Problems that are easily overlooked in thermocouple calibration

During the process of calibrating thermocouples, calibrators can usually promptly discover and easily handle inaccurate measured data caused by common problems such as loose terminal posts, short circuits in thermocouples, or bundling that deviates from the geometric center. However, they tend to forget some problems that are easily overlooked and affect the test results.

1. Thermocouple length

The JJG351-1996 "Cheap Metal Thermocouples for Work" verification regulations clearly stipulate that the length of the thermocouple should not be less than 750mm. The reason for stipulating the length of the thermocouple is that the thermocouple should have a sufficiently wide temperature gradient zone after leaving the temperature measurement area. The thermoelectric electromotive force of the thermocouple is also generated in this area. To effectively prevent the heat from the hot end (measuring end) of the thermocouple from being transferred to the cold end (wiring end), the most basic method is to keep the cold end of the thermocouple away from the hot end. Generally speaking, the error caused by the insufficient length of the thermocouple is negative, and the correction value is positive. The shorter the length, the greater the error. Therefore, the length of the thermocouple needs to be determined before the furnace is installed for verification.

2. Bending of Thermocouple Wire

Thermocouple wire is thin and soft, and is very easy to deform. When the thermocouple wire undergoes plastic deformation such as folding and twisting, which causes stress in the thermocouple wire, the thermoelectric properties of the thermocouple are changed, thus affecting the accuracy of the measurement results of the deformed thermocouple. Therefore, the thermocouple wire must be straightened before calibration.

3. Thermocouple wire is contaminated

If the thermocouple wire is contaminated or even oxidized, the surface of the thermocouple wire will become dull, dark and black. At this time, the thermoelectric characteristics of the thermocouple are extremely unstable and the accuracy of the measured data is poor. Therefore, the contaminated electrode must be cleaned to eliminate the contamination layer.

4. Impact of response time

The basic principle of contact temperature measurement is that the temperature measuring element must reach thermal equilibrium with the object being measured. Therefore, a certain period of time must be maintained during temperature measurement to allow the two to reach thermal equilibrium. The length 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 of the sensor and the measurement conditions, and varies greatly. Therefore, in the daily calibration process, it is necessary to select the appropriate heating rate and thermal equilibrium time according to different types of thermocouples.

5. Influence of insulation resistance

When the thermocouple is at high temperature, its insulation resistance decreases sharply as the temperature rises, so leakage current will be generated. This current flows into the instrument through the insulation whose insulation resistance has dropped, making the instrument indication unstable or causing measurement errors. Therefore, do not ignore the insulation resistance test of the thermocouple before installing it in the furnace. Only when it meets the requirements of the verification regulations can the temperature tolerance verification be carried out.