Working principle of thermocouple in electric heater: Two conductors of different materials (called thermocouple wires or thermoelectrodes) are joined at both ends to form a circuit. When the temperatures at the junctions are different, an electromotive force is generated in the circuit. This phenomenon is called the thermoelectric effect, and the electromotive force is called thermoelectromotive force. Thermocouples use this principle for temperature measurement. The end directly used to measure the temperature of the medium is called the working end (also called the measuring end), and the other end is called the cold end (also called the compensating end). The cold end is connected to a display instrument or matching instrument, and the display instrument indicates the thermoelectromotive force generated by the thermocouple.
A thermocouple is actually an energy converter that converts thermal energy into electrical energy and uses the generated thermoelectromotive force to measure temperature. Regarding the thermoelectromotive force of a thermocouple, the following issues should be noted:
1: The thermoelectromotive force of a thermocouple is a function of the difference in temperature at the two ends of the thermocouple, not a function of the temperature difference between the two ends;
2: When the thermocouple material is uniform, the magnitude of the thermoelectromotive force generated by the thermocouple is independent of the length and diameter of the thermocouple, and only depends on the composition of the thermocouple material and the temperature difference between the two ends;
3: Once the material composition of the two thermocouple wires is determined, the magnitude of the thermoelectromotive force is only related to the temperature difference of the thermocouple; if the temperature of the cold end of the thermocouple is kept constant, the thermoelectromotive force is a single-valued function of the working end temperature.
What is a thermocouple? This starts from the temperature measurement principle of thermocouples. A thermocouple is a temperature sensing element, a primary instrument that directly measures temperature and converts the temperature signal into a thermoelectromotive force signal, which is then converted into the temperature of the measured medium by an electrical instrument (secondary instrument).
Basic principle of thermocouple temperature measurement: Two conductors of different materials form a closed circuit, and when there is a temperature gradient at the two ends, a current will flow through the circuit. At this time, there is a Seebeck electromotive force between the two ends—the thermoelectromotive force, which 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. The free end 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 when the free end temperature is 0℃, and different thermocouples have different graduation tables. When a third metal material is inserted into the thermocouple circuit, 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 circuit. Therefore, when measuring temperature with a thermocouple, a measuring instrument can be inserted, and after measuring the thermoelectromotive force, the temperature of the measured medium can be knownelectric heater.
