Measurement principle and advantages of thermocouple

Thermocouples are one of the commonly used temperature detection components in industry. The working principle of thermocouples is based on the Seeback effect, where two conductors with different compositions are connected at both ends to form a loop. Its advantages are: ① high measurement accuracy. Because the thermocouple is in direct contact with the measured object and is not affected by the intermediate medium Wide measurement range. Commonly used thermocouples can measure continuously from -50~+1600 ℃, while some special thermocouples can measure temperatures as low as -269 ℃ (such as gold iron nickel chromium) and as high as+2800 ℃ (such as tungsten rhenium) Simple construction and easy to use. Thermocouples are usually composed of two different types of metal wires, and are not limited by size or opening. They have protective sleeves on the outside, making them very convenient to use.

The basic principle of thermocouple temperature measurement is to weld two different materials of conductors or semiconductors A and B together to form a closed circuit. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, an electromotive force is generated between them, resulting in a current of a certain magnitude in the circuit. This phenomenon is called thermoelectric effect. Thermocouples work by utilizing this effect.

2. Types and Structure Formation of Thermocouples

(1) The commonly used types of thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The standard thermocouple referred to refers to a thermocouple that has a national standard that specifies the relationship between its thermoelectric potential and temperature, allows for errors, and has a unified standard scale. It has a matching display instrument available for selection. Non standardized thermocouples are not as widely used or of the same order of magnitude as standardized thermocouples, and generally do not have a unified calibration table. They are mainly used for measurements in certain special occasions. Since January 1, 1988, all thermocouples and thermistors in China have been produced in accordance with IEC international standards, and seven standardized thermocouples, namely S, B, E, K, R, J, and T, have been designated as unified design thermocouples in China.

(2) In order to ensure the reliable and stable operation of the thermocouple, the structural requirements for it are as follows: ① The welding of the two thermoelectric poles that make up the thermocouple must be firm; ② The two thermoelectric electrodes should be well insulated from each other to prevent short circuits; ③ The connection between the compensating wire and the free end of the thermocouple should be convenient and reliable; ④ The protective sleeve should ensure sufficient isolation between the thermoelectric electrode and harmful media.