Thermocouples are temperature sensitive sensors made using the Seebeck effect in physics. When two different conductors A and B form a closed circuit, they form a thermocouple.
The temperature T end is the temperature sensing part, called the hot end; The temperature T0 is connected to the instrument section, known as the cold end. When the temperature T at the hot end and the temperature T0 at the cold end are different, a thermoelectric potential EAB (T, T0) is generated in the circuit, which is known as the thermoelectric effect and is commonly referred to as the thermoelectric potential. The size of thermoelectric type is related to the difference between T and T0 (known as temperature difference). According to the distribution theory of thermoelectric potential in thermocouple circuits, the thermoelectric potential of a thermocouple is only the difference between the temperature T and T0 at both ends of the thermocouple, that is:
EAB （T, T0）= EAB（T）- EAB（T0） 式（1.1）
That is to say, the thermoelectric potential of a thermocouple is equal to the potential difference caused by the temperatures T and T0 at the hot and cold ends.
In actual temperature measurement, the thermoelectric potential corresponding to the cold end varies with the temperature of the cold end (ambient temperature). It is very difficult to ensure a constant cold end temperature, and to some extent, the measurement accuracy depends on the influence of the cold end temperature. Only when the temperature at the cold end of the thermocouple remains constant, is the thermoelectric potential a function of the measured temperature. The standard specifies that the thermoelectric potential of a node is the thermoelectric potential at 0 ℃.
According to equation (1.1), if T=0, we can obtain:
EAB（T0） = EAB（0）- EAB （0, T0） 式（1.2）
When T0=0, we can obtain:
EAB（T） = EAB（T,0）- EAB （0） 式（1.3）
Substituting equations (1.2) and (1.3) into equation (1.1) yields:
EAB（T，0）= EAB（T,T0）+EAB （T0,0） 式（1.4）
In equation (1.4), EAB (T, 0) is the thermoelectric potential at a cold end temperature of 0 ℃ and a hot end temperature of T. This value is the scale value given by the finished thermocouple; EAB (T, T0) is the thermoelectric potential measured when the hot end temperature is T and the cold end temperature is T0, which is the actual thermoelectric potential value; EAB (T0,0) is the thermoelectric potential obtained by assuming a cold junction temperature of 0 ℃ and an actual cold junction temperature of T0. In actual measurement, T0 is measured using the integrated temperature sensor AD590, and the corresponding thermoelectric potential EAB (T0,0) is automatically retrieved from the calibration table of the corresponding thermocouple. This is the value obtained by checking the table, which is the thermoelectric potential value corresponding to the cold junction temperature compensation. By using a microcontroller to algebraically add the measured EAB (T, T0) value to the cold end temperature compensated EAB (T0,0) value, the thermoelectric potential EAB (T0,0) value at a cold end temperature of 0 ℃ and a hot end temperature of T can be obtained. Then, the temperature value corresponding to EAB (T0,0) can be automatically checked from the calibration table. This is the value obtained from the second table lookup, which is the actual temperature obtained at the hot end of the hot forging couple.
In actual production, the hot end (measuring end) of a thermocouple is far away from the cold end, and the cold end is exposed to the air, making it susceptible to the influence of ambient temperature. Therefore, it is difficult to maintain a constant temperature at the cold end. To achieve this, it is necessary to extend the cold end and perform temperature compensation.