thermocoupleIt is a temperature sensing element and a primary instrument. It directly measures temperature and converts the temperature signal into a thermoelectric potential signal, which is then converted into the temperature of the measured medium through electrical instruments (secondary instruments). The basic principle of thermocouple temperature measurement is that two conductors with different compositions form a closed circuit. When there is a temperature gradient at both ends, current will flow through the circuit.
At this point, there exists an electromotive force - thermoelectric force - between the two ends, which is known as the Seebeck effect. inthermocoupleWhen a third metal material is connected to the circuit, as long as the temperature of the two contacts of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, it will not be affected by the connection of the third metal to the circuit. Therefore, when using a thermocouple for temperature measurement, a measuring instrument can be connected to measure the thermoelectric potential, which can then determine the temperature of the measured medium.
At present, the materials used for thermocouples mainly include platinum, copper, nickel, etc. These materials are mainly used because their temperature to resistance ratio in common temperature ranges is linearly related. Here, we mainly introduce platinum resistance thermometers. Platinum is a precious metal with stable physical and chemical properties, especially strong oxidation resistance. It is easy to purify and has good processability.
It can be made into extremely fine platinum wire, which has higher electrical resistivity and reproducibility compared to metals such as copper and nickel. It is an ideal material for thermal resistance, but the defect is that the temperature coefficient of resistance is small, and it is prone to brittleness in the recovery medium, making it more expensive. Secondly, thermocouples are theoretically an energy converter that converts thermal energy into electrical energy and measures temperature using the generated thermoelectric potential.