Thermocouple (thermocouple) is a commonly used temperature measuring element in temperature measuring instruments. It directly measures the temperature, converts the temperature signal into a thermoelectromotive force signal, and converts it into the temperature of the measured medium through an electrical instrument (secondary instrument). The appearance of various thermocouples is often very different due to needs, but their basic structure is roughly the same. They are usually composed of main parts such as hot electrodes, insulating sleeve protection tubes and junction boxes, and are usually combined with display instruments, recording instruments and electronic adjustments. Used in conjunction with the device.
Main Feature
1. Simple assembly,
Thermocouple (Figure 2)
Thermocouple (Figure 2)
Easy to replace;
2. Compression spring type temperature sensing element, good seismic performance;
3. High measurement accuracy;
4. Large measuring range (-200℃～1300℃, under special circumstances -270℃～2800℃);
5. Fast thermal response time;
6. High mechanical strength and good pressure resistance;
7. High temperature resistance can reach 2800 degrees;
8. Long service life.
To work stably, the structure requirements for it are as follows:
1. The welding of the two thermoelectrodes forming the thermocouple must be firm;
2. The two hot electrodes should be well insulated from each other to prevent short circuit;
3. The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable;
4. The protective sleeve should be able to ensure that the hot electrode is fully isolated from the harmful medium.
Working Principle
Two conductors of different components (called thermocouple wires or thermoelectrodes) are connected at both ends into a loop. When the temperatures of the two junctions are different, an electromotive force will be generated in the loop. This phenomenon is called the thermoelectric effect. This electromotive force is called thermoelectric force. Thermocouples use this principle to measure temperature. Among them, 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 compensation end); the cold end and the display When the instrument or supporting instrument is connected, the display instrument will point out the thermoelectric potential generated by the thermocouple.
Thermocouple is actually a kind of energy converter, it converts thermal energy into electrical energy, and uses the generated thermoelectric potential to measure temperature. For the thermoelectric potential of thermocouple, the following issues should be paid attention to:
1. The thermoelectric potential of a thermocouple is the temperature difference between the two ends of the thermocouple, not the temperature difference between the cold and working ends of the thermocouple;
2. The size of the thermoelectric potential generated by the thermocouple. When the material of the thermocouple is uniform, it has nothing to do with the length and diameter of the thermocouple, but is only related to the composition of the thermocouple material and the temperature difference between the two ends;
3. After the two thermocouple wires of the thermocouple are determined, the thermoelectric potential of the thermocouple is only related to the temperature difference of the thermocouple; if the temperature of the cold junction of the thermocouple remains constant, the thermoelectric potential of the thermocouple is only Single-valued function of working end temperature. Solder two conductors or semiconductors A and B of different materials to form a closed loop, as shown in the figure. When there is a temperature difference between the two attachment points 1 and 2 of the conductors A and B, an electromotive force is generated between the two, thus forming a large current in the loop. Thermocouples use this effect to work.