Basic Principles of Thermocouple Temperature Measurement

Basic Principles of Thermocouple Temperature Measurement

Soldering two different materials of conductors or semiconductors A and B together forms a closed circuit. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, an electromotive force is generated between them, resulting in a current of a certain magnitude in the circuit. This phenomenon is called the thermoelectric effect. Thermocouples work by utilizing this effect.

What is a thermocouple

Both thermocouples and thermal resistors belong to the contact type temperature measurement in temperature measurement. Although their functions are to measure the temperature of objects, their principles and characteristics are not the same

Thermocouples are the most widely used temperature devices in temperature measurement. Their main characteristics are wide measurement range, stable performance, simple structure, good dynamic response, and the ability to remotely transmit 4-20mA electrical signals, making it easy for automatic control and centralized control. The temperature measurement principle of thermocouples is based on the thermoelectric effect. When two different conductors or semiconductors are connected into a closed circuit, when the temperatures at the two contacts are different, a thermal electric potential will be generated in the circuit. This phenomenon is called the thermoelectric effect, also known as the Seebeck effect. The thermoelectric potential generated in a closed circuit consists of two types of potentials; Thermoelectric potential and contact potential. Thermoelectric potential refers to the potential generated by the two ends of the same conductor due to different temperatures. Different conductors have different electron densities, so the potential they generate is also different. Contact potential, as the name suggests, refers to the potential formed when two different conductors come into contact with each other due to their different electron densities, resulting in a certain amount of electron diffusion. When they reach a certain equilibrium, the potential is formed, The magnitude of the contact potential depends on the material properties of two different conductors and the temperature at their contact points. At present, there is a standard specification for thermocouples used internationally, which stipulates that thermocouples are divided into eight different divisions, namely B, R, S, K, N, E, J, and T. Their measurement temperature can be as low as -270 degrees Celsius and as high as 1800 degrees Celsius. Among them, B, R, S belong to the platinum series of thermocouples, as platinum is a precious metal, So they are also known as precious metal thermocouples, and the remaining few are called cheap metal thermocouples. There are two types of thermocouple structures, ordinary type and armored type. Ordinary thermocouples are generally composed of thermal electrodes, insulation pipes, protective sleeves, and junction boxes, while armored thermocouples are a solid combination of thermocouple wires, insulation materials, and metal protective sleeves that are assembled and stretched. However, the electrical signal of thermocouples requires a special wire for transmission, which we call compensation wire. Different thermocouples require different compensation wires, and their main function is to connect with the thermocouple to keep the reference end of the thermocouple away from the power source, thereby stabilizing the temperature of the reference end. Compensation wires can be divided into two types: compensation type and extension type. The chemical composition of the extension wire is the same as that of the compensated thermocouple. However, in practice, the extension wire is not made of the same material as the thermocouple, and is generally replaced by a wire with the same electron density as the thermocouple.

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