Thermocouple

Thermocouple is a commonly used temperature measuring element in temperature measurement instruments It directly measures temperature and converts the temperature signal into a thermoelectric electric force signal, which is converted into the temperature of the measured medium through PLC DCS or electrical instruments (secondary instruments) The appearance of variable thermocouples is often very different due to needs, but their basic structure is generally the same They are commonly composed of main parts such as thermal electrodes, insulation sleep protection tubes, and junction boxes, and are commonly used in connection with display instruments, recording instruments, and electronic regulators

When there is a temperature gradient at both ends, there will be current passing through the circuit, and there is an electric force - thermoelectric electric force - between the two ends, which is called the Seebeck effect Two homogenic conductors with different compositions are thermoelectric electrodes, with the working end at the higher temperature and the free end at the lower temperature The free end is commonly at a constant temperature Recording to the functional relationship between thermoelectric electric force and temperature, a thermocouple graduation table is made; The graduation table is observed under the condition of a free end temperature of 0 ℃, and different thermocouples have different graduation tables

When a third metal material is connected to the thermocouple circuit, as long as the temperature of the two contacts of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, unaffected by the connection of the third metal to the circuit There are more, when measuring temperature with a thermocouple, a measuring instrument can be connected, and after measuring the thermoelectric electric force, the temperature of the measured medium can be known When measuring temperature, a thermocouple requires the temperature of its cold end (the measuring end is the hot end, and the end connected to the measuring circuit through a lead is called the cold end) to remain constant, so that its thermoelectric potential is professional to the measured temperature If the temperature of the cold end changes during measurement, it will serially affect the accuracy of the measurement Taking certificate measures at the cold end to compensate for the impact caused by temperature changes at the cold end is called normal cold end compensation for thermocouples Special compensating wires are used to connect with measuring instruments

Calculation method for thermocouple cold junction compensation:

From mills to temperature: Measure the cold junction temperature, convert it to the corresponding mill value, and add it to the mill value of the thermocouple to calculate the temperature;

From temperature to mills: Measure the actual temperature and cold end temperature, convert them into mills, subtract them to retain mills, and retain the temperature

Two Conditions for Thermocouple Temperature Measurement

It is a temperature sensing element, a primary instrument, and a thermocouple directly measures the temperature A closed circuit composed of two conductors with different composition materials Due to different materials, different electron identities generate electron diffusion, and after stable equilibrium, potential is generated When there is a gradient temperature at both ends, a current is generated in the circuit, generating thermoelectric electric force The larger the temperature difference, the greater the current After measuring the thermoelectric electric force, the temperature value can be determined Thermocouples are essential energy converters that convert thermal energy into electrical energy

The technical advantages of thermocouples include a wide temperature measurement range and stable performance; High measurement accuracy, thermocouple in direct contact with the measured object, not affected by intermediate medium; Fast thermal response time, flexible response of thermocouples to temperature changes; The measurement range is large, and thermocouples can continuously measure temperature from -40 to 1600 ℃; Thermocouples have reliable performance and good mechanical strength Long service life and conservative installation

The electric coil must be composed of two types of conductor (or semiconductor) data with different properties that meet certain requirements to form a circuit There must be a temperature difference between the measuring end and the reference end of the thermocouple

Solder two different types of conductors or semiconductors A and B together to form a closed circuit When there is a temperature difference between the two attachment points 1 and 2 of conductor A and B, an electromagnetic force will be generated between them, so a current of one size will be formed in the circuit This phenomenon is called the Thermoelectric effect Thermocouples work by applying this effect

Main features

1. Easy assembly and easy replacement;

2. Spring loaded temperature sensing element with good seismic performance;

3. High measurement accuracy;

4. Large measurement range (-200 ℃~1300 ℃, in special cases -270 ℃~2800 ℃);

5. Fast thermal response time;

6. High mechanical strength and good pressure resistance;

7. High temperature resistance up to 2800 degrees;

8. Long service life

Structural requirements

The structural form of the thermocouple is to ensure its reliability

The structural requirements for stable operation are as follows:

