During the verification of thermocouples, the verification personnel can generally find and handle the inaccurate measured data caused by common problems such as unreliable terminal, thermocouple short circuit or bundle deviation from the geometric center in time, but it is easy to ignore some problems affecting the detection results.
1. The length of the thermocouple
JJG351-1996 "Cheap Metal Thermocouples for Work" clearly stipulates that the length of thermocouples should not be less than 750mm. The reason for specifying the length of thermocouples is to take into account that the thermocouple should have a sufficiently wide temperature after leaving the temperature measurement area. Gradient zone. The thermoelectromotive force of the thermocouple is also generated in this area. To effectively prevent the heat from the hot end (measurement end) of the thermocouple from being transferred to the cold end (terminal), the most basic method is that the cold end of the thermocouple has enough Keep away from the hot end. Generally speaking, the error caused by insufficient thermocouple length is negative, and the correction value is positive. The shorter the length, the greater the error caused. Therefore, the length of the thermocouple needs to be determined before the furnace verification.
2. The thermocouple wire is bent
The thermocouple wire is thin and soft, and it is easily deformed. When the thermocouple wire of the thermoelectrode is stressed due to plastic deformation such as folding and twisting of the thermocouple wire, the thermoelectric characteristics of the thermocouple are changed, so that the measurement result of the deformed thermocouple is accurate. affected. Therefore, the thermocouple wire must be straightened before verification.
3. The thermocouple wire is contaminated
If the thermocouple wire is contaminated or even oxidized, the surface of the thermocouple wire will be dark, dark and black. At this time, the thermoelectric characteristics of the thermoelectrode are extremely unstable, and the accuracy of the measurement data is poor. Therefore, it must be cleaned and polluted. The electrode eliminates the pollution layer.
4. The impact of response time
The basic principle of contact temperature measurement is that the temperature measuring element must reach thermal equilibrium with the measured object. Therefore, it is necessary to maintain a certain period of time during temperature measurement to achieve thermal equilibrium between the two. 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, which vary greatly. Therefore, in the daily verification process, the appropriate heating rate and thermal equilibrium time should be selected according to different types of thermocouples.
5. The influence of insulation resistance
At high temperatures, the insulation resistance of a thermocouple decreases sharply as the temperature rises, so leakage current will occur. This current flows into the meter through the insulation whose insulation resistance has fallen, causing the meter to be unstable or cause measurement errors. Therefore, do not neglect the insulation resistance test of the thermocouple before installing the furnace. Only when the requirements of the verification regulations are met, can the temperature tolerance verification be performed.
