution of equations (30) and (31), we obtain

. (32)

To determine the resistance of the resistor R 9, it is necessary to substitute the obtained value of R PR into equation (30). After transformations, we get the following quadratic equation:

. (34)

The reduced resistance of the reochord as the resistance of a parallel circuit is

, (35)

. (36)

Let's determine the value of the current I 0 in the power supply circuit:

;

. (37)

Knowing the current I 0, you can determine the resistance of the resistor R 8:

To check the correctness of the calculation, it is necessary to check the value of the coefficient η according to the formula

. (39)

The resistance of the measuring circuit resistors must be calculated with an accuracy: R 3 , R 6 - ±0.05 Ohm; R 7, R 8, R 9, R 10 - ± 0.5 Ohm.

LITERATURE

1. GOST 2.001-70 ESKD. General provisions.

2. Guidelines for the design of course and diploma projects for students of the specialty "Autoultization of heat and power processes". - Kyiv: KPI, 1982.

3. GOST 2.301-68. (ST. SEV 1181-78) ESKD. Formats.

4. GOST 2.302-68. (ST. SEV 118C-78). ESKD. Scales.

5. GOST 24.302-80. Technical documentation system for automated control systems. General requirements for the implementation of schemes.

6. State Committee for Science and Technology. Industry-wide guidelines teaching materials on the creation of process control systems in industries (ORMM-2 process control systems). - M., 1979.

7. Klyuev A.S., Glazov B.V., Dubrovsky A.Kh. Design of automation systems for technological processes: a reference guide. - M.: Energy, 1980.

8. GOST 24.206-80. Technical documentation system for automated control systems. Requirements for the content of technical support documents.

9. ST SEV 1986-79. Conditional graphic designations in schemes. Main energy equipment and pipelines.

10. ST SEV 1178-78. Lines. Basic rules for implementation.

11. GOST 21.404-85. System of design documents for construction. Automation of technological processes. Conventional designations of devices and automation equipment in diagrams.

12. GOST 2.304-81. Drawing fonts.

13. GOST 2.307-68. Application of dimensions and limit deviations.

14. GOST 2.303-68. Lines.

15. Canary B.D. etc. Automatic devices, regulators and computing systems. - D.: Mashinostroenie, 1976.

16. Glinkov G.M., Makovsky V.A., Dotman S.D. Design of control systems and automatic regulation metallurgical processes: A guide for course and diploma design. - M.: Metallurgy, 1970.

17. Shipetin L.I. Technique for designing automation systems for technological processes. - M.: Mashinostroenie, 1976.

18. Rules for measuring the flow of gases and liquids by standard narrowing devices RD-50-2/3-80. - M.: Publishing house of standards, 1982. -318 p.

19. Rules 28-64. Flow measurements of liquids, gases and vapors with standard charts and nozzles. - M.: Publishing house of standards, 1980.

20. Industry norms. Installation of measuring instruments and automation equipment. T. 3. (Measuring narrowing devices). Ministry of Energy and Electrification of the USSR, 1967.

21. GOST 24.203-80. Technical documentation system for automated control systems. Requirements for the content of system-wide documents.

22. GOST 24.301-80. Technical documentation system for automated control systems. General requirements for the execution of text documents.

23. Album of graphs to rules 28-64 for measuring the flow of liquids, gases and vapors with standard diaphragms and nozzles. - M.: Publishing house of standards, 1964.

24. Nesterenko A.D. etc. Commissioning guide automatic devices control and regulation. - Kyiv: Naukova Dumka, 1976.

25. Preobrazhensky V.P. Thermotechnical measurements and devices. -II.: Energy, 1978.

26. Andreev A.A. Automatic indicating, self-recording and control devices. - L .: Mashinostroenie, 1973.

27. GOST 2.105-79 (ST SEV 2667-80).

28. GOST 2.501-68. Accounting and storage rules.

29. State system of industrial devices and means of automation: Nomenclature catalogue. Part I. - M.: TsNIITEPtsriborostroeniya, means of automation and control systems, 1984. - 171 p.

30. State system of industrial devices and means of automation: Nomenclature catalogue. Part 2. - M.: TsNIITEPtsriborostroeniya, means of automation and control systems, 1984. - 155 p.

31. State system of industrial Instruments and means of automation: Nomenclature catalog. Part 3. - M .: TsNIITEPtsriborostroeniya, means of automation and control systems, 1984. - 52 p.

32. Devices, means of automation and computer technology for nuclear power: Nomenclature catalog GSP. Add. to Ch. I. - M.: TsNIITEPtsriborostroeniya, means of automation and control systems, 1983. - 167 p.

33. Ivanova G.M., Kuznetsov N.D., Chistyakov B.C. Thermotechnical measurements and devices. - M.: Energoatomizdat, 1984. - 232 p.

Introduction

1. The composition of the course work

2. Choice of technical means of measurement

3. Explanations for the graphic part

4. Explanations for the calculation part

4.1 Calculation of the narrowing device

4.2 The order of calculation of the throttle device

4.3 Orifice design

4.4 Calculation of the measuring circuit of the automatic potentiometer

4.5 Calculation of the measuring circuit of the automatic bridge

Literature

INTRODUCTION

Future specialists studying in the specialty 140104 "Industrial Heat Power Engineering" receive this knowledge when studying the discipline "Heat Engineering Measurements". The course work, provided for by the work program of this discipline, contributes to the consolidation, deepening and generalization of the knowledge gained by students during the training, and the application of this knowledge to the integrated solution of specific engineering problems for the development of schemes for thermal control of thermal power plants.

1. COMPOSITION OF COURSE WORK

Course work on the design of the measuring channel for monitoring the physical parameter of the process consists of an explanatory note and a graphic part.

The text part (explanatory note) of the course work includes the following main sections:

· Introduction;

· Choice of technical means of measurement;

· Calculation of measurement channel error;

· Calculation of the narrowing device (measuring circuit of the secondary device);

The graphic part of the work includes:

· functional diagram of the developed measuring channel;

· drawing of the narrowing device (assembly drawing of the installation of the primary converter on the process equipment).

2. CHOICE OF TECHNICAL INSTRUMENTS OF MEASUREMENT

Specific types of measuring instruments are selected taking into account the characteristics of the technological process and its parameters.

First of all, they take into account such factors as fire and explosion hazard, aggressiveness and toxicity of the environment, the range of information signal transmission, the required accuracy and speed. These factors determine the choice of methods for measuring technological parameters, the required functionality of instruments (indication, recording, etc.), measurement ranges, accuracy classes, type of remote transmission, etc.

Devices and transducers should be selected according to reference literature, based on the following considerations:

To control the same parameters of the technological process, it is necessary to use the same type of measuring instruments, mass-produced;

With a large number of identical parameters, it is recommended to use multi-point devices;

The accuracy class of instruments must comply with technological requirements;

The most common types of industrial secondary devices included in the State system of industrial devices and automation equipment (GSP) are presented in table 1.

Table 1

PV devices are secondary devices of the "Start" pneumatic system and are used to measure any technological parameters previously converted into compressed air pressure (unified pneumatic signal).

KSM-4 balanced bridges work with one or more standard calibration resistance thermometers, and KSU-4 milliammeters with one or more DC signal sources.

Secondary KSD devices work together with primary measuring transducers equipped with differential transformer sensors.

Each type of instrument mentioned above is available in various modifications that differ in size, measurement ranges, number of input signals, availability of auxiliary devices, etc.

When choosing secondary devices for joint operation with the same type of sensors of the same calibration and with the same measurement limits, it should be taken into account that KSP, KSM, KSD devices are produced with a number of points of 3,6,12. Multi-point devices have a switch that automatically and alternately connects the sensor to the measuring circuit. The printer, located on the carriage, prints points on the diagram with the serial number of the sensor.

When choosing sensors and secondary devices for joint operation, attention should be paid to matching the output signal of the sensor and the input signal of the secondary device.

table 2

The most common intermediate GSP converters

The intermediate converter NP-3 is used as a normalizing converter for converting the output signal of a differential transformer converter into a unified current signal.

Converter EPP-63 carry out the transition from the electric branch of the GSP to the pneumatic one.

Thermal measurements

1. The concept of measurement

Measurement is the process of obtaining by experience a numerical relationship between the measured value and some of its value, taken as a unit of comparison.

Numerical value of the measured value

The number expressing the ratio of the measured quantity to the unit of measurement is called the numerical value of the measured quantity; it can be integer or fractional, but is an abstract number. The value of a quantity taken as a unit of measurement is called the size of this unit.

The smaller the selected unit, the larger the numerical value for the measured quantity. The result of any measurement is a named number. As a result, for the definiteness of writing the measurement result, next to the numerical value of the measured quantity, the abbreviated designation of the accepted unit is placed. When choosing units of measurement, it is necessary to take into account the "convenience" factor - the measurement result, if possible, should be expressed in a "convenient" number: not too large and not too small.

If the unit of measurement is presented in the form of a specific sample, called a measure, then the measurement process is reduced to a direct comparison of the measured value with the measure, as a material expression of the unit of measurement.

In those cases when a direct comparison is impossible or difficult to carry out, the measured value is converted into some other physical quantity that is uniquely related to the measured value and is more convenient for measurement. For example, temperature measurement with a liquid-glass thermometer is reduced to determining the length of the liquid column, expressed in scale divisions, and temperature measurement using a resistance thermometer is reduced to determining electrical resistance, etc.

Direct measurements

According to the method of obtaining the numerical value of the desired value, measurements can be divided into two types: direct and indirect.

Direct measurements are those whose results are obtained directly from experimental data. In this case, the value of the desired quantity is obtained either by direct comparison with the measures, or by means of measuring instruments graduated in the appropriate units. With direct measurements, the result is expressed directly in the same units as the measured value. Direct measurements are a very common type of technical measurements. These include measurements of length - with a meter, temperature - with a thermometer, pressure - with a manometer, etc.

Indirect measurements

Indirect measurements include those whose result is obtained on the basis of direct measurements of several other quantities associated with the desired value by a certain dependence.

Indirect measurements include the determination of the flow rate of liquid, gas and steam from the pressure drop in the narrowing device, etc.

Indirect measurements are used in engineering and scientific research in cases where the desired value cannot be or is difficult to measure directly by direct measurement or when indirect measurement yields more accurate results.

Measurement methods

The measurement method is understood as a set of methods for using the principles and means of measurement.

The principle of measurement is understood as a set of physical phenomena on which measurements are based, for example, temperature measurement using the thermoelectric effect, measurement of the flow of liquids by pressure drop in a narrowing device.

The process of measurement, methods of carrying it out and the means of measurement with which it is carried out depend on the measured quantity, existing methods and measurement conditions.

In metrological practice, in addition to the considered types of measurements, cumulative and joint types of measurements are used.

Depending on the purpose and the accuracy required for them, measurements are divided into laboratory (accurate) and technical.

When performing thermotechnical measurements, the method of direct assessment, the method of comparison with the measure and the zero method are widely used.

The direct evaluation method is understood as a measurement method in which the value of the measured quantity is determined directly from the reading device of a direct measuring instrument, for example, pressure measurement with a manometer, temperature measurement with a thermometer, etc. It is the most common, especially in industrial settings.

Method of comparison with a measure - a method in which the measured value is compared with the value of a reproducible measure, for example, measurement e. d.s. thermoelectric thermometer or DC voltage on the compensator by comparison with e. d.s. normal element. It is often referred to as compensatory.

