The basic principle of thermocouple temperature measurement
Conductors or semiconductors A and B of two different materials are welded together 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, and thus the loop A current of one size is formed, which is called a thermoelectric effect. Thermocouples use this effect to work. What is a thermocouple thermocouple and thermal resistance are contact temperature measurement in temperature measurement, although the same effect is to measure the temperature of the object, but their principles and characteristics are not the same. Thermocouple is the most used in temperature measurement A wide range of temperature devices, his main features are wide range of kisses, stable performance, simple structure, good dynamic response, and can remotely transmit 4-20mA electrical signals, facilitating automatic control and centralized control. The thermometry principle of thermocouples is based on the thermoelectric effect. Connecting two different conductors or semiconductors into a closed loop, when the temperatures at the two junctions are 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 the closed loop consists of two potentials: the temperature difference potential and the contact potential. The thermoelectric potential refers to the potential generated by the temperature difference between the two ends of the same conductor. Different conductors have different electron densities, so the potentials they generate are different. The contact potential, as the name implies, refers to the contact of two different conductors. Because of their different electron densities, they produce a certain amount of electron diffusion. When they reach a certain equilibrium, the potential of the contact potential depends on the material properties of the two different conductors and the temperature of their contact points. At present, the internationally applied thermocouple has a standard specification. The international thermocouple is divided into eight different scales, namely B, R, S, K, N, E, J and T, which can measure the lowest temperature. Measuring 270 degrees Celsius, up to 1800 degrees Celsius, where B, R, S belong to the platinum series of thermocouples, because platinum is a precious metal, so they are also called precious metal thermocouples and the remaining few are called cheap metal thermoelectric I. There are two types of thermocouples, normal and armored. Ordinary thermocouples are generally composed of hot electrodes, insulating tubes, protective sleeves and junction boxes, while armored thermocouples are assembled by combining thermocouple wires, insulating materials and metal protective sleeves. Stretched into a solid combination. However, the electrical signal of the thermocouple requires a special wire for transmission. This wire is called a compensation wire. Different thermocouples require different compensation wires, the main function of which is to connect with the thermocouple, so that the reference end of the thermocouple is far away from the power supply, so that the temperature of the reference junction is stable. The compensation wire is 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 type wire is not made of the same material as the thermocouple, and is generally used and thermoelectric. Instead of wires with the same electron density. The connection of the compensation wire to the thermocouple is generally clear. The positive terminal of the thermocouple is connected to the red line of the compensation wire, and the negative electrode is connected to the remaining color. Most of the materials for the compensation wires are made of copper-nickel alloy.
R type thermocouple: platinum rhodium 13-platinum thermocouple
R type thermocouple specifications: for precious metal thermocouples. The diameter of the filament is specified to be 0.5mm, the allowable deviation is -0.015mm, and the nominal chemical composition of the positive electrode (RP) is platinum-rhodium alloy, which contains 13% bismuth, 87% platinum, and pure platinum (RN). The maximum operating temperature is 1300 ° C, and the short-term maximum operating temperature is 1600 ° C.
Advantages of R-type thermocouple: It has the highest accuracy, the best stability, wide temperature range and long service life. It has good physical and chemical properties, good thermoelectric potential stability and high oxidation resistance at high temperatures, and is suitable for oxidizing and inert atmospheres. Since the comprehensive performance of the R-type thermocouple is equivalent to that of the S-type thermocouple, it has been difficult to promote in China. In addition to the application of temperature measurement on imported equipment, domestic temperature measurement is rarely used. Between 1967 and 1971, a collaborative study between the UK NPL, the US NBS and the Canadian NRC research institutes showed that the stability and reproducibility of the R-type thermocouples are better than those of the S-type thermocouples. Research in this area has not yet been carried out.
R type thermocouple is insufficient: it is a thermoelectric potential, the thermoelectric potential rate is small, the sensitivity is low, the mechanical strength is lowered at high temperature, it is very sensitive to pollution, and the precious metal material is expensive, so the one-time investment is large. There are very few manufacturers to reuse.