The working principle and structure of thermocouple1

1. What is a thermocouple Thermocouple is one of the most commonly used temperature detection components in the industry. The working principle of thermocouple is based on the Seeback effect, that is, two conductors with different components are connected to form a loop, such as the temperature of the two connecting ends. Different, the physical phenomenon of thermal current occurs in the loop. 2. Basic principle of thermocouple temperature measurement A thermocouple is a temperature sensing element and a primary surface. It directly measures the temperature, converts the temperature signal into a thermoelectromotive force signal, and then converts it into the measured medium through the electrical surface (secondary surface). temperature. The basic principle of thermocouple temperature measurement is that two conductors of different compositions form a closed loop. When there is a temperature gradient at the two ends, a current will flow through the loop. At this moment, there is an electromotive force between the two ends—thermoelectromotive force, which is called the Seebeck effect. The two homogeneous conductors of different compositions are the hot electrodes, the end with the higher temperature is the working end, and the end with the lower temperature is the free end, and the free end is generally 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 temperature of the free end is at 0°C, and different thermocouples have different graduation tables. When the third metal material is connected to the thermocouple circuit, 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 third metal being connected to the circuit. . Therefore, when measuring the temperature of the thermocouple, the measuring surface can be connected, and the temperature of the measured medium can be known after measuring the thermoelectromotive force. When there is a temperature difference between the two sticking points 1 and 2 of the conductors A and B, an electromotive force occurs between the two, thus forming a large current in the loop, which is called the thermoelectric effect. Thermocouples use this effect to work. Two conductors of different compositions (called thermocouple wires or hot electrodes) are joined at both ends to form a loop. When the temperature of the junction is different, an electromotive force will occur in the loop. This phenomenon is called the thermoelectric effect, and this electromotive force called thermoelectric potential. Thermocouples use this principle to measure temperature. Among them, the end that is directly used to measure the temperature of the medium is called the working end (also called the measurement end), and the other end is called the cold end (also called the compensation end); the cold end and the The display surface or matching surface connection, the display surface will indicate the thermoelectric potential generated by the thermocouple. A thermocouple is actually an energy converter, which converts thermal energy into electrical energy, and uses the generated thermoelectric potential to measure the temperature. For the thermoelectric potential of the thermocouple, the following issues should be paid attention to: 1: The thermoelectric potential of the thermocouple is a thermoelectric The difference of the temperature function between the two ends of the couple, not the function of the temperature difference between the two ends of the thermocouple; 2: The size of the thermoelectric potential generated by the thermocouple, when the thermocouple data is uniform, it has nothing to do with the length and diameter of the thermocouple, only the thermocouple The composition of the couple material is related to the temperature difference between the two ends; 3: When the composition of the two thermocouple wires of the thermocouple is determined, the size of the thermoelectric potential of the thermocouple is only related to the temperature difference of the thermocouple; if the temperature of the cold end of the thermocouple remains constant , the thermoelectric potential of the thermocouple is only a single-valued function of the temperature at the operating end.

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