1, thermocouple temperature measurement principle and verification method 1.1 thermocouple temperature measurement principle The principle of temperature measurement is based on the principle of thermoelectric effect, that is, the two different components of the conductor are welded or twisted together, when the two ends in different temperature field At this time, a thermoelectromotive force (composed of a thermoelectric potential and a contact potential) is generated in the circuit, and the temperature to be measured is directly or indirectly obtained by measuring the magnitude of the thermoelectromotive force.
1.2 Thermocouple verification method The common method of verification for industrial thermocouples is a comparison method that uses a standard thermocouple of a higher level and a thermocouple to be directly compared. In operation, the thermocouple is bundled together with a standard thermocouple. Put in the verification furnace. The comparison method is divided into bipolar method, polar method of the same name, and differential method. In the actual work of calibration laboratories, we use a two-stage method: after the thermocouples and standard thermocouples are bound, they are placed at the same temperature of the test furnace. Next, the thermoelectric values ​​of the standard thermocouples and the thermocouples under test are respectively measured by the electric measuring device, and then the thermoelectric potentials on the index table and the potential values ​​at the corresponding temperature points on the standard certificate and various thermocouples are used in various points. The differential thermoelectric potential at the point can be used to determine the deviation of the thermocouple to be calibrated at the temperature point from the index table.
In the formula, e is the arithmetic mean value of the thermoelectromotive force measured at the temperature near the test point of the thermocouple under test; e is the thermoelectromotive force value of the temperature at a certain verification point on the certificate of the standard thermocouple; e standard thermocouple At the temperature of a certain test point, the measured arithmetic mean of the thermoelectromotive force; e is divided into the thermodynamic value of the temperature of a certain test point checked on the thermocouple index table; S standard thermocouple at a certain test point temperature The differential thermal emf; S is the differential thermo-electromotive force at the temperature of a certain thermocouple thermocouple detected.
2. Analysis of existing problems and countermeasures 2.1 The practice of combined thermo-electromotive force measurements with large or small values ​​and the structure and characteristics of thermocouples may be comprehensively considered in the following six reasons:
1) The electrode bends. Thermocouple wire is thin and soft, and it is easily deformed. When the double wire is folded and distorted, plastic deformation causes stress in the thermocouple wire, which changes the thermoelectric characteristics of the thermocouple. The accuracy of the results of the deformation thermocouple measurement is affected. For this reason, the wire must be straightened before verification.
2) The hot electrode is contaminated. The thermocouple wire is contaminated or even oxidized, which makes the surface of the thermocouple wire unlit and dark and dark. At this time, the thermoelectric characteristics of the thermode are extremely unstable, and the accuracy of the measured data is poor, so it is necessary to perform Clean, eliminate pollution layers.
3) Improper depth of thermocouple insertion. The national inspection regulations JJG141-2000 "Working with precious metal thermocouples" and JJG351-1996 "Working with inexpensive metal thermocouples" have strict regulations on the insertion depth of thermocouples. It must be ensured that the measuring end of the thermocouple is placed in the highest temperature zone of the test furnace (at least the measuring end is placed in an effective uniform temperature field). Due to the actual operation, the position of the thermocouple may move somewhat, and the amplitude is too large to cause the insertion depth. Improperly, the measurement end of the thermocouple is not placed in the highest temperature zone of the test furnace, resulting in inaccurate measurement results and the error is beyond the allowable range. Therefore, the thermocouples must be re-installed in accordance with the requirements of the verification procedures, and they must be tested again.
4) The reference temperature of the thermocouple is too high. In the thermocouple detection, we usually place the reference end of the thermocouple in a 0°C thermostat so that the reference junction temperature of the thermocouple is 0°C. In the actual verification work, the reference junction temperature may occur due to an increase in the furnace temperature. Changes affect the measurement results. For this case, you can use a zero-degree thermostat that has been qualified by metrology and maintains a constant temperature of (0 ± 0.1) °C. You can also continuously configure the ice-water mixture to adjust the reference junction temperature, or use the thermoelectric correction method. E(t, O) = E(t, t1) + E(t1, O) calculates the thermoelectromotive force value when the reference terminal temperature deviates from 0°C. In the formula, E is the thermo-electromotive force value of the thermocouple, t is the measuring terminal temperature of the thermocouple, and t1 is the actual temperature of the reference terminal in the thermocouple calibration process. In addition, there is an electronic freezing device made by using the principle of semiconductor refrigeration, which can keep the reference temperature of the thermocouple at (0±0.05)°C, and it is small in size, easy to operate, and easy to use.