1. The welding of the two thermoelectric electrodes that make up the thermocouple must be firm;

2. The two thermoelectric electrodes should be well insulated from each other to prevent short circuits;

3. The connection between the compensation wire and the free end of the thermocouple should be considered and related;

4. The protective sleep should ensure sufficient isolation between the thermal electric code and hazardous media

Operational principle

Two different types of conductors (called thermocouple wires or thermoelectric electrodes) are connected at both ends to form a circuit

When the temperature of two junction points is different, the electromagnetic force will be generated in the circuit This phenomenon is called Thermoelectric effect, and this electric force is called thermoelectric potential Thermocouples use this principle for temperature measurement, where the end directly used for measuring the temperature of the medium is called the working end (also known as the measurement end), and the other end is called the cold end (also known as the compensation end); The cold end is connected to a display instrument or supporting instrument, and the display instrument will indicate the thermoelectric potential generated by the thermocouple

A thermocouple is actually an energy converter that converts thermal energy into electrical energy, and uses the generated thermoelectric potential to measure temperature For the thermoelectric potential of a thermocouple, the following issues should be noted:

1. The thermoelectric potential of a thermocouple is the difference in temperature function between the working end of the thermocouple, compared to the difference in temperature between the cold end and the working end of the thermocouple;

2. The magnet of the thermoelectric potential generated by thermocouples, when the material of the thermocouple is uniform, is not related to the length and diameter of the thermocouple, but only to the composition of the thermocouple material and the temperature difference at both ends;

3. When the material composition of the two thermocouple wires of the thermocouple is determined, the magnet of the thermoelectric potential of the thermocouple is only related to the temperature difference of the thermocouple; If the temperature of the cold end of the thermocouple remains constant, the thermoelectric potential of the thermocouple is only a single valued function of the working end temperature Weld two different materials of conductors or semiconductors A and B together to form a closed circuit, as shown in the figure. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, an electromotive force is generated between them, resulting in a current of a ceramic magnet in the circuit Thermocouples work by utilizing this effect

Common types

Commonly used thermocouples can be divided into two categories: standard thermocouples and non standard thermocouples The so called standard thermocouple refers to a thermocouple that has a national standard that specifies the relationship between its thermal potential and temperature, allowable errors, and a unified standard graduation table It has accompanying display instruments for selection Non standardized thermocouples are not as good as standardized thermocouples in terms of usage range or order of magic, and generally do not have a unified schedule table They are mainly used for measurement in certificate special cases Standardized thermocouples Since January 1, 1988, China has produced thermocouples and thermal resistors in accordance with IEC international standards, and designed seven standardized thermocouples S, B, E, K, R, J, and T as Chinese communication's unified design thermocouples

Thermocouple graduation number Thermoelectric electrode material

Positive and negative electrodes

S Platinum Rhodium 10 Pure Platinum

R Platinum Rhodium 13 Pure Platinum

B Platinum Rhodium 30 Platinum Rhodium 6

K nickel chromium nickel silicon

T pure copper, copper, and nickel

J iron copper nickel

N nickel chromium silicon nickel silicon

E nickel chromium copper nickel

In theory, any two different conductors (or semiconductors) can be configured as thermocouples, but as a practical temperature measuring element, there are variable requirements for it. In order to ensure reliability and sufficient measurement accuracy in engineering technology, not all materials can form thermocouples Generally, the basic requirements for electronic materials of thermocouples are:

1. Within the temperature measurement range, the thermoelectric properties are stable, do not change over time, have sufficient physical and chemical stability, and are not easily oxidized or corroded;

2. Low temperature coefficient of resistance, high conductivity, and low specific heat;

3. The thermoelectric potential generated in temperature measurement should be large, and the relationship between the thermoelectric potential and temperature is a linear or near linear single valued function;

4. Good material reproducibility, high mechanical strength, simple manufacturing process, and low price

How to install thermocouples

In production, due to different measured objects, environmental conditions, measurement requirements, and installation methods and measures of thermal resistors, there are many issues that need to be considered How, in principle, temperature measurement accuracy, safety, and maintenance convention can be considered from three aspects

