In order to ensure that the Mineral insulated thermocouple calibration can meet the relevant requirements of the specification, the main work is to reduce the uncertainty of the thermocouple calibration as much as possible. Precision thermocouple calibration furnace has a wide uniform temperature zone, wide corresponding jack temperature uniform temperature zone and good temperature stability.
1. Issues
Issues with the previous calibration furnace is that the furnace body used a horizontal tubular heating furnace structure, while it used a single-section heating wire made of unequal distances for heating. This heating method can effectively ensure that the central area obtains a wide isothermal working area. However, there are serious problems in the actual application process. The worst issue is short distance of the isothermal zone. Therefore, when performing calibration work, not only the heat conduction of the tested thermocouple will affect the actual temperature field, but also the external heat transfer of the test furnace itself, which will lead to certain errors in the results of the thermocouple test. For the above effects, it can be specifically manifested as inconsistent heat exchange conditions between the calibration furnace and each tested thermocouple, and at the same time, there are certain differences in the heat balance points at different times.
(1) .Influence of temperature field
For the heat exchange status and heat balance point between the furnace and the tested thermocouple, the change of the temperature field in the calibration furnace working area will affect the results. Specifically, the most direct result of the change in the temperature field is the impact on the temperature of the measuring end of the standard thermocouple and the calibrated thermocouple, temperature will also vary with the change of position. The first thing to do when calibrating an assembled thermocouple is to disassemble the protective tube and then place the thermal electrode and insulating tube with the standard thermocouple in the calibration fire for comparison. Through a certain comparison, it can be known that the temperature of the measuring end between the standard thermocouple and the calibrated thermocouple is so small that it is almost negligible, which will not cause very large uncertainty. When the calibration diameter of the calibrated MI thermocouple is very large, the measuring end node of the thermocouple will need a very large distance because it is in the metal protective tube, and then the measuring end of the standard thermocouple will appear, which will cause temperature differences. In the actual calibration process, when MI thermocouple are tied together, the phenomenon of axial heat conduction will occur at this time, and the temperature field at this time will be much worse than the temperature field environment at no-load. At the same time, in the working area of the horizontal furnace itself, there will be radial convection, which will cause the temperature of the calibrated thermocouple to be different at different positions on a cross section. In order to really ensure the reduction of the radial temperature difference during calibration, it is necessary to use an equal temperature block effectively.
(2) .Differences in thermal equilibrium points at different times
When the heat temperature zone is very small, the external heat transfer of the tested thermocouple and the calibration furnace itself will cause the actual temperature field to change, which will affect the thermocouple test results. For the same temperature point, the test at different times will also be different, because the heat balance point of the calibration furnace is different at different times. Therefore, in the actual thermocouple inspection process, the experimental data obtained by subsequent measurements are usually higher than the the last measurement.
When the isothermal zone of the calibration furnace itself is large enough, it can be ensured that the heat conduction of the thermocouple to be tested and the calibration furnace itself will not affect the thermocouple test results, and the thermal balance point of the non-isothermal distribution area of the calibration furnace and the position change between the calibrated and standard thermocouple will not affect the test value.
2. Improvements
A. It is necessary to apply the internal and external secondary furnace body structure and temperature control system to the precision thermocouple calibration furnace, so that the expansion of the isothermal working area can be effectively guaranteed, and at the same time, the calibration of the thermocouple to be calibrated and the calibration furnace body can be ensured. When the working temperature of the outer stage can be very close to the working temperature of the inner stage, it can ensure that the heat transfer of the inner stage will be reduced to the minimum, thus ensuring a wider uniform temperature working area of the calibration furnace;
B. For the precision thermocouple calibration furnace, in order to ensure the reduction of the data dispersion in the furnace, it is necessary to use the socket method to place the thermocouple, so as to ensure that each consistency of the heat exchange results between a calibrated thermocouple and the calibration furnace is effectively improved;
C. In the actual calibration process, in order to reduce the impact on the external heat conduction of the calibrated thermocouple, you can also set a compensation heater. The specific method is to set the compensation heater at the furnace mouth of the calibration furnace.
D. The calibration furnace is equipped with a digital temperature controller to indicate and control the working temperature. For the temperature of the furnace tube and temperature equalizing block in the working area, it is detected by different temperature control thermocouple. We can divide them into temperature control thermocouple 1 and 2 in order. The temperature controller is used to realize the control, which are output 1 and 2 respectively; after that, the control of the heating power of the uniform temperature block and the furnace tube heater is carried out through the thyristor. Under normal circumstances, the temperature of the furnace tube is lower than the temperature during the working process of the uniform temperature block, so as to ensure the occurrence of external heat conduction during the working process of the uniform temperature block, and then fundamentally achieve the improvement of the calibration furnace zone temperature uniformity. https://www.super-instrument.com/
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