5) The axial temperature field of the test furnace does not meet the requirements. The temperature field of the thermocouple test furnace has requirements in the verification procedures, requiring that the highest uniform temperature field center and the geometric center of the furnace should not deviate more than 10mm along the axis, the uniform temperature field length should not be less than 60mm, and the radius should be within 14mm, between any two points The temperature difference is not more than 1°C. Thermocouple test furnace temperature field, especially whether the axial temperature field meets the requirements is the key factor to determine the quality and accuracy of the test, thermocouple test furnaces are generally resistance furnace, after a certain period of time the resistance furnace after the use of heating elements have changed Therefore, the performance of the furnace changes, and even the technical specifications and requirements for the uniform temperature field of the furnace can not be met. For this purpose, the axial temperature field temperature distribution of the furnace needs to be tested periodically, and if necessary, the longitudinal temperature field temperature distribution can be simultaneously performed. The measurement ensures that the performance and technical specifications of the temperature field comply with the requirements of the national regulations.
6) Destruction of the insulation layer, causing external power supply to enter or cause leakage, and there may be interference signals. At this time, it is necessary to find out the reasons for the targeted repair or replacement of insulating materials, check the source of interference, and try to eliminate.
2.2 Data zero value appears during the verification process According to the analysis, this situation may be caused by the following three factors:
1) Compensate for short circuit between wires. This is more likely to occur in the thermocouple test. When the thermocouple is installed in the furnace and wiring, the compensating wire is short-circuited. At this time, the read data is zero. In this case, first check the compensation wire, or replace the compensation wire or re-insulate the short-circuited part.
2) Local short circuit within the thermocouple. At this point, the thermocouple is taken out and the short-circuit part is inspected. There are two cases. First, there may be disconnection at the working end of the welding. The hot electrodes are not actually welded together, but are simply hinged together. The articulation is not performed according to the standard. Or the joint is a virtual weld. Second, the insulation is damaged and the hot electrode is short-circuited. At this time, the insulation should be repaired and the test should be installed again.
3) The circuit is broken and the terminal is loose. Check the wiring circuit, find a broken or loose place, re-wiring or tightening.
2.3 In the process of verification, the thermal electromotive force of the thermocouple in the thermodynamic instability test is not stable. After analysis, there are three possible reasons:
1) The thermocouple terminal and the hot electrode are in poor contact. If this happens during thermocouple verification, first check whether the contact between the terminal and the hot electrode is firm and reliable.
2) The thermocouple thermoelectrode or measurement terminal will be broken and there will be intermittent connections. When this happens, the measured thermo-electromotive force tends to drift constantly and is extremely unstable. If the breakpoint is at the measurement end, it must be re-welded and then retested.
3) External vibration can also make the measured value unstable. When this happens, vibration reduction measures should be taken to firmly install the thermocouple.
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1.2 Thermocouple verification method The common method of verification for industrial thermocouples is a comparison method that uses a standard thermocouple of a higher level and a thermocouple to be directly compared. In operation, the thermocouple is bundled together with a standard thermocouple. Put in the verification furnace. The comparison method is divided into bipolar method, polar method of the same name, and differential method. In the actual work of calibration laboratories, we use a two-stage method: after the thermocouples and standard thermocouples are bound, they are placed at the same temperature of the test furnace. Next, the thermoelectric values ​​of the standard thermocouples and the thermocouples under test are respectively measured by the electric measuring device, and then the thermoelectric potentials on the index table and the potential values ​​at the corresponding temperature points on the standard certificate and various thermocouples are used in various points. The differential thermoelectric potential at the point can be used to determine the deviation of the thermocouple to be calibrated at the temperature point from the index table.
In the formula, e is the arithmetic mean value of the thermoelectromotive force measured at the temperature near the test point of the thermocouple under test; e is the thermoelectromotive force value of the temperature at a certain verification point on the certificate of the standard thermocouple; e standard thermocouple At the temperature of a certain test point, the measured arithmetic mean of the thermoelectromotive force; e is divided into the thermodynamic value of the temperature of a certain test point checked on the thermocouple index table; S standard thermocouple at a certain test point temperature The differential thermal emf; S is the differential thermo-electromotive force at the temperature of a certain thermocouple thermocouple detected.
2. Analysis of existing problems and countermeasures 2.1 The practice of combined thermo-electromotive force measurements with large or small values ​​and the structure and characteristics of thermocouples may be comprehensively considered in the following six reasons:
1) The electrode bends. Thermocouple wire is thin and soft, and it is easily deformed. When the double wire is folded and distorted, plastic deformation causes stress in the thermocouple wire, which changes the thermoelectric characteristics of the thermocouple. The accuracy of the results of the deformation thermocouple measurement is affected. For this reason, the wire must be straightened before verification.
2) The hot electrode is contaminated. The thermocouple wire is contaminated or even oxidized, which makes the surface of the thermocouple wire unlit and dark and dark. At this time, the thermoelectric characteristics of the thermode are extremely unstable, and the accuracy of the measured data is poor, so it is necessary to perform Clean, eliminate pollution layers.