The null method is one in which the effect of the measured quantity is completely balanced by the effect of the known quantity, so that as a result their mutual action is reduced to zero. The device used in this case serves only to establish the fact that equilibrium has been achieved, and at this moment the reading of the device becomes equal to zero. The instrument used in the null method does not, by itself, measure anything and is therefore commonly referred to as the null instrument. The zero method has a high measurement accuracy. Zero devices used for the implementation this method should have high sensitivity. The concept of accuracy is not applicable to zero instruments. The accuracy of the measurement result produced by the zero method is determined mainly by the accuracy of the exemplary measure used and the sensitivity of the zero instrument.

General information about measuring instruments

Measuring instruments are called technical means used in measurements and having normalized metrological characteristics - characteristics of the properties of measuring instruments that affect the results and measurement errors.

Types of measuring instruments

The main types of measuring instruments are measures, measuring instruments, measuring transducers and measuring devices.

Measure - a measuring instrument designed to reproduce a physical quantity of a given size. For example, a weight is a measure of mass; measuring resistor - a measure of electrical resistance; temperature lamp - a measure of brightness or color temperature.

A measuring device is a measuring instrument designed to generate a signal of measuring information in a form accessible to direct perception by an observer.

A measuring device whose readings are a continuous function of changes in the measured quantity is called an analog measuring device. If the readings of a device that automatically generates discrete signals of measuring information are presented in digital form, then such a device is called digital.

An indicating measuring device is a device that allows only the reading of indications. If the measuring instrument provides for recording readings, then it is called a recording instrument.

A self-recording measuring device is a recording device in which readings are recorded in the form of a diagram. The registering device, which provides for printing readings in digital form, is called a printing device.

A direct-acting measuring device is a device in which one or more conversions of the measuring information signal in one direction are provided, i.e. without the use of feedback, for example, indicating pressure gauge, mercury-glass thermometer.

A measuring device in which the input value is integrated with respect to time or another independent variable is called an integrating measuring device.

A measuring transducer is a measuring instrument designed to generate a measurement information signal in a form convenient for transmission, further transformation, processing and (or) storage, but not amenable to direct perception by an observer. Measuring transducers, depending on their purpose and functions, can be divided into primary, intermediate, transmitting, scale and others.

The primary converter is the measuring transducer, to which the measured value is connected, i.e. the first in the measuring chain. Examples include thermoelectric thermometer, resistance thermometer, flow meter constriction device. The measuring transducer, which occupies a place in the measuring circuit after the primary one, is called intermediate.

A transmitting measuring transducer is a measuring transducer intended for remote transmission of a measuring information signal.

A scale measuring transducer is a measuring transducer designed to change a value by a given number of times, for example, a measuring current transformer, a voltage divider, a measuring amplifier, etc.

Measuring devices are called measuring instruments, consisting of measuring instruments and measuring transducers. Measuring devices, depending on their purpose and functions, can be divided into primary and intermediate measuring devices (devices).

Under the primary measuring device (primary device) understand the measuring instrument, which summed up the measured value. An intermediate measuring device (intermediate device) is a measuring instrument to which the output signal of the primary converter is connected (for example, the pressure drop created by the narrowing device). Primary and intermediate devices equipped with transmitting converters can be made with reading devices or without them.

Secondary measuring devices (secondary devices) are measuring instruments that are designed to work in conjunction with primary or intermediate devices, as well as with some types of primary and intermediate converters.

In addition to the considered measuring instruments, more complex measuring devices of automatic action, the so-called measuring information systems, are used. Such systems are understood as devices with automatic multi-channel (at many points) measurement, and in some cases, information processing according to some given algorithm.

It should be noted that one of the important features of new developments of measuring instruments and elements for automation devices (automatic control, regulation and control) is the unification of the output and input signals of converters, primary, intermediate and secondary devices. Unification of output and input signals ensures the interchangeability of measuring instruments, reduces the variety of secondary measuring devices. In addition, unified devices and elements significantly increase the reliability of the operation of automation devices and open up broad prospects for the use of information computers.

Depending on the purpose, and at the same time on the role that various measuring instruments (measures, measuring instruments and transducers) perform in the measurement process, they are divided into three categories:

1) working measures, measuring instruments and transducers;

2) exemplary measures, measuring instruments and transducers;

3) standards.

Working measuring instruments are all measures, devices and converters intended for practical everyday measurements in all sectors of the national economy. They are divided into measuring instruments of increased accuracy (laboratory) and technical.

Exemplary are called measures, instruments and primary converters (for example, thermoelectric thermometers, resistance thermometers) intended for verification and calibration of working measures, measuring instruments and converters. The upper measurement limit of the reference instrument must be equal to or greater than the upper measurement limit of the instrument under test. The permissible error of the exemplary instrument or measuring device in the case when corrections to its readings are not taken into account should be significantly less (4-5 times) than the permissible error of the tested instrument.

Working measures, measuring instruments and transducers are verified at the institutes of measures and measuring instruments and in the control laboratories of the system of the State Committee for Standards, Measures and Measuring Instruments.

Exemplary measures, measuring instruments and primary transducers intended for verification of workers are verified at the State Institutes of Measures and Measuring Instruments and in the State Control Laboratories of the 1st category for even more accurate exemplary measures, instruments and transducers, i.e. exemplary measuring instruments of a higher category (for example, exemplary instruments of the 2nd category are verified by comparison with exemplary instruments of the 1st category). Exemplary measures, instruments and converters of the highest category (1st category) in this field of measurement are verified at the State Institutes of Measures and Measuring Instruments according to the relevant working standards,

Measures, measuring instruments and primary transducers that serve to reproduce and store units of measurement with the highest (metrological) accuracy achievable at a given level of science and technology, as well as to verify measures, instruments and transducers of the highest category, are called standards.

Measurement error

When measuring any quantity, no matter how carefully we make the measurement, it is not possible to obtain a result free from distortion. The reasons for these distortions may be different. Distortions can be caused by the imperfection of the applied measurement methods, measuring instruments, variability of measurement conditions, and a number of other reasons. The distortions that result from any measurement cause the measurement error - the deviation of the measurement result from the true value of the measured quantity.

The measurement error can be expressed in units of the measured value, i.e. in the form of an absolute error, which is the difference between the value obtained during the measurement and the true value of the measured quantity. The measurement error can also be expressed as a relative measurement error, which is the ratio to the true value of the measured quantity. Strictly speaking, the true value of the measured quantity always remains unknown; one can only find an approximate estimate of the measurement error.

The error of the measurement result gives an idea of which figures in the numerical value of the quantity obtained as a result of the measurement are doubtful. It is necessary to round the numerical value of the measurement result in accordance with the numerical digit of the significant figure of the error, i.e. the numerical value of the measurement result must end with a digit of the same digit as the error value. When rounding, it is recommended to use the rules of approximate calculations.

Types of measurement error

Measurement errors, depending on the nature of the causes that cause their appearance, are usually divided into: random, systematic and gross.

Random error is a measurement error that changes randomly with repeated measurements of the same quantity. They are caused by causes that cannot be determined by measurement and cannot be influenced. The presence of random errors can only be detected by repeating measurements of the same quantity with the same care.

Random measurement errors are not constant in value and sign. They cannot be determined separately and cause inaccuracies in the measurement result. However, with the help of probability theory and statistical methods, random measurement errors can be quantified and characterized in their totality, and the more reliable, the greater the number of observations.

A systematic error is understood as a measurement error that remains constant or regularly changes during repeated measurements of the same value. If systematic errors are known, i.e. have a specific meaning and a specific sign, they may be deleted by amendment.

Usually, the following types of systematic errors are distinguished: instrumental, measurement methods, subjective, installations, methodological.

Instrumental errors are understood as measurement errors that depend on the errors of the measuring instruments used.

The error of the measurement method is understood as the error resulting from the imperfection of the measurement method.

Subjective errors (occurring in non-automatic measurements) are caused individual features observer, for example, delay or advance in registering the moment of any signal, incorrect interpolation when reading readings within one division of the scale, from parallax, etc.

Installation errors occur due to incorrect installation of the arrow of the measuring instrument at the initial mark of the scale or careless installation of the measuring instrument, for example, not on a plumb line or level, etc.

Methodological measurement errors are such errors that are determined by the conditions (or methodology) for measuring a quantity (pressure, temperature, etc. of a given object) and do not depend on the accuracy of the measuring instruments used. A methodological error can be caused, for example, by the additional pressure of a liquid column in connecting line if the pressure measuring device is installed below or above the pressure tap. When performing measurements, especially accurate ones, it must be borne in mind that systematic errors can significantly distort the measurement results. Therefore, before proceeding with the measurement, it is necessary to find out all possible sources of systematic errors and take measures to exclude or determine them. In non-automatic measurements, much depends on the knowledge and experience of the experimenter.

Careful and correct installation of measuring instruments is necessary to eliminate installation errors in both precise and technical measurements.

12. Measurement accuracy

Depending on the purpose and requirements for measurement accuracy, measurements are divided into precise (laboratory) and technical. Accurate measurements, as a rule, are carried out repeatedly and with the help of measuring instruments of increased accuracy. By repeating the measurements, the influence of random errors on their result can be weakened, and, consequently, the accuracy of the measurement can be increased. At the same time, it must be borne in mind that even under favorable conditions, the accuracy of measurement cannot be higher than the accuracy of verification of the measuring instruments used.

When performing technical measurements that are widely used in industry, and sometimes in laboratory conditions, working measuring instruments are used, which are not supplied with amendments during their verification.

When performing accurate measurements, they use measuring instruments of increased accuracy, and at the same time, more advanced measurement methods are used. However, despite this, due to the inevitable presence of random errors in any measurement, the true value of the measured quantity remains unknown and instead of it we take some average arithmetic value, with respect to which, with a large number of measurements, as probability theory and mathematical statistics show, we have reasonable confidence to consider that it is the best approximation to the true value. Technical measurements of practically constant values, widely used in industry and in laboratory conditions, are understood as measurements performed once with the help of working (technical or increased accuracy) measuring instruments graduated in the appropriate units. When performing direct technical measurements, a single reading of the readings on the scale or diagram of the measuring device is taken as the final result of measuring this quantity. The accuracy of the direct measurement result when using a direct-acting measuring instrument can be estimated by the approximate maximum (or limiting) error,

When performing technical measurements, random errors in most cases are not determining the accuracy of the measurement and therefore there is no need for multiple measurements and calculation of the arithmetic mean of the measured value, since the results of individual measurements will coincide within the permissible errors of working measuring instruments. It should also be noted that technical measurements allow measurements of various quantities with least cost means and forces, in the shortest possible time and with sufficient accuracy.

13. General information about temperature

Temperature is one of the most important parameters of technological processes. It has some fundamental features, which necessitates the use of a large number of methods and technical means for its measurement.

Temperature can be defined as a thermal state parameter. The value of this parameter is determined by the average kinetic energy of the translational motion of the molecules of a given body. When two bodies come into contact, for example, gaseous ones, the transfer of heat from one body to another will occur until the values of the average kinetic energy of the translational motion of the molecules of these bodies are equal. With a change in the average kinetic energy of the motion of the body's molecules, the degree of its heating changes, and at the same time, the physical properties of the body also change. At a given temperature, the kinetic energy of each individual molecule of the body can differ significantly from its average kinetic energy. Therefore, the concept of temperature is statistical and is applicable only to a body consisting of a sufficiently large number of molecules; when applied to a single molecule, it is meaningless.

