The application of thermocouples in daily work and the basic principles of temperature measurement

The application of thermocouples in daily work and the basic principle of temperature measurement:

Two conductors of different materials form a closed loop. When there is a temperature gradient at both ends, current will pass through the loop. At this time, there is an electromotive force between the two ends - thermoelectric electromotive force, which is the so-called Seebeck effect. Two homogeneous conductors of different components are thermocouples. The end with higher temperature is the working end, and the end with lower temperature is the free end. The free end is usually at a constant temperature. According to the functional relationship between thermoelectric electromotive force and temperature, a thermocouple scale is made; the scale is obtained when the temperature of the free end is 0℃, and different thermocouples have different scales. When a third metal material is connected to the thermocouple loop, 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 loop.

Therefore, when measuring the temperature of the thermocouple, a measuring instrument can be connected. After measuring the thermoelectric electromotive force, the temperature of the measured medium can be known. When measuring temperature, the temperature of the cold end (the measuring end is the hot end, and the end connected to the measuring circuit through the lead is called the cold end) of the thermocouple must remain unchanged, so that the magnitude of its thermoelectric potential is proportional to the measured temperature.

If the (ambient) temperature of the cold end changes during measurement, it will seriously affect the accuracy of the measurement. Taking certain measures at the cold end to compensate for the impact caused by the change in the cold end temperature is called normal cold end compensation of the thermocouple. Use a special compensation wire to connect to the measuring instrument.