To avoid damage to temperature measuring elements, sufficient mechanical strength should be ensured To protect temperature sensing elements from wear and tear, protective screens or tubes should be added To ensure safety and reliability, the installation method of temperature measuring elements should depend on the specific situation (such as the temperature and pressure of the medium to be measured, the length of the temperature measuring element, its installation position, form, etc.). Here are just a few examples to draw attention to:

All temperature measuring elements that bear pressure must be installed to ensure their sealing Thermocouples working at high temperatures should generally be installed vertically to prevent deformation of the protective tube at high temperatures If horizontal installation is necessary, it should not be too long, and a bracket should be used to protect the thermocouple If the temperature measuring element is installed in a pipeline with a high medium flow rate, it should be installed objectively To prevent excessive exposure of the temperature measuring element, it is best to install it at the bend of the pipeline When the medium pressure exceeds 10MPa, a protective jacket must be added to the measuring element The installation location of thermocouples/thermal resistors should also consider sufficient space and space for their disassembly, maintenance, and calibration Thermocouples and thermal resistors with long protective tubes should be able to be easily disassembled and assembled

Measurement method for thermal response time of thermocouples

[3] The thermal response time of the thermocouple is relatively complex, and different test conditions will result in different measurement results This is because it is affected by the heat exchange rate between the thermocouple and the surrounding medium If the heat exchange rate is high, the thermal response time is short In order to make the thermal response time of thermocouple products comparable, the national standard simulations that the thermal response time should be conducted on a dedicated water flow testing device The water flow speed of the device should be maintained at 0.4 ± 0.05m/s, with an initial temperature range of 5-45 ℃ and a temperature step value of 40-50 ℃ During the experience, the temperature change of water should not exceed ± 1% of the temperature step value The insertion depth of the tested thermocouple is 150mm or the designed insertion depth (select the smaller value and indicate it in the test report)

Due to the complexity of the device, only a few units currently have this set of equipment There are more, the national standards allow manufacturers to negotiate with users and use other testing methods, but the provided data must indicate the test conditions

Due to the small thermoelectric potential of B-type thermocouples near room temperature and the diversity in measuring their thermal response time, the national standard simulations that the thermoelectric component of S-type thermocouples of the same specification can be used to replace their own thermoelectric component, and then tested

During the test, the output change of the thermocouple should be recorded to a time T0.5 equivalent to a 50% temperature step change If necessary, the thermal response time T0.1 with a 10% change and T0.9 with a 90% change can be recorded The recorded thermal response time should be the average of at least three test results from the same test, and the deviation of each measurement result from the average value should be within ± 10% In addition, the time required to form a temperature step change should not exceed one tension of the T0.5 of the tested thermocouple The response time of the recording instrument or instrument should not exceed one tension of the T0.5 of the tested thermocouple

Main classification

1. Classification by fixed device type

As the main temperature measurement method, thermocouples have a wide range of applications, and there are variable requirements for fixed devices and technical performance Theoretical, the fixed devices of thermocouples can be divided into six types: non fixed device type, threaded type, fixed flange type, movable flange type, movable flange square type, and conductive protective tube type

2. Classification by assembly and structural method

According to the performance and structure of thermocouples, they can be divided into special purpose thermocouples such as detachable thermocouples, flameproof thermocouples, armored thermocouples, and spring fixed thermocouples

Installation requirements

The installation of thermocouples and thermal resistors should be noted to facility accurate temperature measurement

Safe and easy to maintain, without affecting equipment operation and production operations To meet the above requirements, the following points should be noted when selecting the installation location and insertion depth of thermocouples and thermal resistors:

1. In order to ensure sufficient heat exchange between the measurement end of thermocouples and thermal resistors and the measured medium, the measurement point position should be reasonably selected, and the installation of thermocouples or thermal resistors near dead corners of valves, tubes, pipelines, and equipment should be avoided as many as possible

2. Thermocouples and thermal resistors with protective sleeps have heat transfer and heat dispersion losses In order to reduce measurement errors, thermocouples and thermal resistors should have sufficient insertion depth:

(1) For thermocouples measuring the fluid temperature at the center of the pipeline

