During the calibration process of thermocouple, people could find and deal with the inaccuracy of measured data in a timely manner, which is caused by the common problems such as looseness of terminals, short circuit of thermocouple or deviation of the binding from the geometric center, but they tend to ignore some problems that affect the test results.
1. Thermocouple wire is bent
Thermocouple wire is thin and soft, and easy to deform. Plastic deformation such as folding and twisting occurs, it will cause stress on the wire of thermal electrode, which leads to the change of thermoelectric characteristics of wire, and the accuracy of the measurement result is affected. Therefore, the thermocouple wire must be straightened before calibration.
2. Thermocouple wire is contaminated
Thermocouple wire is polluted or even oxidized, which will make the surface of thermoelectric electrode wire not bright and dark. At this time, thermoelectric characteristics of the thermal electrode are extremely unstable, and accuracy of the measurement data is poor. Therefore, it is necessary to clean the polluted electrode and eliminate the pollution layer.
3. Impact of response time
The basic principle of temperature measurement by contact method is that temperature measuring element should reach thermal balance with the measured object. Therefore, when measuring temperature, it is necessary to keep a certain time to make both reach thermal equilibrium. The holding time is related to the thermal response time of the temperature measuring element. The thermal response time mainly depends on the sensor structure and measurement conditions, which varies greatly. Therefore, it is important to choose appropriate heating rate and heat balance time according to different types of thermocouple during daily verification process.
4. Effect of insulation resistance
Under high temperature, insulation resistance of thermocouple decreases sharply with the rise of temperature, so it will produce leakage current, which flows into instrument through the insulation whose insulation resistance has decreased, and makes the instrument indication unstable or producing measurement errors. Therefore, insulation resistance test should not be ignored before thermocouple is installed. Only when requirements of the verification regulation are met, the temperature tolerance verification can be carried out.
