When two different conductors or semiconductors A and B form a loop with their two ends connected, as long as the temperatures at the two junctions are different, with one end at temperature T, called the working end or hot end, and the other end at temperature T0, called the free end (also reference end) or cold end, an electromotive force will be generated in the loop. The direction and magnitude of this electromotive force are related to the materials of the conductors and the temperatures of the two junctions. This phenomenon is called the "thermoelectric effect", the loop composed of two conductors is called a "thermocouple", the two conductors are called "thermoelectrodes", and the generated electromotive force is called "thermoelectromotive force".
The thermoelectromotive force consists of two parts: one is the contact electromotive force between the two conductors, and the other is the temperature difference electromotive force of a single conductor.
The magnitude of the thermoelectromotive force in a thermocouple loop is only related to the conductor materials forming the thermocouple and the temperatures of the two junctions, and has nothing to do with the shape and size of the thermocouple. When the materials of the two electrodes of the thermocouple are fixed, the thermoelectromotive force is a function of the temperatures t and t0 of the two junctions [1]. That is,
Formula
Formula
This relationship has been widely used in practical temperature measurement. Because the cold end t0 is constant, the thermoelectromotive force generated by the thermocouple only changes with the temperature of the hot end (measuring end), that is, a certain thermoelectromotive force corresponds to a certain temperature. We can achieve the purpose of temperature measurement by measuring the thermoelectromotive force.
The basic principle of thermocouple temperature measurement is that two conductors of different materials form a closed loop,
When there is a temperature gradient between the two ends, a current will flow through the loop, and an electromotive force—thermoelectromotive force—will exist between the two ends. This is the so-called Seebeck effect. Two homogeneous conductors of different materials are the thermoelectrodes, the end with higher temperature is the working end, and the end with lower temperature is the free end, which is usually at a constant temperature. According to the functional relationship between thermoelectromotive force and temperature, a thermocouple graduation table is made; the graduation table is obtained under the condition that the free end temperature is 0°C, and different thermocouples have different graduation tables.
When a third metal material is inserted into the thermocouple loop, as long as the temperatures at the two junctions of this material are the same, the thermoelectromotive force generated by the thermocouple will remain unchanged, i.e., it is not affected by the insertion of the third metal into the loop. Therefore, when measuring temperature with a thermocouple, a measuring instrument can be connected. After measuring the thermoelectromotive force, the temperature of the measured medium can be known. When measuring temperature with a thermocouple, it is required that the temperature of its cold end (the measuring end is the hot end, and the end connected to the measuring circuit through leads is called the cold end) remains constant, so that the thermoelectromotive force is proportional to the measured temperature. If the (ambient) temperature of the cold end changes during measurement, the accuracy of the measurement will be seriously affected. Taking certain measures at the cold end to compensate for the effects caused by changes in cold end temperature is called cold junction compensation of the thermocouple. Special compensation wires are used for connection with the measuring instrument.
Calculation method for thermocouple cold junction compensation:
From millivolts to temperature: Measure the cold end temperature, convert it to the corresponding millivolt value, add it to the millivolt value of the thermocouple, and then convert to temperature;
From temperature to millivolts: Measure the actual temperature and the cold end temperature, convert them to millivolt values respectively, subtract them to obtain the millivolt value, and then obtain the temperature.
