What is the working principle and internal structure analysis of thermocouples?

Thermocouples are one of the most commonly used temperature detection components in industry. Its advantages are: ① high measurement accuracy. Due to the direct contact between the thermocouple and the measured object, it is not affected by the central medium The measurement scale is wide. Commonly used thermocouples can measure continuously from -50~+1600 ℃, while some special thermocouples can measure -269 ℃ (such as gold iron nickel chromium) and up to+2800 ℃ (such as tungsten rhenium) The structure is simple and easy to use. Thermocouples are usually composed of two different types of metal wires, and are not constrained by size or size. They have maintenance 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 occurs between them, thus forming a large and thin current in the circuit. This phenomenon is called thermoelectric effect. Thermocouples operate using this effect.

2. Types and structural composition 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 is a thermocouple that has a national standard that specifies the relationship between its thermoelectric potential and temperature, allows for errors, and has a consistent standard scale. It has a matching visible surface for selection. Non standardized thermocouples are not as large in scale or order of magnitude as standardized thermocouples, and generally do not have a consistent calibration table. They are mainly used for measurement in certain special occasions. Since January 1, 1988, all thermocouples and thermal resistors in China have been produced in accordance with IEC international standards, and seven standardized thermocouples (S, B, E, K, R, J, T) have been designated as China's unified planning thermocouples.

(2) To ensure the reliable and stable operation of the thermocouple, the following construction requirements are required: 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 maintenance sleeve should ensure sufficient isolation between the hot electrode and harmful media