Generally, the measuring end should be inserted into the center of the pipeline (vertically or objectively installed) If the pipeline diameter of the measured fluid is 200mm, the insertion depth of the thermocouple or thermal resistor should be selected as 100mm;

(2) For temperature measurement of high temperature, high pressure, and high speed fluids (such as main steam temperature), in order to reduce the resistance of the protective sleep to the fluid and prevent the protective sleep from breaking under the action of the fluid, the shallow insertion method of the protective tube or the use of a hot sleep type thermocouple can be adapted The shallow insertion type thermocouple protective sleep tube should be inserted into the main steam pipeline at a depth of not less than 75mm; The standard insertion depth of the hot sleep thermocouple is 100mm;

(3) If it is necessary to measure the temperature of the flow gas inside the flow, although the flow diameter is 4m, a thermocouple or thermal resistor can be inserted at a depth of 1m;

(4) When the insertion depth of the measuring element exceeds 1m, it should be installed vertically as many as possible, or a support frame and protective sleep should be added

Correct use

The correct use of thermocouples can not only accurately observe the temperature value, but also ensure that the product is qualified

More over, it can also save material consumption for thermocouples, saving money and ensuring product quality Incorrect installation, thermal conductivity, and time lag are the main errors in the use of thermocouples

1. Errors caused by incorrect installation

If the installation position and insertion depth of the thermocouple cannot reflect the true temperature of the furnace, in other words, the thermocouple should not be installed too close to the door and heating, and the insertion depth should be at least 8-10 times the diameter of the protective tube; The gap between the protective sleep of the thermocouple and the wall is not filled with insulation material, causing heat to overflow or color air to invade the furnace Theremore, the gap between the thermocouple protective sleep and the furnace wall hole should be blocked with insulation materials such as refinery mud or asbestos rope to prevent cold and hot air measurement from affecting the accuracy of temperature measurement; The cold end of the thermocouple is too close to the furnace body, causing the temperature to exceed 100 ℃; The installation of thermocouples should avoid strong magnetic and electric fields as many as possible, so thermocouples and power cable wires should not be installed in the same period to avoid introducing interference and causing errors; Thermocouples cannot be installed in areas where the measured medium ray flows When measuring the gas temperature inside a tube using a thermocouple, it is necessary to install the thermocouple in the alternative direction of the flow rate and make full contact with the gas

2. Errors introduced due to insulation determination

If the thermocouple is insulated, exceptional dirty or salt residue on the protective tube and pull plate can lead to poor insulation between the thermocouples and the furnace wall, which is even more severe at high temperatures This is not only causes loss of thermoelectric potential but also introduces interference, resulting in errors that can sometimesreach up to hundreds of degrees

3. Error introduced by thermal inertia

Due to the thermal inertia of the thermocouple, the indicated value of the instrument lags behind the change in the measured temperature

This impact is particularly significant when conducting rapid measurements Theremore, thermocouples with thinner thermoelectric electrodes and smaller protective tube diameters should be used as many as possible When the temperature measurement environment permits, the protective tube can even be removed Due to measurement lag, the example of temperature fluctuations detected by thermocouples is smaller than that of furnace temperature fluctuations The larger the measurement lag, the smaller the example of thermocouple fluctuations and the greater the difference from the actual furnace temperature When using thermocouples with large time constants for temperature measurement or control, regardless of the temperature displayed by the instrument instruments very little, the actual furnace temperature may fluid greatly In order to accurately measure temperature, thermocouples with small time constants should be selected The time constant is inversely proportional to the Heat transfer coefficient and directly proportional to the diameter of the hot end of the thermocouple, the density of the material and the specific heat If you want to reduce the time constant, in addition to increasing the Heat transfer coefficient, the most effective way is to minimize the size of the hot end In use, materials with good thermal conductivity and protective sleeps with thin pipe walls and small inner diameters are commonly used In more precise temperature measurements, bare wire thermocouples without protective sleeps are used, but thermocouples are protected to damage and should be corrected and replaced in a timely manner

4. Thermal resistance error

At high temperatures, if there is a layer of coal ash on the protective tube and due courses to it, the thermal resistance increases, hinting the conduction of heat At this point, the temperature reading is lower than the true value of the measured temperature Thereforee, the external cleanliness of the thermocouple protection tube should be maintained to reduce errors

