Common thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to the thermocouple that the national standard stipulates the relationship between its thermoelectric potential and temperature, the allowable error, and has a unified standard indexing meter, and it has a supporting display instrument available. Non-standardized thermocouples are not as good as standardized thermocouples in terms of scope or order of magnitude, and there is generally no unified indexing table, mainly used for measurement in some special occasions. Standardized thermocouples Since January 1, 1988, thermocouples and thermal resistors are all produced according to IEC international standards, and seven standardized thermocouples (S, B, E, K, R, J, T) are designated as unified design thermocouples in China.

In order to ensure the reliable and stable operation of the thermocouple, its structural requirements are as follows:

(1) The welding of the two thermal electrodes of the thermocouple must be firm;

(2) The two thermal electrodes should be well insulated from each other to prevent short circuit;

(3) The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable;

④ The protective sleeve should ensure that the thermal electrode is fully isolated from harmful media.