Thermocouple is a widely used temperature sensor in industrial field. Thermocouple temperature measurement system is composed of thermocouple, compensation wire, display control instrument, PLC system or DCS system. Thermocouple compensation wire is a wire with the same nominal value of thermoelectric potential as the matched thermocouple within a certain temperature range. The following are ten common cases of wrong use of compensation conductor in industrial field summarized by Xiaobian. I hope you will gain something after reading the case analysis.

Ordinary wires are used as thermocouple signal wires without compensation wires. According to the thermocouple temperature measurement principle, the thermoelectric potential of thermocouple loop is related to the measured temperature and thermocouple reference end temperature. The temperature of thermocouple reference end (referring to the temperature at thermocouple junction box) installed on the use site changes with the ambient temperature and cannot be constant. When the temperature of the reference end of the thermocouple fluctuates, the compensation wire is used to extend the reference end to an environment with stable temperature or away from the heat source to compensate the error caused by the temperature change of the reference end of the thermocouple. Ordinary wires can transmit the MV signal generated by thermocouple temperature measurement, but they can not compensate for extending the temperature of thermocouple reference end to the instrument control room, resulting in inaccurate temperature compensation of thermocouple temperature measurement system. Correct method: thermocouple compensation wire must be used for thermocouple signal transmission. It is forbidden to replace the compensation wire with cable.

Thermocouples with different graduation numbers and thermocouple compensation wires are mixed to introduce measurement error. A unit used S-type thermocouple to measure the furnace temperature. The staff knew that the thermocouple must use the compensation wire, so they used the inventory K-type thermocouple compensation wire to connect the Platinum Rhodium 10 platinum thermocouple signal to the display instrument (as shown in Figure 2). During use, it was found that the actual furnace temperature deviated greatly from the measured value. After replacing the compensation wire with SC, the temperature measurement returned to normal. According to the regulations of the State Administration of quality and technical supervision, the thermoelectric potential and allowable error of thermocouple compensation wires shall comply with the provisions of JJG 351-1996 verification regulation of low metal thermocouples for work and relevant standards. The thermoelectric potential generated by thermocouple compensation wires corresponding to different graduation numbers under the same ambient temperature is different, so thermocouples with different graduation numbers shall be mixed with thermocouple compensation wires, The temperature compensation error of thermocouple reference end must be introduced into the thermocouple measurement system. Correct method: all kinds of thermocouple compensation wires must be matched with thermocouples with corresponding graduation numbers. The above is what Xiaobian talked about today. If you want to know more wonderful information, please check our website update in time.