Fault handling

Fault diagnosis method for thermocouple input generation:

After correctly wiring and powering up according to the instrument wiring diagram, the instrument first displays the thermocouple procedure number of the instrument

Then the range of the instrument is displayed, and then the Nixie tube in the lower row of the instrument displays the set temperature, and the Nixie tube in the upper row of the instrument displays the measured temperature If the upper Nixie tube of the instrument displays ^ ^ OVER ^ ^, ^ ^ 0000 ^ ^ or ^ ^ 000 ^ ^ install of the temperature of the heater, it indicates that the input part of the instrument has a fault, and the following tests should be performed:

1) Remove the thermocouple from the input end of the instrument thermocouple and short circuit the input end of the instrument thermocouple with any wire When the instrument is powered on, when the display value of the upper Nixie tube of the instrument is about room temperature, it indicates that the internal wiring of the thermocouple is open circuit, and the thermocouple of the same type should be replaced If the above situation persist, it indicates that the input terminal of the instrument has been damaged during transportation and needs to be replaced

2) Remove the thermocouples of the above faulty instruments and replace them with thermocouples connected to the instruments with the same graduation number that operates normally next by After power on, when the upper row of Nixie tube of the original faulty instrument displays the temperature of heating, it indicates that the thermocouple wiring is open, and replace the same type of thermocouple

3) Remove the faulty thermocouple from the instrument and use a multiplier to measure Ohm (R) * 1

Use a multiplier with two measuring rods to measure both ends of the thermocouple If the resistance value displayed on the multimeter is large, it indicates an open circuit in the internal connection of the thermocouple Replace the same type of thermocouple Otherwise, if there is a certain resistance value, it indicates that there is a problem with the input terminal of the instrument and the instrument should be replaced

4) The wiring is correct according to the instrument wiring diagram If the instrument is powered on and the Nixie tube in the upper row of the instrument displays negative values, it indicates that the thermocouple ^ ^+^ ^ and ^ ^ - ^ ^ connected to the instrument are wrongly connected Just exchange it again

5) When the instrument is running with correct wiring, the temperature displayed by the upper Nixie tube of the instrument differences from the actual measured temperature by 40 ℃ to 70 ℃ Even greater differences indicate that the graduation number of the instrument is incorrect compared to the graduation number of the thermocouple Recording to the corresponding relationship between the temperature and millivolt (MV) value of thermocouples with graduation numbers B, S, K, E, etc., under the same temperature, the generated millivolt (MV) value is the smallest in B graduation number, the second smallest in S graduation number, the larger in K graduation number, and the largest in E graduation number This principle is used to differentiate

Common fault analysis and handling:

Possible causes of malfunction and handling methods

If the thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is lower) and the thermoelectric electrode is short circuited due to moisture, dry it; If the insulator is damaged, replace it

Due accumulation at the junction post of the thermocouple, causing due accumulation during short circuit cleaning

Short circuit between compensating wires, identify the short circuit point, strengthen insulation or replace compensating wires

If the thermocouple thermal electrode deteriorates and the length allows, cut off the deteriorated section and weld it again, or replace it with a new thermocouple

The compensation wire and thermoelectric dipole are reversed and reconnected correctly

Replace the matching compensation wire if the compensation wire does not match the thermocouple

The installation position of the thermocouple is not recorded or the insertion depth does not meet the requirements Reinstall it according to regulations

The thermocouple cold junction temperature compensation does not meet the requirements Adjust the cold junction compensator

Thermocouples and display instruments do not match Replace thermocouples or display instruments to match

The thermoelectric potential is higher than the actual value (the indicated value of the display instrument is higher) The display instrument is not compatible with the thermocouple, and the thermocouple should be replaced to match it

Thermocouple and compensation wire do not match Replace the compensation wire to make it match

DC interference signal enters to terminate DC interference

Unstable output of thermoelectric potential Poor contact between thermocouple terminal and thermoelectric electrode Tighten the terminal screws

The insulation of the thermocouple measurement line is damaged, causing intermittent short circuits or grounding Identify the fault point and repair the insulation