It is known that with the development of science and technology, the concept of "temperature" is expanding. For example, when studying high-temperature plasma, the concept of "electron temperature" was introduced, which characterizes the flow of electrons in plasma.

Temperature scales

The ability to measure temperature with a thermometer is based on the phenomenon of heat exchange between bodies with different degrees of heating and on changes in the thermometric (physical) properties of substances when heated. Consequently, in order to create a thermometer and build a temperature scale, it would seem possible to choose any thermometric property that characterizes the state of a substance and, based on its changes, build a temperature scale. However, it is not so easy to make such a choice, since the thermometric property must change unambiguously with temperature, be independent of other factors, and allow the measurement of its changes to be relatively simple and convenient way. In fact, there is not a single thermometric property that could fully satisfy these requirements in the entire range of measured temperatures.

Let us use, for example, to measure temperature by the volumetric expansion of bodies when heated, and take the mercury and alcohol thermometers of the usual type. If their scales between the points corresponding to the temperatures of boiling water and melting ice at normal atmospheric pressure are divided into 100 equal parts (counting as 0 the melting point of ice), then it is obvious that the readings of both thermometers - mercury and alcohol - will be the same at points 0 and 100, because these temperature points were taken as reference points to obtain the main scale interval. If these thermometers measure the same temperature of any medium not at these points, then their readings will be different, since the coefficients of volumetric thermal expansion of mercury and alcohol depend differently on temperature.

In liquid-glass thermometers currently used, one does not have to deal with such a discrepancy in readings, since all modern thermometers have a single International practical temperature scale, which is built on a completely different principle (the method for constructing this scale is described below).

We would meet with the same difficulties if we tried to implement a temperature scale on the basis of some other physical quantity, for example, the electrical resistance of metals, etc.

Thus, when measuring temperature on a scale built on an arbitrary assumption of a linear relationship between a property of a thermometric body and temperature, we still do not achieve an unambiguous numerical measurement of temperatures. Therefore, the temperature measured in this way (i.e., by the volumetric expansion of some liquids, by the electrical resistance of metals, etc.) is usually called conditional, and the scale on which it is measured. - conditional scale.

It should be noted that among the old conditional temperature scales, the centigrade Celsius temperature scale, the degree of which is equal to a hundredth of the main temperature interval, is most widely used. The main points of this scale are the melting point of ice (0) and the boiling point of water (100) at normal atmospheric pressure.

In order to further improve the conditional temperature scale, work was carried out to study the possibility of using a gas thermometer to measure temperatures. For the manufacture of gas thermometers, they used real gases (hydrogen, helium, and others), and at the same time such gases that, in their properties, differ relatively little from the ideal.

The way to create a unified temperature scale, not associated with any particular thermometric properties and suitable for a wide temperature range, was found in the use of the laws of thermodynamics. Independent of the properties of the thermometric substance is the scale based on the second law of thermodynamics. It was proposed in the middle of the last century by Kelvin and was called the thermodynamic temperature scale.

The Kelvin thermodynamic temperature scale was the initial scale for constructing temperature scales that do not depend on the properties of the thermometric substance. In this scale, the interval between the melting point of ice and the boiling point of water (to maintain continuity with the hundred-degree Celsius temperature scale) was divided into 100 equal parts.

DI. Mendeleev in 1874 for the first time scientifically substantiated the expediency of constructing a thermodynamic temperature scale not by two reference points, but by one. Such a scale has significant advantages and makes it possible to determine the thermodynamic temperature more accurately than a scale with two reference points.

However, the thermodynamic temperature scale, which is purely theoretical, did not open the way for its practical use even at first. For this purpose, it was necessary to establish a connection between the thermodynamic scale and real instruments for measuring temperatures. Of the temperature meters, gas thermometers deserve the most attention, the readings of which can be related to the thermodynamic temperature scale by introducing the concept of an ideal gas scale. The thermodynamic scale, as is known, coincides with the scale of an ideal gas, if we take the melting point of ice as 0 and the boiling point of water as 100 at normal atmospheric pressure. This scale was given the name centigrade thermodynamic temperature scale.

However, gas thermometers can be used to reproduce the thermodynamic centigrade temperature scale only up to temperatures not exceeding 1200°C, which cannot meet the modern requirements of science and technology. The use of gas thermometers for higher temperatures encounters great technical difficulties, which are currently insurmountable. In addition, gas thermometers are rather bulky and complex devices and are very inconvenient for everyday practical purposes. As a result, in order to more conveniently reproduce the thermodynamic centigrade temperature scale, a practical scale was adopted in 1927, which was called the International Temperature Scale of 1927 (ITS-27).

The Regulations on ITS-27, adopted by the Seventh General Conference on Weights and Measures as provisional, after some clarifications, were finally adopted in 1933 by the Eighth General Conference on Weights and Measures. In the USSR, MTSh-27 was introduced on October 1, 1934 by the All-Union Standard (OST VKS 6954).

In subsequent years, work was carried out to revise the ITS-27 in order to achieve a more accurate agreement with the thermodynamic scale in the form in which it was adopted, but with some improvements based on refined and newly obtained experimental data. As a result of the work carried out by the Advisory Committee on Thermometry, a draft Regulation on the International Practical Temperature Scale of 1948 (IPTS-48) was developed, approved by the ninth General Conference on Weights and Measures.

For a scale with one reference point, it is necessary to assign a certain numerical value to its only experimentally realized point. The absolute zero point will then serve as the lower boundary of the temperature interval.

The maximum error in reproduction of the boiling point of water is 0.01°C, the melting point of ice is 0.001°C. The triple point of water, which is the equilibrium point of water in the solid, liquid and gaseous phases, can be reproduced in special vessels with a marginal error of no more than 0.0001°C.

With all this in mind, and having carefully considered all the numerical results obtained in various metrological laboratories in a number of countries, the Advisory Committee on Thermometry has recognized that the best value for the temperature of the triple point of water, lying above the melting point of ice by 0.01 ° C, is a value of 273.16 K. The Tenth General Conference on Weights and Measures in 1954 based on this established a thermodynamic temperature scale with one reference point - the triple point of water.

The new definition of the thermodynamic temperature scale was reflected in the "Regulations on IPTS-48. Edition 1960", adopted by the Eleventh General Conference on Weights and Measures. This scale provides for the use of two temperature scales: the thermodynamic temperature scale and the practical temperature scale. The temperature on each of these scales can be expressed in two ways: in degrees Kelvin (K) and in degrees Celsius (°C), depending on the origin (zero position) on the scale.

In foreign literature, along with the expression of temperature in Kelvin (K) and degrees Celsius (°C), degrees Fahrenheit (°P) and degrees Rankine (°Ka) are sometimes used. It should be borne in mind that earlier the degree Fahrenheit was characteristic of the scales of mercury-glass thermometers, and now, like the degree Celsius, it means that the temperature is expressed according to the IPTS, but with a different numerical value.

The unit kelvin is defined as 1/273.16 of the thermodynamic temperature of a flat point of water. Celsius is equal to Kelvin. Temperature differences (intervals) are expressed in Kelvin, but can also be expressed in degrees Celsius instead of the previously used deg (deg).

Liquid glass thermometers

Basic information. Liquid glass thermometers are used to measure temperatures in the range from -200 to +750 C. Despite the fact that, in addition to glass liquid thermometers, there are a number of other temperature measuring devices that meet the requirements of modern process control technology to a large extent, glass thermometers have become widespread both in laboratory and industrial practice due to ease of handling, sufficiently high measurement accuracy and low cost.

The principle of operation of liquid-in-glass thermometers is based on the thermal expansion of the thermometric liquid contained in the thermometer. In this case, obviously, the readings of a liquid thermometer depend not only on the change in the volume of the thermometric liquid, but also on the change in the volume of the glass container in which this liquid is located. Thus, the observed (visible) change in the volume of the liquid is underestimated by a size correspondingly equal to the increase in the volume of the reservoir (and partly of the capillary).

Mercury, toluene, ethyl alcohol, kerosene, petroleum ether, pentane, etc. are used to fill liquid thermometers. The scope of their application, as well as the values of the coefficients of actual and apparent expansion of liquids are given in Table 3-1-1.

Mercury thermometers are the most widely used liquid thermometers. They have a number of advantages due to the significant advantages of mercury, which does not wet glass, is relatively easy to obtain in a chemically pure form and remains liquid at normal atmospheric pressure over a wide temperature range (from -38.87 to +356.58 ° C). It should also be noted that the saturated vapor pressure of mercury at a temperature exceeding 356.58 ° C is small compared to the saturated vapor pressure of other liquids. This makes it possible, by a relatively small increase in pressure over mercury in the capillary, to noticeably increase its boiling point, and at the same time to expand the temperature range for the use of mercury thermometers.

Among the shortcomings of mercury from the point of view of thermometry is a relatively small coefficient of expansion (see table).

When measuring temperature with thermometers filled with organic liquids, it must be borne in mind that they wet the glass, and as a result, the reading accuracy decreases.

Thermometers, depending on the purpose and range of temperature measurements, are made of glass of various grades.

Thermometric liquids

liquid	Possible applications, o C	Average coefficient of volumetric thermal expansion, K -1
		valid


Ethanol

Petroleum ether

Notes:

The apparent expansion coefficient of mercury in thermometric borosilicate glass is 0.000164 K - 1 , and in quartz glass 0.00018 K -1 .

Under the apparent coefficient of volumetric thermal expansion understand the difference between the coefficients of volumetric thermal expansion of thermometric liquid and glass.

Liquid level measurement. Instruments for measuring the level of liquid.

Level measurement of liquids plays an important role in process automation in many industries. These measurements are especially important in cases where maintaining a certain constant level, for example, the water level in the steam generator drum, the liquid level in tanks, apparatus and other devices, is associated with the conditions for the safe operation of the equipment. The technical means used to measure the level of a liquid are called level gauges. Devices designed to signal the limiting levels of liquids are called level switches. Level gauges are also widely used in various industries to measure the level of the amount of liquid in tanks, tanks and other devices.

Level gauges, designed to measure the level of a liquid in order to maintain it constant, have a double-sided scale. The scales and chart paper of these level gauges are calibrated in centimeters or meters, and the instruments used to measure the water level in the steam generator drum are calibrated in millimeters.

Level gauges used to measure the level of the amount of liquid in tanks, tanks and other devices have a one-sided scale. The scales and chart paper of these level gauges are graduated in centimeters and meters, and sometimes in percentages.

Level gauges used to measure the liquid level in order to maintain it constant within certain limits are equipped with a device for signaling the maximum level deviations from the set value.

For liquid level detectors, the contact device is triggered at a given level value for a given object.

Depending on the requirements for automation of technological processes, various methods of measuring the liquid level are used. If there is no need for remote transmission of readings, the liquid level can be measured with sufficient accuracy and reliability using index glasses or showing differential. level gauges.

Measurement of the liquid level with index glasses is based on the principle of communicating vessels. The design of the fittings and the material of the indicator glasses depend on the pressure and temperature of the liquid, the level of which must be controlled.

For remote measurement of the level of liquids under atmospheric, vacuum or excess pressure, the method of measuring the pressure difference using differential pressure is used. pressure gauges. Many industries also use the method of controlling the level of liquids using a float (or displacer).

