Thermocouple working principle

When there are two different conductors or semiconductor A and B to form A loop, its both ends are connected, as long as the temperature of the two nodes is different, the end temperature of T, called end or hot end work, on the other end temperature T0, known as the free end (also known as the reference side) or the cold end, circuit will generate an electromotive force, the direction and size of the electromotive force is related to the conductor material and the temperature of the two contact. This phenomenon is known as the "thermoelectric effect", a loop made up of two conductors called a "thermocouple", the two conductors called a "hot electrode", and the resulting electromotive force called a "thermoelectric electromotive force".

Thermoelectric EMF consists of two parts: one is the contact emf of two conductors, and the other is the thermoelectric EMF of a single conductor.

The size of the thermoelectric electromotive force in the thermocouple loop is only related to the conductor material of the thermocouple and the temperature of the two contacts, and has nothing to do with the shape and size of the thermocouple. When the two electrode materials of the thermocouple are fixed, the thermoelectric EMFs are the two contact temperatures T and T0. Delta function of delta. 

This relation has been widely used in practical temperature measurement. Since the cold end T0 is constant, the thermoelectric EMF produced by the thermocouple changes only with the change of the temperature of the hot end (measuring end), that is, a certain thermoelectric EMF corresponds to a certain temperature. We can measure the temperature only by measuring thermoelectric electromotive force.

The basic principle of thermocouple temperature measurement is that two different material conductors form a closed loop,

When there is a temperature gradient at both ends, an electric current flows through the loop, and between the two ends there is an electromotive force -- the thermoelectric emf -- known as the Seebeck effect. Two homogeneous conductors of different composition are thermal electrodes, with the working end at a higher temperature and the free end at a lower temperature, which is usually at a constant temperature. According to the function relation of thermoelectric electromotive force and temperature, the thermocouple dividing meter is made. The indexing table is obtained at the temperature of the free end at 0℃. Different thermocouples have different indexing tables.

When the third metal material is connected in the thermocouple circuit, the thermoelectricity generated by the thermocouple will remain unchanged as long as the temperature of the two contact points of the material is the same, that is, it will not be affected by the third metal access circuit. Therefore, in the thermocouple temperature measurement, access to the measuring instrument, measured thermoelectric emf, you can know the temperature of the measured medium. When measuring the temperature of a thermocouple, it is required that the temperature of its cold end (the measuring end is the hot end, and the end connected with the measuring circuit through the lead wire is called the cold end) remain unchanged, and the thermoelectric potential is proportional to the measured temperature. If the cold end (environment) temperature changes during measurement, the accuracy of measurement will be seriously affected. Some measures should be taken at the cold end to compensate for the influence caused by the temperature change at the cold end. Special compensation wire for connection with measuring instrument.

Calculation method of thermocouple cold end compensation:

From millivolt to temperature: measure the cold end temperature, convert it to the corresponding millivolt value, and add it to the thermocouple's millivolt value to convert the temperature;

From temperature to millivolt: the actual temperature and the cold end temperature are measured and converted into millivolt values respectively. After subtraction, the millivolt values are obtained and the temperature is obtained.