Shunt error of sheathed thermocouple

Shunt error of armoured thermocouple 1) Shunt error The armoured thermocouple used for carburizing furnace of Wazhou Group can not be used for only one week. In order to explore the reasons, the author visited the site, but found no abnormalities, so he had to remove it from the furnace and pass the verification result of the measurement room. So what's the problem? Later, according to the on-site installation characteristics of the thermocouple, it was discovered through research that the above-mentioned problem was caused by the shunt error of the armored thermocouple. The so-called shunt error refers to the phenomenon that when the temperature distribution in the middle part of the thermocouple exceeds 800°C exists when the temperature distribution of the thermocouple is over 800°C, the indication value of the thermocouple is abnormal, which is called shunt error. According to the homogeneity circuit rule, the temperature measurement with a thermocouple is only related to the temperature at the two ends of the measurement terminal and the reference terminal, and has nothing to do with the intermediate temperature distribution. However, because the insulation of the sheathed thermocouple is powdered MgO, the insulation resistance drops by an order of magnitude every time the temperature rises by 100°C. When the temperature in the middle part is high, there must be leakage current, which causes the output potential of the thermocouple There is a shunt error. (2) Conditions for shunting errors Insert the sheathed thermocouple horizontally into the furnace. Its specifications and experimental conditions are: diameter 4.8mm, length 25m, length of heating belt in the middle part is 20m, temperature is 1000℃. In this experiment, the temperature difference between the measuring end of the thermocouple and the middle part is 200°C. If the temperature at the measuring end is higher than the middle part, a negative error will occur; on the contrary, a positive error will occur. If the temperature difference between the two is 200°C, then the shunt error is about 100°C. This must not be ignored. The conditions for generating shunt errors are related to factors such as the type and diameter of the sheathed thermocouple.

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