In the chemical, petrochemical and a number of other industries, in addition to the above methods for measuring the level of liquids, capacitive, ultrasonic, acoustic and radioisotope level gauges are used. Piezometric level gauges are used to measure the level of aggressive crystallizing liquids and slurries in open containers.

Measurement of the water level in the drum of steam generators. Types of level gauges.

Normal operation of drum steam generators can only be carried out if the water level in the drum is strictly maintained within certain allowable limits. Therefore, measuring the water level in the drum, especially in modern powerful steam generators with a very limited water supply, is an important and responsible task during their operation.

Control of the water level in the drum of steam generators with low steam production and low steam pressure in the drum is carried out by direct observation of the level using a water meter supplied with the steam generator. In some cases, for greater reliability, a reduced water level indicator in the drum is additionally installed directly at the steam generator. In this case, the showing differential is used. level gauges or a reduced Igema level indicator.

Steam generators with a capacity of 35 t / h and above, along with water-indicating devices on the drum, supplied together with them, are additionally equipped with differential. level gauges. Secondary indicating and self-recording instruments of level gauges are installed on the control panel of the steam generator or unit. These devices are usually equipped with a contact device for signaling an unacceptable change in the water level in the steam generator drum.

On modern powerful steam generators of thermal power plants, in addition to level gauges for measuring the water level in the drum, an additional differential is installed. level gauges with secondary indicating devices equipped with a contact device. With the help of these level gauges, technological protection is carried out when the steam generator is overfilled with water and when the level in its drum is lost. In this case, the contacts of the secondary devices of the level gauges are included in the protection device according to the "two out of two" or "two out of three" scheme.

Differentials are widely used as level gauges. membrane pressure gauges of the DM type complete with secondary devices of the differential transformer system or differential pressure gauges-level gauges of the DME type with a direct current output signal, operating in conjunction with secondary devices of the KSU type,

KPU, etc., as well as with automatic regulators, information - computing and control machines.

Rice. 19-2-1. Scheme for measuring the water level in the drum with a differential pressure gauge using a two-chamber surge vessel.

To connect a differential level gauges, special leveling vessels of various designs are used to the drum of steam generators. Calculation of the scale differential. level gauges or their secondary devices are usually produced for the working (nominal) steam pressure in the drum, taking into account the type of surge vessel.

On fig. 19-2-1 shows the scheme for measuring the water level in the drum of the differential steam generator. pressure gauge using a standard two-chamber surge vessel (thermal insulation on the outer surface of the vessel not shown). In the wide part of the vessel connected to the steam space of the drum, the water (condensate) level is kept constant. In pipe 2 connected to the water space of the drum, the water level changes as the water level in the drum changes. When installing a shut-off valve on the pipe connecting the steam space of the drum with the equalizing vessel, it is necessary that its spindle be in a horizontal position. Otherwise, a water lock may form, which can cause unstable operation of the differential. pressure gauge.

All types of surge vessels used to measure the water level in the drum of steam generators using dif. pressure gauge, allow you to provide reliable control of it in a wide range (from +315 to - 315 mm) only at a nominal value of steam pressure, subject to certain conditions. Level gauges operating with these surge vessels at varying steam pressure in the steam generator drum over a wide range (from the nominal value to 0.2 MPa) have a limited error only in the region of one fixed level value.

Water level measurement in steam turbine condensers

Measurement of the level of condensate (water) in the turbine condenser has importance during their operation. An increase in the water level in the condenser leads to flooding of the lower rows of cooling pipes, which causes supercooling of the condensate. A significant decrease in the level of condensate impairs the operation of the condensate pump due to a decrease in the back pressure from the suction pipe of the pump.

For greater reliability, the water level in the turbine condenser is monitored locally and remotely. Local level control is carried out using a water-indicating glass or an indicating level gauge, installed in the first case directly on the condenser, and in the second - near it. To remotely measure the water level in the condenser, differential level gauges are used. pressure gauges equipped with a converter with an output electrical signal. Secondary indicating instruments of level gauges are installed on the control panel of the turbine or unit. Indicating instruments must be equipped with a contact device for signaling an increase and decrease in the level in the condenser.

Deviation of the parameters from the nominal values for which the differential scale was calculated. pressure gauge, leads to a change in the readings of level gauges, as well as when measuring the water level in the drum of steam generators.

Measurement of the level of liquids in tanks, apparatuses and reservoirs.

To measure the level of liquids in tanks, apparatuses and reservoirs, the method of measuring the pressure difference using differential is widely used. pressure gauge. Depending on the requirements for automation of technological processes, apply different types diff. pressure gauges. If there is no need for remote transmission of level readings, then it is advisable to use differential. manometers with reading device. These diff. pressure gauges can be equipped with a contact device for signaling limit values of the level. For remote level measurement can be used dif. pressure gauges with electrical or pneumatic output signal, complete with the appropriate secondary instrument.

Since the liquid to be measured may be under atmospheric, vacuum or gauge pressure, this must be taken into account when choosing the type and model of differential. pressure gauge, as they are produced for different maximum permissible operating overpressure. Limit nominal differential pressure dif. pressure gauge is selected depending on the level measurement range.

To connect a differential pressure gauge to a tank or other device, various types of surge vessels are used. This vessel should have such a size at which it would be possible to neglect the additional error of the dif. pressure gauge.

The method for measuring the level of a neutral, non-viscous liquid in a pressurized tank, reservoir or apparatus is in principle similar to the method for measuring the water level in a steam generator drum. To connect a differential pressure gauge to a tank or to another device, a single-chamber equalizing vessel is usually used, and less often, vessels of other types. If in this case it is necessary to use separation vessels, then they are installed additionally in the differential lines. pressure gauge at the low level mark.

If, when measuring the level of a liquid, its density can vary within small limits, then the calculation of the differential scale. pressure gauge or its secondary device, it is advisable to produce for the average value of the density of this liquid.

If the properties of the liquid whose level is to be measured do not allow the connection of a dif. pressure gauge, it is necessary to use separating vessels or separating devices of other types instead of a surge vessel, which should be located in the connecting lines as close as possible to the tank or tank.

The dimensions of equalizing and separating vessels are usually chosen depending on the volume of the plus and minus chambers of the differential. pressure gauge. When using separating devices of another type, it is necessary to take into account the possible change in the readings of the level gauge.

Level measurement of liquids with float and displacer level gauges

The simplest technical tool for measuring the level of liquid in tanks is a float level indicator. The level in this case is judged by the position of the pointer attached to the counterweight, connected to the float with a cable thrown over the blocks. This method of measurement allows you to control the level of liquid in the tank under atmospheric pressure, in the case when the object is located relatively close to the observation post.

For remote measurement of the level of a liquid under atmospheric, vacuum or overpressure, displaced level gauges with a unified DC output signal 0-5 are widely used in various industries; 0-20 mA of the UB-E type or pneumatic with a pressure of 0.2-1 kgf / cm 2 (0.02-0.1 MPa) of the UB-P type. The action of level gauges UB-E and UB-P, respectively, is based on the principle of electric power or pneumatic power compensation of the force developed by the sensitive element (displacer) of the level gauge measuring unit, immersed in the liquid, the level of which is measured. Level transmitters of the UB-E type use a linear transducer with electric power compensation PLE, and UB-P level transmitters use a transducer with pneumatic power compensation.

Rice. 19-4-1. Scheme of the device of the buoy level gauge.

In addition to the considered level gauges UB-E and UB-P, other types of displaced level meters with a pneumatic output signal and level indicators with a differential transformer converter of the accuracy class are also used.

Float level gauges with an additional device are used for remote measurement of the water level in open reservoirs, the pressure created by the difference in the levels of the upper and lower pools, and the position of various types of gates. In level gauges of this type and in their secondary devices, synchros are used as measuring transducers.

To signal the limit values of the liquid level in tanks or tanks, float level switches of various types are used.

Capacitive level transmitters

Capacitive level gauges are widely used for signaling and remote measurement of the level of homogeneous liquids in various objects in the chemical, petrochemical and other industries. Capacitive level gauges can be used to measure the level of liquids under pressure up to 25-60 kgf / cm 2 (2.5-6.0 MPa) and having a temperature of - 40 to 200 C. These limitations are due to the reliability of the insulation used for manufacturing general industrial primary converters of capacitive level gauges.

Capacitive level gauges cannot be used to measure the level of viscous (more than 0.980 Pa-s), film-forming, crystallizing and precipitating liquids, as well as explosive environments.

The action of the considered level gauges is based on measuring the electrical capacitance of the primary converter, which changes in proportion to the change in the controlled level of the liquid in the tank. The primary transducer, which converts the change in liquid level into a proportional change in capacitance, is, for example, a cylindrical capacitor, the electrodes of which are arranged coaxially. For each value of the liquid level in the tank, the capacitance of the primary transducer is defined as the capacitance of two capacitors connected in parallel, one of which is formed by a part of the transducer electrodes and the liquid whose level is measured, and the second by the rest of the transducer electrodes and air or liquid vapor.

When using capacitive level gauges, it must be borne in mind that the measured liquid level is functionally related to the dielectric constant of substances. Therefore, when measuring the level of a liquid with a capacitive level gauge, it should be taken into account that the value of the dielectric constant of a liquid changes with a change in its temperature.

Depending on the electrical characteristics of the liquid, the level of which is measured by the capacitive method, are divided into non-conductive and electrically conductive. Such a division of liquid dielectrics has some conventionality, but is practically expedient.

Some types of capacitive level gauges are used for signaling and remote measurement of the level of bulk solids with constant moisture.

Capacitive level transmitters perform cylindrical and lamellar type, as well as in the form of a rigid rod or cable. In the latter case, the metal wall of the tank serves as the second electrode. To ensure the constancy of the characteristics of the transducer and improve the accuracy of level measurement, it is advisable to use transducers with a rod or cable located in a steel pipe, which is the second electrode of the transducer.

Acoustic and ultrasonic level gauges

In acoustic and ultrasonic level gauges, a method is implemented based on the use of the effect of reflection of ultrasonic vibrations from the interface between two media with different acoustic impedances.

The level gauges, called acoustic, use the method of locating the liquid level through a gaseous medium. The advantage of this method is that the acoustic energy sent to the object to measure the liquid level propagates through the gaseous medium. This provides versatility for a variety of liquids to be measured as well as high reliability for non-liquid contact sensors.

In level gauges called ultrasonic, a method is used based on the reflection of ultrasonic vibrations from the interface between media on the liquid side.

Depending on the sound wave parameter used to measure the liquid level, there are frequency, phase and pulse methods for measuring the level, as well as some combinations of them, such as pulse-frequency, etc. Each of these methods, having a common acoustic (ultrasonic) method measurement merits, has its advantages and disadvantages.

Acoustic level gauges are widely used for remote measurement of the level of liquids in various objects in the chemical, paper, food and other industries. Level gauges of this type can be used to measure the level of various liquids (homogeneous and inhomogeneous, viscous, aggressive, crystallizing, precipitating) under pressure up to 40 kgf / cm2 (4 MPa) and having a temperature of 5 to 80 ° C. Acoustic level gauges cannot be used to measure the level of liquids under high overpressure and vacuum pressure. If the liquid, the level of which must be measured, will be under a vacuum pressure of up to 0.5 kgf/cm2 (0.05 MPa), then acoustic level gauges can be used.

