A thermocouple is a commonly used temperature sensing element in temperature measuring instruments. It directly measures temperature and converts the temperature signal into a thermoelectric potential signal, which is then converted into the temperature of the measured medium by an electrical instrument (secondary instrument). The external appearance of various thermocouples often varies considerably depending on the application, but their basic structure is generally the same. They typically consist of main parts such as thermoelectric electrodes, an insulating sleeve, a protective tube, and a junction box. They are usually used in conjunction with display instruments, recording instruments, and electronic regulators. is a commonly used temperature sensing element in temperature measuring instruments. It directly measures temperature and converts the temperature signal into a thermoelectric potential signal, which is then converted into the temperature of the measured medium by an electrical instrument (secondary instrument). The external appearance of various thermocouples often varies considerably depending on the application, but their basic structure is generally the same. They typically consist of main parts such as thermoelectric electrodes, an insulating sleeve, a protective tube, and a junction box. They are usually used in conjunction with display instruments, recording instruments, and electronic regulators.
In industrial production processes, temperature is one of the important parameters that needs to be measured and controlled. Thermocouples are widely used in temperature measurement due to their many advantages, including simple structure, ease of manufacture, wide measurement range, high accuracy, low inertia, and output signals that are easy to transmit remotely. Furthermore, because thermocouples are passive sensors, they do not require an external power supply for measurement, making them very convenient to use. Therefore, they are often used to measure the temperature of gases or liquids in furnaces and pipes, as well as the surface temperature of solids.
Working Principle: Two conductors of different compositions (called thermocouple wires or thermoelectrodes) are joined at both ends to form a circuit. When the temperatures of the two junctions are different, an electromotive force is generated in the circuit. This phenomenon is called the thermoelectric effect, and this electromotive force is called the thermoelectric potential. Thermocouples utilize this principle for temperature measurement. One 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 is connected to a display instrument or a matching instrument, which indicates the thermoelectric potential generated by the thermocouple.A thermocouple is essentially an energy converter that converts thermal energy into electrical energy, using the generated thermoelectric potential to measure temperature. Regarding the thermoelectric potential of a thermocouple, the following points should be noted:
1. The thermoelectric potential of a thermocouple is the difference between the functions of the temperatures at the two ends of the thermocouple's working end, not a function of the temperature difference between the cold end and the working end;
2. The magnitude of the thermoelectric potential generated by a thermocouple, when the thermocouple material is uniform, is independent of the length and diameter of the thermocouple, and only related to the composition of the thermocouple material and the temperature difference between the two ends;
3. Once the materials of the two thermocouple wires are determined, the magnitude of the thermocouple's thermoelectric potential is only related to the temperature difference of the thermocouple; if the temperature of the cold end of the thermocouple is kept constant, then the thermoelectric potential of the thermocouple is a single-valued function of the working end temperature. Two conductors or semiconductors A and B of different materials are welded together to form a closed circuit, as shown in the figure. When there is a temperature difference between the two junctions 1 and 2 of conductors A and B, an electromotive force is generated between them, thus forming a current in the circuit. Thermocouples work using this effect.
