Types and structures of thermocouples

Thermocouples are one of the most commonly used temperature detection elements in the industry. The working principle of thermocouples is based on the Seebeck effect, that is, when two conductors of different properties are connected at both ends to form a loop, if the temperatures of the two connecting ends are different, a physical phenomenon of thermal current will be generated in the loop.

Types and structural composition of thermocouples

(1) Types of thermocouples

Common thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The standard thermocouples used refer to thermocouples whose national standards define the relationship between their thermoelectric potential and temperature, allowable deviations, and have a unified standard scale. They have matching display instruments available for selection. Non-standard thermocouples are not as good as standard thermocouples in terms of application range or number of levels, and some have different scales. They are mainly used for measurements in certain special places.

Standardized thermocouples Since January 1, 1988, my country has produced all thermocouples and thermal resistors according to IEC standards, and seven types of standardized thermocouples, S, B, E, K, R, J, and T, are unified design thermocouples in my country.

(2) The structure of thermocouples In order to ensure that the thermocouple works reliably and stably, the structural requirements for it are as follows:

① The welding of the two thermocouples that form the thermocouple must be firm;

② The two thermocouples should be well insulated from each other to prevent short circuits;

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

④ The protective sleeve should be able to ensure that the thermocouple is fully isolated from the harmful medium.

3. Temperature compensation of the cold end of thermocouple

Since the materials of thermocouples are generally relatively precious (especially when precious metals are used), and the distance between the temperature measurement point and the instrument is very far, in order to save thermocouple materials and reduce costs, a compensating wire is usually used to extend the cold end (free end) of the thermocouple to the control room where the temperature is relatively stable, and connect it to the instrument terminal. It must be pointed out that the role of the thermocouple compensating wire is only to extend the hot electrode and move the cold end of the thermocouple to the instrument terminal in the control room. It itself cannot eliminate the influence of the cold end temperature change on the temperature measurement, and does not play a compensatory role. Therefore, other modification measures are required to compensate for the influence of the cold end temperature on the temperature measurement when t0≠0℃.