Ultrasonic level gauges can only be used to measure the level of homogeneous liquids and are not widely used in industry. However, ultrasonic level transmitters can measure the level of homogeneous liquids under high overpressure.

In the ECHO-1 acoustic level gauge, generator 9 generates electrical pulses with a certain repetition rate, which are converted into ultrasonic pulses using an acoustic transducer 1 installed on the tank lid. Propagating along the acoustic path, ultrasonic pulses are reflected from the interface plane and fall on the same transducer 1.

Rice. 19-6-1. Scheme of acoustic level gauge ECHO-1.

Ultrasonic level gauge. The ultrasonic level gauge uses a pulsed level measurement method based on the reflection of ultrasonic vibrations from the interface between media on the liquid side. In this case, the measure of the liquid level is also the time of passage of ultrasonic vibrations from the piezometric transducer (emitter) to the interface plane (liquid-gas) and back to the receiver. The limit of permissible basic error of the ultrasonic level gauge does not exceed 2.5% of the liquid level measurement range,

23. Level measurement of bulk solids

The measurement of the level of bulk solids in bunkers and other devices is significantly different from the measurement of the level of liquids, since the nature of the location of the material in the object does not allow us to speak of its level as a horizontal surface. The wide variety of materials that need to be measured in the energy and industrial sectors require different methods and designs of level gauges.

At thermal power plants, level gauges are needed to measure the level of lumpy (raw) coal and coal dust in bunkers. In industry, level gauges are used to measure the level of charge, coal, rock, and various powdered materials. When measuring the level of bulk solids, in particular solid fuel, it is necessary to know the nature of the movement of the material in the object (bunker) and the shape of the object. When choosing technical means for automatic level control, it is necessary to take into account the possible explosiveness of the material, the level of which is to be measured.

Bunkers for lumpy and pulverized fuel at thermal power plants in most cases have the shape of a truncated pyramid with a top directed downwards. They are made of reinforced concrete or steel. This form of the bunker has a certain effect on the nature of the movement of the fuel. With a bunker height of 8-10 m, the fuel layer in it is subjected to a sufficiently large horizontal compression, which causes a noticeable deterioration in its bulk properties. In this regard, in the bunker of any capacity in the zone of maximum pressure, the appearance of hangs and arch formation is possible. Due to the possibility of these phenomena, there should not be any protrusions on the inner surface of the hopper (especially in the zone of maximum pressures) that can distort the nature of the movement of the fuel.

Usually in the bunker, the fuel is partially located on the inner walls in the form of layers of various thicknesses. As the central layers of fuel actuate, the thickness of the layer on the walls of the bunker also decreases. As a result, the actual capacity of the bunker is reduced by 20-25% compared to the nominal one. The size of the layer of fuel on the walls depends on the angle of inclination of the walls of the hopper, the moisture content of the fuel and the coefficient of internal friction. To eliminate fuel hang-ups in the bunker, various caving devices are used.

In bunkers with lumpy fuel, the lowest point of the funnel from the side of the bunker cover is conventionally taken as the level. Coal dust, due to its high fluidity, is located in the form of a more or less even horizontal layer, however, when coal dust loses its fluid properties and caking, the level decreases with distortions, accompanied by the formation of funnels, "wells" and sticking of a layer of dust on the walls of the bunker.

To automate the loading of bunkers or other objects, it is necessary at least to provide, using signaling level gauges, automatic control of the presence of material in two sections along the height in the lower part of each bunker - to receive a signal to turn on the loading devices and in the upper part - to receive a signal to turn off the loading devices.

To ensure greater reliability of the process, it is often necessary to continuously monitor the level in bunkers or other objects. In this case, for remote measurement of the level of bulk solids in technological objects, level gauges are used, equipped with secondary devices, which must have a contact device for signaling the level limit values. The contact device of secondary devices can also be used to automate the loading of hoppers or other objects.,

Technical means intended for measuring and signaling the level of bulk solids are divided into electromechanical, electrical, electronic, pneumatic, radioactive and weight. Currently, the range of signaling devices and level meters commercially manufactured for use at TPPs is limited, some types of them have been introduced on an experimental basis, but they are not mass-produced. Radioactive level gauges, pneumatic and weight gauges at thermal power plants have not received distribution.

Bulk solids level switches

To signal the limiting levels of bulk solids and automate the loading of bunkers and other containers, various types of signaling devices are used.

In the chemical industry, level switches with sensitive transducer elements that perceive the pressure of bulk solids, the level of which is controlled, are used. This group of electromechanical devices includes membrane and pendulum level indicators. AT Food Industry membrane level switches are used, commercially available and used in control systems for the supply of flour, grain and other bulk materials in order to prevent accidental accumulation of material in the inlet and outlet gravity flows of grain processing machines.

Operating experience at thermal power plants of membrane detectors of the level of coal dust in bunkers has shown that they do not provide reliable level control due to the formation of dust layers on the walls. For the same reason, pendulum-type signaling devices cannot be recommended for monitoring coal dust.

It should be noted that in order to ensure reliable control and automation of loading bunkers with coal and dust at thermal power plants, more advanced level indicators should be created.

Devices for measuring the level of bulk solids

For continuous remote measurement of the level of bulk solids, level gauges equipped with secondary devices are used. Among the devices discussed above for remote measurement of the level of bulk solids with constant humidity, electronic capacitive level indicators EIU-2 are used. To measure the level of bulk solids, other types of capacitive level gauges are also produced. Note that capacitive devices at thermal power plants do not provide the necessary reliability for measuring the level of coal and dust in bunkers and have not been widely used.

In some industries, in particular the chemical industry, weight meters for the level or mass of bulk material in a bunker are used. As a transducer in these level gauges, a mass dose is used, which is the support of one of the legs of the bunker. The mess dose has a steel body with a piston sealed with a metal membrane. The mass dose, the connecting line and the inner cavity of the tubular spring of the manometer are filled with liquid. The measured pressure in the mass dose with a manometer is equal to the gravity of the hopper with the material in it, divided by the area of the piston.

In weighing level gauges, in addition to the mass dose, more advanced magnetoelastic transducers are also used, which provide higher measurement accuracy. To convert the gravitational force of the hopper with the material filling it into an electrical signal, magnetoelastic transducers are installed under its supports. The action of these transducers is based on the change in the magnetic permeability of the transducer steel plate during elastic mechanical deformation.

thermotechnical measurement level gauge error

principled circuit diagram weight level gauge for measuring the mass of material in the bunker using magnetoelastic transducers is shown in fig. 20-3-1.

Instruments for measuring gas composition

Measuring instruments designed to quantify the composition of a gas are called gas analyzers and gas chromatographs. These technical means, depending on their purpose, are divided into portable and automatic. Portable gas analyzers and chromatographs are used in laboratory conditions for the quantitative determination of gas composition in research work, as well as in special surveys, testing and adjustment of various industrial heat engineering installations (steam generators, furnaces, etc.). Instruments of this type are widely used to test automatic gas analyzers.

Automatic gas analyzers designed for continuous automatic measurement of the volume percentage of one analyte in gas mixture, are widely used in various industries, in particular energy. Modern automatic gas analyzers allow you to determine the content of carbon dioxide (CO,), oxygen (0 2), carbon monoxide and hydrogen (CO + H 2), CO, H 2, methane (CH 4) and other gases in a gas mixture.

Automatic gas analyzers are widely used to control the combustion process in the combustion devices of steam generators, furnaces and other units, to analyze technological gas mixtures, to determine the hydrogen content in hydrogen cooling systems for turbine generator windings, etc.

For the correct maintenance of the combustion mode, it is necessary to maintain a certain ratio between the quantities of fuel and air supplied to the furnace of the steam generator (or furnace). Insufficient amount of air leads to incomplete combustion of the fuel and entrainment of unburned products into the chimney. An excess amount of air ensures complete combustion, but requires a large amount of fuel to heat the additional volume of air. In both cases, the useful heat output of the steam generator furnace decreases. The required fuel-air ratio depends on various factors and primarily on the type of fuel. For various types of fuel, the optimal value of the excess air coefficient is set, which ensures economical operation of the installation.

Continuous monitoring of the combustion regime under operating conditions at modern thermal power plants is carried out using automatic gas analyzers according to the content of 0 2 in combustion products (flue gases). In industry and on low-power steam generators, the combustion process is sometimes controlled by analyzing the combustion products for CO 2 content. The content of CO 2 in the products of complete combustion is an unambiguous function of excess air only for a certain type of fuel with a constant composition.

At incomplete combustion the content of CO 2 in combustion products is not an unambiguous function even at a constant fuel composition. When burning a mixture of two types of fuel, control of combustion products by CO 2 cannot be carried out, since a small change in the ratio of the mixture of these fuels leads to a change in the optimal value of CO 2

When the combustion process is controlled by 02, changes in the composition of the fuel or in the quantitative ratio of the mixture of different types of fuel have practically no effect on the content of 02 in the combustion products. To control the combustion mode when burning fuel oil and gas with small excesses of air, it is necessary to use automatic gas analyzers with a measurement range of 0 to 2% 0 2 .

For greater reliability, along with the content of 0 2 in the combustion products, it is also advisable to control the content of CO, H 2 and CH 4; it is desirable to additionally control the density of smoke using a smoke meter. Smoke density control is also necessary for sanitary reasons to ensure the purity of the atmospheric air. However, at present smoke meters are not mass-produced.

Gas analyzers are usually calibrated as a percentage by volume. This method of calibrating the scale of gas analyzers is convenient, since the percentage of individual components in the total volume remains unchanged when the pressure and temperature of the gas mixture change.

Chemical gas analyzers

Chemical gas analyzers, belonging to the group of mechanical devices, are based on measuring the reduction in the volume of a taken gas sample after removal of the analyzed component. The removal of the component is carried out by methods of selective absorption or separate afterburning.

So, for example, from a taken gas sample, carbon dioxide is absorbed by an aqueous solution of caustic potash, which has the ability to selectively absorb CO 2:

KOH + CO 2 \u003d K 2 C0 3 + H 2 0.

The unabsorbed residue of the analyzed gas enters the gas measuring device, where the decrease in volume corresponding to the absorbed CO 2 is measured.

This method is used both in portable manual gas analyzers of the type GKhP2 and GKhPZ (GOST 6329-52), often called Orsa devices, and in automatic gas analyzers.

The method of selective absorption in combination with the method of separate afterburning of combustible components of the analyzed gas sample makes it possible to determine the percentage of the following components of the gas mixture CO 2 (S0 2), 0 2, CO, H 2, C m H n (the sum of unsaturated hydrocarbons), the amount of methane CH 4 and other saturated hydrocarbons. This method is used in a portable gas analyzer of the VTI-2 type (GOST 7018-54).

Automatic chemical gas analyzers are currently not used at TPPs. The main disadvantage of these gas analyzers is that they are intermittent devices, giving 20-30 analyzes per hour.

Optical gas analyzers

Optical gas analyzers are based on the use of the dependence of a change in one or another optical property of the analyzed gas mixture on a change in the concentration of the measured component.

Gas analyzers based on the absorption of infrared rays are widely used in various industries and are used to determine the concentration of carbon monoxide (CO), carbon dioxide (CO 2), methane (CH 4), ammonia (CH 3) in complex gas mixtures, and as well as other gases. This is explained by the fact that in the infrared region of the spectrum, gases have very intense absorption bands that differ from each other in position in the spectrum.