Loose installation of thermocouples or external vibration accelerating of thermocouples to terminate vibration or take shock absorption measures

The thermoelectric electrode will be disconnected, repaired or replaced with a thermocouple

Identify the source of external interference (such as AC leakage, electromagnetic field induction, etc.) and opt shielding measures

Thermocouple has a large error in thermoelectric potential, and the hot electrode has deteriorated Replace the hot electrode

Improve installation position of thermocouples, changing installation position

Due removal on the surface area of the protective tube

Temperature compensation

Due to the fact that the materials of thermocouples are generally related expense (specifically when using precise metals)

The distance from the temperature measurement point to the instrument is very far In order to save thermocouple materials and reduce costs, compensation wires are commonly used to extend the cold end (free end) of the thermocouple to a control room with correlated stable temperature and connect it to the instrument terminals It must be pointed out that the function of the thermocouple compensation wire is only to extend the thermoelectric code, causing the cold end of the thermocouple to move to the instrument terminal in the control room It itself cannot limit the impact of temperature changes at the cold end on temperature measurement and does not have a compensation effect There are other correction methods needed to be used to compensate for the impact of the cold end temperature t0 ≠ 0 ℃ on temperature measurement When using thermocouple compensation wires, it is necessary to pay attention to the model matching, the polarity cannot be connected incorrectly, and the temperature difference between the compensation wire and the thermocouple connection end cannot exceed 100 ℃

Main advantages

1. High measurement accuracy Due to direct contact with the tested object, it is not affected by the intermediate medium

2. Wide measurement range Commonly used thermocouples can measure continuously from minus 50 degrees to 1600 degrees Some special thermocouples can measure as low as -269 degrees (such as gold iron nickel chromium) and as high as 2800 degrees (such as tungsten and rhenium)

3. Simple construction and conservative use Thermocouples are commonly composed of two different types of metal wires, and are not limited by size or beginning They have a protective sleep outside, making them very conservative to use

Selection method

A thermocouple is formed by connecting two different conductors together

When the measurement and reference connection points are at different temperatures, the so called thermoelectric magnetic force (EMF) is generated The purpose of the connection point is to measure the connection point of a thermocouple located at the measured temperature The reference connection point is the part of the thermocouple connection point that remains at a known temperature or can automatically compensate for temperature changes

In conventional industrial applications, thermocouple components are generally terminated at joints; How, the reference connection point is randomly located on the joint, and installed, appropriate thermocouple extension wires are used to connect to a controlled environment with correlated stable temperature The type of connection point is the physical connection (welding) between the shell type thermocouple connection point and the probe wall, which can achieve good heat transfer - that is, heat is transferred from the outside to the thermocouple connection point through the probe wall It is recommended to use shell type thermocouples to measure the temperature of static or flowing corrosive gases and liquids, as well as some high pressure applications In an insulated thermocouple, the thermocouple connection point is separated from the probe wall and surrounded by a soft powder Although the response speed of insulated thermocouples is slower than that of shell type thermocouples, they can provide electrical insulation It is recommended to use insulated thermocouples to measure corrosive environments, which can be completely electrically insulated from the surrounding environment through a shield Exposed end thermocouples allow the top of the connection point to penerate into the surrounding environment, providing the best response time, but are limited to use in non corrosive, non hazardous, and non pressurized applications The response time is represented by a time constant, which is defined as the time required for the sensor to change 63.2% between the initial and final values in the controlled environment The exposed end thermocouple has the fast response speed, and the smaller diameter of the probe sheath, the fast response speed, but its maximum allowable measurement temperature is also lower An extension line thermocouple extension line is a pair of lines with the same temperature and electrical frequency characteristics as the connected thermocouple When the connection is appropriate, the extension line transfers the reference connection point from the thermocouple to the other end of the line, which is manually located in the controlled environment

When selecting thermocouples, the following factors need to be considered:

1. Temperature range to be measured;

2. Required response time;

3. Connection point type;

4. Chemical corrosion resistance of thermocouples or shear materials;

5. Resistance to wear or vibration;

6. Installation and restriction requirements, etc