The basic principle of thermocouple temperature measurement

- Mar 21, 2020-

Welding two different materials of conductors or semiconductors A and B to form a closed loop. 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 the two. The formation of a large current in this phenomenon is called the thermoelectric effect. Thermocouples use this effect to work.

 

What is a thermocouple

Thermocouples and thermal resistors are both contact temperature measurement in temperature measurement. Although their functions are the same to measure the temperature of objects, their principles and characteristics are not the same.

 

Thermocouple is the most widely used temperature device in temperature measurement. Its main characteristics are wide temperature measurement range, stable performance, simple structure, good dynamic response, and the ability to transmit 4-20mA electrical signals remotely, which is convenient for automatic control and concentration control. The thermocouple temperature measurement principle is based on the thermoelectric effect. Two different conductors or semiconductors are connected into a closed loop. When the temperature at the two junctions is different, a thermoelectric potential will be generated in the loop. This phenomenon is called the thermoelectric effect, also known as the Seebeck effect. The thermoelectric potential generated in a closed loop is composed of two potentials; the temperature difference potential and the contact potential. The temperature difference 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 they generate different potentials. The contact potential, as its name implies, is when two different conductors are in contact. Because of their different electron densities, there is a certain amount of electron diffusion. When they reach a certain equilibrium, the potential formed. The size of the contact potential depends on the material properties of the two different conductors and the temperature of their contact points. At present, the thermocouples used in the world have a standard specification. The international regulations stipulate that the thermocouple is divided into eight different graduations, which are B, R, S, K, N, E, J, and T. The lowest possible measurement temperature is Measured at minus 270 degrees Celsius, up to 1800 degrees Celsius, where B, R, and S belong to platinum series thermocouples. Since platinum is a precious metal, they are also called noble metal thermocouples and the remaining ones are called cheap metal thermoelectrics I. There are two types of thermocouples, ordinary and armored. Ordinary thermocouples are generally composed of thermal electrodes, insulating tubes, protective sleeves, and junction boxes. Armored thermocouples are assembled by combining the thermocouple wire, insulating material and metal protective sleeve. It is a solid combination made by drawing. However, the electrical signal of the thermocouple needs a special kind of wire to transmit. This kind of wire is called a compensation wire. Different thermocouples require different compensation wires. Their main function is to connect the thermocouple to keep the reference end of the thermocouple away from the power supply, so that the temperature of the reference end is stable. Compensation wires are divided into compensation type and extension type. The chemical composition of extension wires is the same as that of the thermocouple being compensated. However, in practice, extension wires are not made of the same material as thermocouples. Replace the wires with the same electron density. The connection between the compensation wire and the thermocouple is generally clear. The positive electrode of the thermocouple is connected to the red wire of the compensation wire, while the negative electrode is connected to the remaining color. Most of the materials of general compensation wires are copper-nickel alloys.