Photocolorimetric gas analyzers, based on the absorption of rays in the visible region of the spectrum, are divided into liquid and tape. Liquid gas analyzers are devices with direct (direct) absorption of radiation by the determined component during the interaction of the analyzed component with a liquid reagent. In gas analyzers of the second type, light absorption is measured by the surface of a paper or textile tape, previously impregnated or moistened with an appropriate reagent. Photocolorimetric gas analyzers are widely used to measure the microconcentration of various gases in air environment and complex gas mixtures. These gas analyzers are also widely used to determine the toxic concentration of various gases and vapors harmful to humans in the air. Photocolorimetric gas analyzers are not used to determine high concentrations. It should be noted that the photocolorimetric method is widely used for the analysis of liquids, in particular for the analysis of water at thermal power plants.

Spectrophotometric gas analyzers based on the method of emission spectral analysis of a gas mixture are used to analyze argon, helium, nitrogen, hydrogen and oxygen.

Gas analyzers based on the absorption of ultraviolet rays are used in the chemical, oil and food industries. Due to their high sensitivity, they are widely used to determine the toxic and explosive concentrations of various gases in the air of industrial enterprises. Gas analyzers of this type make it possible to determine the content of mercury vapor, chlorine and other gases and vapors both in the air and in technological gas mixtures.

Gas chromatographs

Gas chromatographs designed for the quantitative analysis of gas mixtures are widely used as laboratory instruments in various industries (chemical, gas, petrochemical, energy, etc.). In recent years, in our country and abroad, great attention has been paid to the creation of industrial gas chromatographs. The use of these devices in the chemical and petrochemical industries for the control and automation of technological processes has made it possible to improve the grade of products and achieve greater economic efficiency.

In the power industry, laboratory-type chromatographs are used for periodic analysis of combustion products of various types of fuel, when conducting research on the combustion process in furnace devices and testing steam generators; chromatographs with an additional device are used to determine the amount of hydrogen dissolved in water and steam, as well as the moisture content of hydrogen in cooling systems for turbine generator windings.

Chromatographs are used for periodic analysis of combustion products of various fuels in industrial steam generators, ovens and other installations. In addition, chromatographs can be used to determine the concentration harmful impurities(CO, CH 4, etc.) in the air of industrial premises. Here chromatography is used to separate gas mixtures by physical methods based on the distribution of one or more components of the mixture between two phases. One of these phases, fixed on the adsorbent (the surface of a solid body or a thin layer of liquid), is washed by the mobile phase (carrier gas together with the analyzed gas) moving in free space not occupied by the stationary phase. In this case, repeated repetition of elementary acts of adsorption and desorption occurs. Since the individual components of the gas mixture are absorbed and retained by this adsorbent differently, the distribution of the components between the two phases, and at the same time their movement relative to each other, is carried out in a certain sequence at a rate characteristic of each component. This allows one-by-one determination of the concentration of each component of the gas mixture.

The method of chromatographic separation of substances using adsorbents was first discovered in 1903 by the Russian scientist M.S. Color and applied by him in the study of pigments involved in plant photosynthesis. During the research M.S. Color dealt with colored substances and therefore he called the separation method he used chromatography. At present, chromatographic methods are also used for the separation of colorless substances, but the name of the methods has remained the same.

Gas chromatography as a method of qualitative and quantitative analysis various substances has become widely known in recent years. The development of gas chromatography was greatly facilitated by the method of gas-liquid chromatography proposed in 1952 by A. Martin and A. James.

Gas chromatography is divided into gas-adsorption and gas-liquid.

The gas adsorption method for separating the components of a gas mixture is based on the different adsorbability of the components by solid adsorbents, which are porous substances with a large surface. Adsorbents widely used in gas adsorption chromatography are activated carbons, silica gels, aluminum gels, and molecular sieves (zeolites). Other adsorbents are also used, such as finely porous glasses.

In gas-liquid chromatography, the separation of complex mixtures of substances is based on the difference in the solubility of the components of the analyzed mixture in a thin layer of liquid deposited on the surface of a solid chemically inert carrier. The solid carrier does not participate directly in the adsorption process, but only serves to create the required solvent surface. The choice of liquid (stationary phase) is determined by the nature of the mixture of substances to be separated. Various liquids are used to separate substances, for example, vaseline oil (a mixture of liquid paraffins of high purity), silicone oil (DS-200, DS-703), high-boiling aviation oil, polyethylene glycol of various grades, etc. A variety of gas-liquid chromatography is capillary gas chromatography, proposed in 1957 g.M. Go-leam. In capillary chromatography, long capillary tubes are used as a solid carrier of the stationary phase, the inner surface of which is covered with a thin uniform layer of non-volatile liquid. Capillary chromatography provides a clearer separation of the components of the gas mixture.

It should be noted that modified adsorbents have recently begun to be used in gas chromatography. In this case, the mobile phase is a gas, and the stationary phase is a solid adsorbent modified with a small amount of liquid. When such an adsorbent is used, the separation of the components of the gas mixture occurs both due to adsorption on a solid carrier and due to solubility in a liquid. Here, gas-adsorption and gas-liquid methods are simultaneously used.

The chromatographic process can be carried out by one of the following methods: developing, frontal or displacement. In the developing method of gas-adsorption and gas-liquid chromatography, a non-sorbing carrier gas continuously flows along the adsorbent layer, and a dose of the analyzed gas mixture is periodically introduced into the flow. This method has been widely used for analytical purposes. Frontal and displacement methods were not found wide application for analytical purposes and will not be considered.

In addition to these methods for implementing the chromatographic process, the method of developing analysis is used with a programmed temperature increase along the entire length of the separating column. The thermodynamic method can be used to analyze microimpurities in gases that are inert with respect to the adsorbent.

In gas chromatography, helium, argon, hydrogen, nitrogen, air, and other gases are commonly used as carrier gases.

Developing gas adsorption chromatography is widely used in power engineering and other industries to separate mixtures of low-boiling substances that are part of combustion products (H 2 , 0 2 , CO, CH 4 , N 2 , etc.); the gas-liquid chromatography method does not provide a good separation of these substances due to their poor solubility in the liquid phase. Recently, the gas adsorption method has also been used for the analysis of high-boiling substances and light hydrocarbon gases.

Gas-liquid chromatography is used to separate high-boiling substances, which include most hydrocarbons. Chromatographic methods allow the analysis of gas mixtures, liquid substances, as well as solid substances not dissolved in a liquid. In the latter case, the separating column of the chromatograph is equipped with a device for evaporating the analyzed liquid.

Methods and technical means for monitoring the quality of water, steam, condensate and concentration of solutions

The widespread introduction of powerful power units for high and supercritical parameters into the energy sector has led to the need to organize reliable automatic continuous and periodic chemical control of the water regime of power plants and the operation of water and condensate treatment plants. The importance of automation of water treatment processes has also increased.

The manual methods of chemical control of some quality indicators used at many power plants do not meet modern increased requirements. These methods require a lot of time, have insufficient accuracy of analysis results and are unsuitable for operational control of the water regime and automation of water preparation processes.

The use of automatic measuring instruments (liquid analyzers) at power plants increases the reliability of chemical monitoring of the quality indicators of steam generator feed water, steam and condensate, and the processes of chemical desalination of feed water and purification of turbine condensate.

To control the water regime of power plants and the operation of water and condensate treatment plants, it is necessary to measure various quality indicators of media differing in chemical composition. These media are under different overpressure, have different temperatures, differ in the amount of mechanical and other impurities. As a result, in many cases, in order to reduce pressure and temperature, as well as to remove mechanical impurities or dissolved gases from a sample of a controlled environment, it is necessary to install special additional devices in front of the primary converter. Various sampling devices are used to take a representative sample of the medium. The use of these additional devices allows you to create the same normal operating conditions for the primary measuring transducers, and at the same time improve the accuracy of measurements.

Measurement of the specific electrical conductivity of aqueous solutions

The measurement of the specific electrical conductivity of aqueous solutions has become widespread in laboratory practice, with automatic chemical control of the water regime of steam power plants, the efficiency of water purification plants and industrial heat exchange and other installations, as well as various quality indicators characterizing chemical and technological processes.

Technical means designed to measure the specific electrical conductivity of aqueous solutions are commonly called conductometric liquid analyzers. The scale of secondary instruments of liquid conductometers (laboratory and industrial) for measuring electrical conductivity is graduated in units of Siemens per centimeter (S-cm-1) or micro-Siemens per centimeter (µS-cm-1). Liquid conductometers, which are used in industrial conditions to measure quality indicators characterizing the salt content in steam, condensate and feed water of steam generators, are usually called salt meters. The scale of secondary instruments of salt meters is graduated (for the conditional content of these salts in a solution) in the following units: milligram per kilogram (mg / kg), microgram per kilogram (mcg / kg) or milligram per liter (mg / l) and microgram per liter (mcg /l). Liquid conductometers used to measure the concentration of solutions of salts, acids, alkalis, etc. are often called concentration meters. The scale of the secondary devices of the concentrators is calibrated as a percentage of the mass concentration value. Conductometric liquid analyzers are also used as signaling devices.

With increased requirements for the quality of feed water, steam and condensate, it is necessary to measure low values of electrical conductivity, not exceeding 5-6 μS-cm-1

In the steam condensate and feed water of steam generators, in addition to a small amount of salts, dissolved gases are usually present - ammonia (CH 3) and carbon dioxide(C0 2) - and hydrazine. The presence of dissolved gases and hydrazine changes the electrical conductivity of the condensate and feed water, and the readings of the liquid conductometer (salt meter) do not unambiguously correspond to the conditional salt content, i.e. the value of the dry residue obtained by evaporation of condensate or feed water. This leads to the need to make corrections to the instrument readings or to use an additional device to remove dissolved gases and hydrazine from the sample.

An additional device in the form of a degasser for removing dissolved gases from the sample does not exclude the effect on the readings of the hydrazine conductometric analyzer. The currently used filter filled with KU-2 cation exchanger makes it possible to eliminate the effect of ammonia and hydrazine on the readings of the instrument.

Electrode conductometric transducers. Electrode transducers used to measure the electrical conductivity of solutions are made for laboratory studies of various solutions and for technical measurements. Measurements in laboratory conditions are made on alternating current. At the same time, it should be noted that the conductometric method of measurement on alternating current remains generally accepted in everyday laboratory practice. Technical measurements of the electrical conductivity of solutions using electrode transducers are usually carried out on alternating current with a frequency of 50 Hz.

The device, dimensions, and, consequently, the constant of the electrode transducers largely depend on the measured value of the electrical conductivity of the solution. In technical measurements, transducers with cylindrical coaxial and, to a lesser extent, flat electrodes are most common. The device of transducers with cylindrical coaxial electrodes is schematically shown in Fig.22-2-2. For the transducer shown in Fig. 22-2-2, a, the outer cylindrical electrode is also its body. The second transducer (Fig. 22-2-2, b) also has a cylinder1 and metal coaxial electrodes, but they are located in its steel case, to which one electrode is welded.

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7.5. Induction and ultrasonic flowmeters.

The above methods for measuring the amount and flow of liquid, steam and gas (air) are characterized by the fact that the sensitive element of the devices is located directly in the medium being measured, i.e., it is subjected to its mechanical and chemical effects and causes an unproductive loss of flow pressure. The continuous influence of the measured medium on the sensitive element of the flow meter has a negative effect over time on the accuracy of the readings, the reliability of the operation and the service life of the device.

