Thermocouples measure temperature based on the seeback effect. The condutors a and b of two different materials are connected into a closed loop. If the two contacts are at different temperatures, t0 and t, there is an electromotive force in the loop, which is called the thermoelectric potential. This phenomenon is called the thermoelectric effect. The thermoelectric potential is a function of temperature t0 and t, and the constant contact temperature t0, then the thermoelectric potential is a single valued function of temperature t, as long as the size of the thermoelectric potential is measured, the measured temperature t can be obtained. The main factors of error of thermocouple are:

1. The influence of thermocouple instability, instability refers to the change of the index value of the thermocouple with the different use time and use conditions. In most cases, it can be a major cause of inaccuracy. The factors affecting the instability are volatilization, oxidation, reduction, embrittlement, crystallization and stain of hot electrode at high temperature.

2. The influence of non-uniformity, generally speaking, if the thermocouple is made of homogeneous conductor, its thermoelectric potential is only related to the temperature at both ends, if the hot electrode material is not uniform, and the hot electrode is in the temperature gradient field , the thermocouple will generate an attached heating potential, the size of which depends on the temperature gradient distribution along the length of the thermal electrode, the non-uniform form and degree of the material and the position of the thermal electrode in the temperature field.

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