Two conductors with different compositions (called thermocouple wires or thermoelectric electrodes) are connected at both ends to form a circuit. When the temperature of the two junction points is different, an electromotive force will be generated in the circuit. This phenomenon is calledThermoelectric effectAnd this electromotive force is called thermoelectric potential. Thermocouples use this principle for temperature measurement, where the end directly used to measure the temperature of the medium is called the working end (also known as the measuring end), and the other end is called the cold end (also known as the compensating end); Connect the cold end to the display instrument or matching instrument, and the display instrument will indicate the thermoelectric potential generated by the thermocouple.

Thermoelectric force generated by thermocouples. Its size is only related to the material of the thermoelectric electrode and the temperature at both ends, and is independent of the length and diameter of the thermoelectric electrode.

The magnitude of thermoelectric potential in a thermocouple circuit is only related to the temperature of the conductor material and the two contact points that make up the thermocouple, and is independent of the shape and size of the thermocouple. When the two electrode materials of the thermocouple are fixed, the thermoelectric potential is the temperature of the two junction pointstandt0The function difference of .

This relationship has been widely used in practical temperature measurement. Because of the cold endt0Constant, the thermoelectric potential generated by the thermocouple is onlyHot end(measurement end)The change in temperature corresponds to a certain temperature, that is, a certain thermoelectric potential corresponds to a certain temperature. We only need to use the method of measuring thermoelectric potential to achieve the purpose of temperature measurement^[1] .

When there is a temperature gradient at both ends, there will be current flowing through the circuit, and an electromotive force will exist between the two ends——Thermoelectric force, this is what is calledSeebeck effect(Seebeck effect）. Two homogeneous conductors with different compositions are thermoelectric electrodes, with the higher temperature end as the working end and the lower temperature end as the free end, which is usually at a constant temperature. Create a thermocouple calibration table based on the functional relationship between thermoelectric potential and temperature;Dividing tableIt is the temperature at the free end0℃Under the given conditions, different thermocouples have different calibration tables.

When connecting a third metal material in a thermocouple circuit, as long as the temperature of the two contacts of the material is the same, the heat generated by the thermocoupleelectric potentialIt will remain unchanged, that is, not affected by the third metal connected to the circuit. Therefore, when measuring temperature with a thermocouple, it can be connectedmeasuring instrumentAfter measuring the thermoelectric potential, the temperature of the measured medium can be determined. When measuring temperature with a thermocouple, it is required to have its cold end (the measuring end is the hot end, which is connected to the lead wire)measuring circuitThe temperature of the connected end (called the cold end) remains constant, and its thermoelectric potential is proportional to the measured temperature. If the temperature changes at the cold end during measurement, it will seriously affect the accuracy of the measurement. Taking certain measures to compensate for the impact caused by temperature changes at the cold end is called normal cold end compensation for thermocouples. andmeasuring instrumentConnect with compensating wires.

Calculation method for thermocouple cold junction compensation:

From millivolts to temperature: measure the cold junction temperature, convert it to the corresponding millivolt value, add it to the millivolt value of the thermocouple, and convert it to temperature;

From temperature to millivolts: Measure the actual temperature and cold end temperature, convert them into millivolts, subtract them to obtain the millivolt value, and then obtain the temperature.