To measure the flow rate of chemically aggressive (acids, alkalis), abrasive (pulp) and other liquids that destroy the material of the parts of the flowmeter in contact with them, the described methods and devices are generally unsuitable.

There are a number of devices for measuring flow, the sensitive element of which does not have direct contact with the measured substance, which allows them to be used in aggressive environments. Such devices include induction and ultrasonic flow meters.

8. Devices for measuring the amount of a substance.

The most accurate and common way to measure the amount of solid fuel is by weighing. The main instrument used for this purpose is a lever (beam) balance that determines the mass of the weighed fuel by comparing it with the mass of calibrated weights (weights).

Types of balances

There are two types of lever scales: manual and automatic, while the hand scales are divided into weight, scale, dial and mixed.

9. Level definitions.

Level gauges.

In modern powerful drums steam boilers there is a very limited water supply (the amount of water in the space between the limit positions of the level in the drum), as a result of which, when the boiler is powered off with water, the level in it may drop beyond the lower limit mark in 1-2 minutes . This shows how important it is to control the level of water in the drum.

Typically, in steam boilers, indicator glasses are used to monitor the water level, which are part of their fittings. Large boilers, due to their considerable height, are also equipped with level gauges installed at the control points for the operation of the units.

Liquid level indicators in tanks.

The simplest device for measuring the level of a liquid in a tank is an indicator glass. However, if the tank is located high or low relative to the observation point, it is difficult to use the index glass. In such cases, special level indicators are used.

10. Devices for monitoring the composition of flue gases and the quality of feed water, steam and condensate.

10.1. Flue gas monitoring

The efficiency of the boiler operation is mainly affected by heat losses due to chemical incompleteness of fuel combustion and with exhaust gases. The size of these losses depends on the air flow rate supplied to the boiler furnace.

Reducing the air supply leads to an increase in losses from chemical incomplete combustion due to lack of oxygen. Each fuel for its combustion needs a certain amount of air, and this amount is the greater, the higher the content of combustible parts in the fuel - carbon and hydrogen. At complete combustion Carbon dioxide forms carbon dioxide, and when hydrogen is burned, water vapor is formed. Incomplete combustion of carbon is associated with the formation of carbon monoxide and a decrease in heat release by almost 3 times.

An increase in the flow rate of the air supplied to the furnace causes an increase in the loss with the outgoing gases, since part of the heat is uselessly spent on heating the additional air. In addition, excessive air supply leads to a decrease in the temperature in the furnace, which is associated with a deterioration in heat exchange conditions.

For each particular case, characterized by the type of boiler, its load and the type of fuel burned, there is an economically most advantageous ratio between the consumption of fuel and air required for combustion. Wherein optimal flow air, the total heat loss from chemical incomplete combustion and with exhaust gases is the smallest value.

Maintaining the optimal operating mode of the boiler furnace requires continuous monitoring of the quantitative composition of flue gases, and the most important is the determination of the content of oxygen or carbon dioxide in them, characterizing the achieved ratio between fuel and air consumption.

Instruments for the quantitative analysis of gases are called gas analyzers. To determine the composition of flue gases, a gas sample taken from the boiler flue is fed into the device. The content of individual components in it is measured by a gas analyzer in volume units, expressed as a percentage of the total volume of the gas mixture.

With complete combustion of fuel, flue gases contain nitrogen (N 2), oxygen (O 2), carbon dioxide (CO 2), water vapor (H 2 O) and, if combustible sulfur (S) is contained in the fuel, sulfur dioxide (SO 2) . With incomplete combustion, combustible gases additionally appear in the flue gases: carbon monoxide (CO), hydrogen (H 2) and methane (CH 4).

Classification of gas analyzers

Exist

- manual;

- automatic gas analyzers.

The former are used for control and laboratory measurements, and the latter for continuous gas analysis in industrial installations.

Handheld gas analyzers are portable control and laboratory devices. Due to their high measurement accuracy, they are widely used in testing and commissioning of boiler units, as well as for verification of automatic gas analyzers.

Automatic gas analyzers are technical devices. They are performed by showing and self-recording and have remote transmission of readings.

According to the principle of operation, gas analyzers used at power plants are divided into chemical, chromatographic, magnetic and electrical.

The scales of gas analyzers are calibrated as a percentage of the volume content of individual components in the gas mixture.

To handheld gas analyzers include portable chemical and chromatographic instruments. Chemical gas analyzers are widely used as very accurate, simple and reliable devices. Recently, many industries have begun to use chromatographic gas analyzers for laboratory measurements, the use of which is also promising for power plants. Chemical gas analyzers according to their purpose are divided into gas analyzers for abbreviated andcomplete (total)gas analysis. Of these, gas analyzers for reduced analysis are especially widely used.

Chemical gas analyzers determine the individual components of a gas mixture by selective absorption (absorption) of their respective chemical reagents. The decrease in the volume of the gas mixture characterizes the content of the desired component in it.

10.2. Methods for determining the quality of water and steam.

The quality of feed water consumed by boilers, characterized by its salinity, hardness, dissolved oxygen content, hydrogen ion concentration and a number of other factors that cause scale formation, sludge precipitation and metal corrosion in boilers, has a significant impact on the operation of a thermal power plant.

The saturated steam produced by the boilers, despite the presence of separation devices, is always; contains some moisture. The humidity of the steam worsens its quality, since the salts contained in it are carried away with the water, the deposition of which on separate sections the steam path causes burnout of superheater pipes, jamming of turbine control valves, a decrease in the power and efficiency of turbine units due to drift of turbine blades, etc.

To ensure reliable and efficient operation of power plant equipment, continuous monitoring of the quality of steam, condensate and feed water is necessary. In operation, for this purpose, a number of permanently operating measuring instruments are used, namely:

To determine the salinity of steam, boiler and feed water -- salt meters,

- water hardness -- hardness gauges,

- concentrations of hydrogen ions in water -- concentrators(pH meters).

Determination of the salinity of the selected sample of steam (condensate) or water in the chemical laboratory of the power plant by evaporation 3--5 . l water in order to obtain a dry residue cannot serve as a method of operational control, as it takes too much time (analysis duration up to 2 days). Labor-intensive is also the determination of the content of oxygen and other substances dissolved in water in the laboratory.

The accuracy of determining the salt content of saturated steam coming from the boiler to the superheater depends to a large extent on the method of taking an average sample, which should most fully characterize the quality of the steam passing through the pipeline. The latter has an uneven distribution of velocities and humidity over the pipe section. Therefore, the steam sampling device must take samples along the entire diameter of the steam pipeline.

For steam sampling, steam sampling tubes (probes) with a number of holes along the generatrix are used, installed horizontally on straight vertical sections of the steam pipeline with a downward steam flow. As an exception, it is allowed to install the steam sampling pipe in vertical sections with an upward flow.

The correctness of the selection of the average sample is influenced not only by the method of installing the steam sampling tube, but also by its design, as well as the dimensions of the steam pipeline.

Salt meters.

Automatic determination of the salinity of steam (condensate) and feed water is carried out by the conductometric method, i.e. by measuring their electrical conductivity.

The electrical conductivity of a solution (electrolyte) is the reciprocal of its electrical resistance, expressed in Ohm -1.

To determine the hardness of water, a photocolorimetric method of analysis is used, based on measuring the intensity of light absorbed by a colored solution. Under the condition of monochromaticity of the absorbed light, the concentration of substances dissolved in water is characterized by its optical density D, which, according to the Lambert-Beer law, is equal to the logarithm of the ratio of the light intensity before and after absorption by the solution or is proportional to the concentration of the colored substance and the thickness of the solution layer. Hardness meters based on the photocolorimetric measurement method have a relatively simple device, are highly sensitive, and allow measuring small concentrations of hardness salts dissolved in water. A photoresistor or photocell serves as a sensitive element of a device that converts light energy into electrical energy. Rigidometers usually use a differential scheme of photocolorimetry, in which the optical density of the studied water is compared with the optical density of a solution of exactly known concentration, and based on the preliminary calibration of the device, the required water hardness is determined.

Hardness meters based on the photocolorimetric measurement method have a relatively simple device, are highly sensitive, and allow measuring small concentrations of hardness salts dissolved in water. A photoresistor or photocell serves as a sensitive element of a device that converts light energy into electrical energy. Rigidometers usually use a differential scheme of photocolorimetry, in which the optical density of the studied water is compared with the optical density of a solution of exactly known concentration, and based on the preliminary calibration of the device, the required water hardness is determined.

oxygen meters .

The degree of solubility of any gas in water depends on its partial pressure in the gaseous medium above the water, regardless of the presence of other gases in this medium. Consequently, if a gas atmosphere free of it is above the surface of water containing dissolved oxygen, then oxygen will be released from the water until a state of equilibrium occurs between the concentrations of O 2 in the gaseous medium and water. Hence, the more O 2 will be contained in the water, the greater its amount will be released into the surrounding gas environment. On the contrary, with a decrease in the concentration of O 2 in water, part of it, previously released, will be reabsorbed by water until a new equilibrium is reached.

Topic: Calculation of measurement errors and instrument accuracy class

1. General information about the accuracy and measurement errors.

2. Estimation and accounting for errors.

3. Metrological characteristics of measuring instruments.

Literature: S. 13-56.

1. When measuring any quantity, no matter how carefully we make the measurement, it is not possible to obtain a result free from distortion. The reasons for these distortions may be different. Distortions can be caused by the imperfection of the applied measurement methods, measuring instruments, variability of measurement conditions, and a number of other reasons. The distortions that result from any measurement cause measurement error -- deviation of the measurement result from the true value of the measured value.

The measurement error can be expressed in units of the measured quantity, i.e., in the form absolute error , which is the difference between the measured value and the true value of the measured quantity. The measurement error can also be expressed as relative error measurement, which is relation to the true value of the measured quantity. Strictly speaking, the true value of the measured quantity always remains unknown; one can only find an approximate estimate of the measurement error.

The error of the measurement result gives an idea of which figures in the numerical value of the quantity obtained as a result of the measurement are doubtful. It is necessary to round the numerical value of the measurement result in accordance with the numerical digit of the significant digit of the error, i.e. the numerical value of the measurement result must end with a digit of the same digit as the error value. When rounding, it is recommended to use the rules of approximate calculations.

Measurement errors, depending on the nature of the causes that cause their appearance, are usually divided into random, systematic and rough.

Under random error understand the measurement error, which changes randomly with repeated measurements of the same quantity. They are caused by causes that cannot be determined by measurement and cannot be influenced. The presence of random errors can only be detected by repeating measurements of the same quantity with the same care. If, when repeating the measurements, the same numerical values are obtained, then this does not indicate the absence of random errors, but the insufficient accuracy and sensitivity of the method or measuring instrument.

Under systematic error understand the measurement error, which remains constant or regularly changes during repeated measurements of the same quantity. If systematic errors are known, i.e., have a certain value and a certain sign, they can be eliminated by making amendments.

Amendment they call the value of the quantity of the same name as the measured one, added to the value of the quantity obtained during the measurement in order to eliminate the systematic error. Note that the correction introduced into the readings of the measuring instrument is called the correction to the reading of the instrument; the correction added to the nominal value of the measure is called the correction to the measure value. In some cases, a correction factor is used, the latter is understood as the number by which the measurement result is multiplied in order to eliminate the systematic error. The following types of systematic errors are usually distinguished:instrumental, measurement method, subjective, installations, methodical.

