Issues to be noted when verifying handle type thermocouples

The calibration of handle type thermocouples is a necessary task, otherwise it will affect the normal operation of thermocouples. However, there are some details during the verification process that may not attract everyone's attention. Today, I have summarized some issues that people tend to overlook in their daily lives, hoping to draw your attention.
The verification regulation JJG351-1996 "Low cost Metal Thermocouples for Work" clearly stipulates that the length of thermocouples should not be less than 750mm. The reason for making the requirement for thermocouple length is to consider that handle type thermocouples need a sufficiently wide temperature gradient zone after leaving the temperature measurement area. The thermoelectric potential of the thermocouple is also generated in this area. To effectively prevent the heat transfer from the hot end (measuring end) of the thermocouple to the cold end (connecting end), the basic method is to keep the cold end of the thermocouple at a sufficient distance away from the hot end. Generally speaking, the error caused by insufficient thermocouple length is negative, while the correction value is positive. The shorter the length, the greater the error, therefore, it is necessary to determine the length of the thermocouple before furnace calibration.
The handle type thermocouple wire is contaminated or even oxidized, which can cause the surface of the thermocouple wire to be dull, dark, and black. At this time, the thermoelectric characteristics of the thermocouple wire are extremely unstable, and the accuracy of the measurement data is poor. Therefore, it is necessary to clean the contaminated electrode and eliminate the pollution layer.
The basic principle of contact temperature measurement is that the temperature measuring element needs to reach thermal equilibrium with the measured object. Therefore, it is necessary to maintain a certain amount of time during temperature measurement in order to achieve thermal equilibrium between the two. The duration of the holding time is related to the thermal response time of the temperature measuring element. The thermal response time mainly depends on the structure and measurement conditions of the sensor, with significant differences. So, in the daily calibration process, it is necessary to choose the appropriate heating rate and thermal equilibrium time according to different types of thermocouples.
The handle type thermocouple wire is thin and soft, and is prone to deformation. When the wire undergoes plastic deformation such as folding or twisting, which causes stress on the wire of the thermal electrode, the thermoelectric characteristics of the thermocouple are changed, thereby affecting the accuracy of the measurement results of the deformed thermocouple. Therefore, the thermocouple wire must be straightened before calibration.
At high temperatures, the insulation resistance of handle type thermocouples rapidly decreases with increasing temperature, resulting in leakage current. This current flows into the instrument through the insulation resistance that has already decreased, causing unstable instrument readings or measurement errors. Therefore, before installing the thermocouple into the furnace, do not neglect the testing of its insulation resistance. Only when it meets the requirements of the verification regulations can temperature tolerance verification be carried out.