Thermocouple verification is easy to ignore
First, the length of the thermocouple JJG351-1996 "working with low-cost metal thermocouple" verification procedures clearly stipulates that the length of the thermocouple is not less than 750mm, the reason for the length of the thermocouple, because it takes into account the thermocouple after leaving the temperature measurement area There must be a wide enough temperature gradient area. The thermocouple's thermo-electromotive force is also generated in this area. To effectively prevent the thermocouple's hot end (measurement) from transferring heat to the cold junction (terminal), the most basic method is to have enough thermocouple cold junction. Distance away from the hot end. In general, the error due to insufficient thermocouple length 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 inspection.
Second, the thermocouple wire bending thermocouple wire is thin and soft, easily deformed, when the wire occurs folding, twisting and other plastic deformation of the thermocouple wire to produce stress, it changes the thermocouple thermoelectric characteristics, so that the deformation of thermoelectric The accuracy of even measurement results is affected. Therefore, the thermocouple wire must be straightened before verification.
3. The thermocouple wire is contaminated by the contaminated thermocouple wire and even oxidized. The surface of the thermocouple wire is not bright and dark and dark. At this time, the thermoelectric characteristics of the thermode are extremely unstable, and the accuracy of the measurement data is poor. Therefore, it is necessary to clean contaminated electrodes and eliminate contaminated layers.
Fourth, the impact of response time The basic principle of contact temperature measurement is that the temperature measurement element and the measured object to achieve thermal equilibrium. Therefore, it takes a certain period of time for the temperature measurement to reach a 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 sensor's structure and measurement conditions. Therefore, in the daily verification process, the appropriate heating rate and time for heat balance should be selected according to different types of thermocouples.
Fifth, the influence of insulation resistance In the high temperature thermocouple, the insulation resistance decreases rapidly with the temperature rise, it will produce a leakage current, the current through the insulation resistance has dropped insulation into the instrument, so that the instrument indicates instability or measurement error. Therefore, do not ignore the test of the insulation resistance before installing the thermocouple. Only when the requirements of the verification regulations are met, can you perform the temperature tolerance verification.
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