Under instrumental errors understand measurement errors that depend on the errors of the measuring instruments used. When using measuring instruments of increased accuracy, instrumental errors caused by the imperfection of measuring instruments can be eliminated by introducing corrections. Instrumental errors of technical measuring instruments cannot be ruled out, since these measuring instruments are not subject to corrections during their verification.

Under measurement method error understand the error resulting from the imperfection of the measurement method. It arises comparatively often in the application of new methods, as well as in the application of approximating equations, which sometimes represent an inaccurate approximation to the actual dependence of quantities on each other. The error of the measurement method must be taken into account when assessing the error of the measuring instrument and, in particular, the measuring installation, and sometimes the error of the measurement result.

Subjective errors (occurring in non-automatic measurements) are caused by the individual characteristics of the observer, for example, delay or advance in registering the moment of a signal, incorrect interpolation when reading readings within one division of the scale, from parallax, etc. Parallax error is understood as a component of the reading error , which occurs due to the sighting of an arrow located at some distance from the scale surface, in a direction not perpendicular to the scale surface.

Installation errors arise due to incorrect installation of the arrow of the measuring instrument at the initial mark of the scale or careless installation of the measuring instrument, for example, not on a plumb line or level, etc.

Methodological errors measurements are such errors that are determined by the conditions (or methodology) for measuring a quantity (pressure, temperature, etc. of a given object) and do not depend on the accuracy of the measuring instruments used. A methodological error can be caused, for example, by the additional pressure of a liquid column in the connecting line, if the pressure measuring device is installed below or above the pressure tapping point, and when measuring temperature with a thermoelectric thermometer complete with a measuring device.

When performing measurements, especially accurate ones, it must be borne in mind that systematic errors can significantly distort the measurement results. Therefore, before proceeding with the measurement, it is necessary to find out all possible sources of systematic errors and take measures to exclude or determine them. However, it is practically impossible to give exhaustive rules for finding and eliminating systematic errors, since the methods for measuring various quantities are too diverse. In addition, in non-automatic measurements, much depends on the knowledge and experience of the experimenter. Below are some general tricks exclusion and detection of systematic errors. To identify possible changes in instrumental errors due to certain malfunctions of the measuring instruments used or their wear and other reasons, all of them must be subjected to regular verification.

Careful and correct installation of measuring instruments is necessary to eliminate installation errors in both precise and technical measurements. If the error is caused by external disturbances (temperature, air movement, vibration, etc.), then their influence must be eliminated or taken into account.

Under gross measurement error is understood as a measurement error that is significantly greater than expected under given conditions.

When measuring a variable in time, the measurement result may turn out to be distorted, in addition to the errors discussed above, by another type of error that occurs only in the dynamic mode and, as a result, has received the name of the dynamic error of the measuring instrument. When measuring a variable in time, a dynamic error may occur due to an incorrect choice of a measuring instrument or a mismatch of the measuring device with the measurement conditions. When choosing a measuring instrument, it is necessary to know its dynamic properties, as well as the law of change of the measured value.

2. Evaluation and accounting for errors in accurate measurements

Usually, in addition to random errors, systematic errors can affect the measurement accuracy. Measurements should be carried out in such a way that there are no systematic errors. In the future, when applying the proposals and conclusions arising from the theory of errors and processing the results of observation, we will assume that the series of measurements do not contain systematic errors, and also gross errors are excluded from them.

Ways of numerical expression of errors of measuring instruments.

Absolute error measuring instrument is determined by the difference between the indication of the instrument and the actual value of the measured quantity. If a? is the absolute error, X- instrument reading, X BUT -- the actual value of the measured quantity, then

? = x-x BUT.

The absolute error of the measure is equal to the difference between the nominal value of the measure and the actual value of the value reproduced by it and is determined by a similar formula.

Absolute error of the transducer input-- the difference between the value of the value at the input of the converter, determined by the actual value of the value at its output using the calibration characteristic assigned to the converter, and the actual value of the value at the input of the converter.

Absolute error of the measuring transducer output-- the difference between the actual value of the value at the output of the converter, which displays the measured value, and the value of the value at the output, determined by the actual value of the value at the input using the calibration characteristic assigned to the converter.

When assessing the quality of measures and measuring instruments, sometimes they use relative errors , expressed in fractions (or percentages) of the actual value of the measured quantity:

Relative error can also be expressed in fractions (or percent) of the nominal value of the measure or instrument reading.

The limits of permissible basic and additional errors of measuring instruments for each of the accuracy classes are established in the form of absolute or reduced errors. The basic and additional errors are expressed in the same way.

The absolute error is expressed:

1) one value

where? - the limit of permissible absolute error; a-- constant number;

2) in the form of a dependence of the maximum permissible error on the nominal value, indication or signal X, expressed by a two-term formula

where b-- constant number;

3) in the form of a table of limits of permissible errors for different nominal values, indications or signals.

The reduced error is determined by the formula

Amendment. The correction is understood as the value of the quantity of the same name as the measured one, added to the value of the quantity obtained during the measurement in order to eliminate the systematic error.

The correction added to the nominal value of the measure is calledcorrection to the value of measures ; the correction introduced into the readings of the measuring instrument is calledamendment to instrument reading . Correction introduced into the readings of the instrument X P, makes it possible to obtain the actual value of the measured quantity X l.

If c is a correction expressed in units of the measured value, then according to the definition

i.e., the correction is equal to the absolute error of the measuring device, taken with the opposite sign.

In some cases, to eliminate the systematic error, a correction factor is used, which is a number by which the measurement result is multiplied.

When checking measuring instruments, only exemplary measuring instruments, as well as working measuring instruments of increased accuracy, are supplied with amendments. Industrial (technical) measuring instruments are not supplied with amendments during their verification, since they are intended for use without amendments. If, as a result of verification of industrial measuring instruments, it is established that their errors do not go beyond the permissible basic and additional errors, then they are recognized as suitable for use.

3. Basic information about the metrological characteristics of measuring instruments.

When evaluating the quality and properties of measuring instruments, knowledge of their metrological characteristics is of great importance, which makes it possible to evaluate errors when operating both in static and dynamic modes.

Accuracy class and permissible errors. The accuracy class of measuring instruments is their generalized characteristic, determined by the limits of permissible basic and additional errors, as well as other properties of measuring instruments that affect accuracy. The limits of permissible basic and additional errors are established in the standards for certain types measuring instruments. It should be borne in mind that the accuracy class of measuring instruments characterizes their properties in relation to accuracy, but is not a direct indicator of the accuracy of measurements performed using these instruments, since accuracy also depends on the measurement method and the conditions for their implementation.

The limits of permissible basic and additional errors of measuring instruments for each of the accuracy classes are established in the form of absolute and reduced errors.

Measuring instruments, the limits of permissible errors of which are expressed in units of the measured quantity, are assigned accuracy classes, denoted by serial numbers, and measuring instruments with a large value of permissible errors are assigned classes of a larger serial number. In this case, the designation of the accuracy class of the measuring instrument is not related to the value of the maximum permissible error, i.e., it is conditional.

Measuring instruments, the limits of the permissible basic error of which are given in the form of reduced (relative) errors, are assigned accuracy classes selected from the series (GOST 13600-68):

K \u003d (1; 1.5; 2.0; 2.5; 3.0; 4.0; 5.0; 6.0) * 10 n; n=1; 0; -one; -2...

Specific accuracy classes are established in the standards for certain types of measuring instruments. How less number, denoting the accuracy class of the measuring instrument, the lower the limits of the permissible basic error. The accuracy classes of measuring instruments, normalized according to the given errors, are related to a specific value of the error limit.

Measuring instruments with two or more ranges (or scales) may have two or more accuracy classes.

Basic error measuring instrument is called the error of the measuring instrument used under normal conditions. Under the limit of permissible basic error is understood the largest (without taking into account the sign) basic error of a measuring instrument, at which it can be recognized as fit and allowed for use. For brevity, this error is often referred to as the allowable basic error.

Normal conditions for the use of measuring instruments are understood as conditions under which the influencing quantities (ambient air temperature, barometric pressure, humidity, supply voltage, current frequency, etc.) have normal values or are within the normal range of values. For measuring instruments, the normal conditions of use are also their certain spatial position, the absence of vibration, external electric and magnetic fields, except for the earth's magnetic field.

As normal values or the normal range of values of the influencing quantities, for example, an ambient temperature of 20±5°C (or 20±2°C) is taken; barometric pressure 760±25 mm Hg. Art. (101.325±3.3 kPa); supply voltage 220 V, frequency 50 Hz, etc. The standard values given as examples or normal ranges of influence quantities are not mandatory for all measuring instruments. In each individual case, normal values or normal ranges of values of the influencing quantities are established in the standards or technical specifications for measuring instruments of this type, in which the value of the permissible basic error does not exceed the established limits.

The specified normal conditions for the use of measuring instruments are usually not working conditions for their use. Therefore, for each type of measuring instruments, standards or specifications establish an extended range of values of the influencing quantity, within which the value of the additional error (change in readings for measuring instruments) should not exceed the established limits.

As an extended range of values of influencing quantities, for example, ambient air temperature from 5 to 50 ° C (or from I-50 to + 50 ° C), relative air humidity from 30 to 80% (or from 30 to 98%) , supply voltage from 187 to 242 V, etc. In some cases, when normalizing the limits of permissible additional errors of measuring instruments, a functional dependence of the permissible additional error on changes in the influencing quantity is given.

A change in instrument readings (additional error of a measure, transducer by input or output) is understood as a change in the error of the instrument (measure, transducer) due to a change in its actual value, caused by the deviation of one of the influencing quantities from the normal value or going beyond the normal range of values.

The limit of permissible additional error (change in indications) is understood as the largest (without taking into account the sign) additional error (change in indications) caused by a change in the influencing quantity within the extended area at which the measuring instrument can be recognized as fit and allowed for use.

It should be noted that the terms basic and additional errors correspond to the actual errors of measuring instruments that occur under given conditions.

We also note that the terms of the limits of the permissible additional (or, respectively, the main) error correspond to the boundary errors within which the measuring instruments, according to the technical requirements, can be considered fit and be allowed for use. All limits of permissible errors are set for the values of the measured quantities lying within the measurement range of the device, and for measuring transducers I-- within the conversion range.

It should also be noted that under operating conditions, external phenomena may occur, the impact of which is not expressed in a direct effect on the readings of the device or the output signal of the transducer, but they can cause damage and malfunction of the measuring unit, mechanism, transducer, etc., for example, devices and converters can be affected by aggressive gases, dust, water, etc. Devices and converters are protected from these factors by means of protective cases, covers, etc.

In addition, measuring instruments can be affected by external mechanical forces (vibration, shaking and shock), which can lead to distortion of instrument readings and the impossibility of counting during these impacts. Stronger impacts can cause damage or even destruction of the instrument and transmitter. Measuring instruments and converters designed to work under conditions of mechanical influences of various intensity and other characteristics, protect them from destructive action with special devices or increase their strength.

Depending on the degree of protection from external influences and resistance to them, devices and transducers are divided (GOST 2405-63) into ordinary, vibration-resistant, dust-proof, splash-proof, hermetic, gas-proof, explosion-proof, etc. This makes it possible to choose measuring instruments in relation to working conditions.

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