3) Improper depth of thermocouple insertion. The national inspection regulations JJG141-2000 "Working with precious metal thermocouples" and JJG351-1996 "Working with inexpensive metal thermocouples" have strict regulations on the insertion depth of thermocouples. It must be ensured that the measuring end of the thermocouple is placed in the highest temperature zone of the test furnace (at least the measuring end is placed in an effective uniform temperature field). Due to the actual operation, the position of the thermocouple may move somewhat, and the amplitude is too large to cause the insertion depth. Improperly, the measurement end of the thermocouple is not placed in the highest temperature zone of the test furnace, resulting in inaccurate measurement results and the error is beyond the allowable range. Therefore, the thermocouples must be re-installed in accordance with the requirements of the verification procedures, and they must be tested again.
4) The reference temperature of the thermocouple is too high. In the thermocouple detection, we usually place the reference end of the thermocouple in a 0°C thermostat so that the reference junction temperature of the thermocouple is 0°C. In the actual verification work, the reference junction temperature may occur due to an increase in the furnace temperature. Changes affect the measurement results. For this case, you can use a zero-degree thermostat that has been qualified by metrology and maintains a constant temperature of (0 ± 0.1) °C. You can also continuously configure the ice-water mixture to adjust the reference junction temperature, or use the thermoelectric correction method. E(t, O) = E(t, t1) + E(t1, O) calculates the thermoelectromotive force value when the reference terminal temperature deviates from 0°C. In the formula, E is the thermo-electromotive force value of the thermocouple, t is the measuring terminal temperature of the thermocouple, and t1 is the actual temperature of the reference terminal in the thermocouple calibration process. In addition, there is an electronic freezing device made by using the principle of semiconductor refrigeration, which can keep the reference temperature of the thermocouple at (0±0.05)°C, and it is small in size, easy to operate, and easy to use.
5) The axial temperature field of the test furnace does not meet the requirements. The temperature field of the thermocouple test furnace has requirements in the verification procedures, requiring that the highest uniform temperature field center and the geometric center of the furnace should not deviate more than 10mm along the axis, the uniform temperature field length should not be less than 60mm, and the radius should be within 14mm, between any two points The temperature difference is not more than 1°C. Thermocouple test furnace temperature field, especially whether the axial temperature field meets the requirements is the key factor to determine the quality and accuracy of the test, thermocouple test furnaces are generally resistance furnace, after a certain period of time the resistance furnace after the use of heating elements have changed Therefore, the performance of the furnace changes, and even the technical specifications and requirements for the uniform temperature field of the furnace can not be met. For this purpose, the axial temperature field temperature distribution of the furnace needs to be tested periodically, and if necessary, the longitudinal temperature field temperature distribution can be simultaneously performed. The measurement ensures that the performance and technical specifications of the temperature field comply with the requirements of the national regulations.
6) Destruction of the insulation layer, causing external power supply to enter or cause leakage, and there may be interference signals. At this time, it is necessary to find out the reasons for the targeted repair or replacement of insulating materials, check the source of interference, and try to eliminate.
2.2 Data zero value appears during the verification process According to the analysis, this situation may be caused by the following three factors:
1) Compensate for short circuit between wires. This is more likely to occur in the thermocouple test. When the thermocouple is installed in the furnace and wiring, the compensating wire is short-circuited. At this time, the read data is zero. In this case, first check the compensation wire, or replace the compensation wire or re-insulate the short-circuited part.
2) Local short circuit within the thermocouple. At this point, the thermocouple is taken out and the short-circuit part is inspected. There are two cases. First, there may be disconnection at the working end of the welding. The hot electrodes are not actually welded together, but are simply hinged together. The articulation is not performed according to the standard. Or the joint is a virtual weld. Second, the insulation is damaged and the hot electrode is short-circuited. At this time, the insulation should be repaired and the test should be installed again.
3) The circuit is broken and the terminal is loose. Check the wiring circuit, find a broken or loose place, re-wiring or tightening.
2.3 In the process of verification, the thermal electromotive force of the thermocouple in the thermodynamic instability test is not stable. After analysis, there are three possible reasons:
1) The thermocouple terminal and the hot electrode are in poor contact. If this happens during thermocouple verification, first check whether the contact between the terminal and the hot electrode is firm and reliable.
2) The thermocouple thermoelectrode or measurement terminal will be broken and there will be intermittent connections. When this happens, the measured thermo-electromotive force tends to drift constantly and is extremely unstable. If the breakpoint is at the measurement end, it must be re-welded and then retested.
3) External vibration can also make the measured value unstable. When this happens, vibration reduction measures should be taken to firmly install the thermocouple.
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