In temperature measurement, thermocouples and RTDs are both contact temperature measurements, and although they both measure the temperature of an object, their principles and characteristics are different.
This paper first introduces the thermocouple, which is a kind of temperature control device widely used in temperature measurement, its main characteristics are wide temperature measurement range, relatively stable performance, simple structure, good dynamic response, and can transmit 4-20 mA electrical signal remotely, which is convenient 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 together to form a closed loop, when the temperature at the two junctions is different, a thermoelectric potential is generated in the loop, a phenomenon called the thermoelectric effect, also known as the Seebeck effect. There are two types of thermopotentials generated in closed circuits: temperature difference potential and contact potential. Temperature difference potential refers to the potential generated by the two ends of the same conductor due to different temperatures, different conductors due to their different electron densities, so the potential produced is also different, the contact potential as the name suggests is that when two different conductors are in contact, due to their different electron densities and produce a certain electron diffusion, when they reach a certain equilibrium, the potential formed, the size of the contact potential depends on the properties of the two different conductor materials and the temperature of their contact points. Now there is a standard specification for thermocouples used in the world, and the thermocouples are divided into eight different degrees in the world, namely B, R, S, K, N, E, J and T, and the temperature they measure is as low as minus 270 degrees and 1800 degrees high, among which B, R, S belong to the platinum series of thermocouples, because platinum belongs to precious metals, so they are also called precious metal thermocouples, and the remaining few are called cheap metal thermocouples. There are two types of thermocouples: ordinary and armoured. The general thermocouple is composed of a thermode, an insulating tube, a protective sleeve and a junction box, while an armored thermocouple is a solid combination of a thermocouple, an insulating material and a metal protective sleeve assembled together and processed by stretching. However, the electromagnetic waves in thermocouples require a special wire to transmit them, which is called a compensation line. Various thermocouples require different compensation wires, and the main function of the compensation wires is to connect the thermocouple, so that the reference end of the thermocouple is far away from the power supply, so that the temperature of the reference end is stable. The compensation wire of the utility model is divided into two kinds of compensation type and extension type again, and the chemical composition of extension wire is the same as the thermocouple that is compensated, but the actual extension wire is not the same metal material as the thermocouple material, and is usually replaced by the wire that has the same electron density with the thermocouple. In general, the connection between the compensation wire and the thermocouple is very clear, the positive pole of the thermocouple is connected to the red wire of the compensation wire, and the negative pole is connected to the remaining color. Copper-nickel alloy is mostly used as the material of ordinary compensation wires.
Second, the introduction of RTD, although RTD is also widely used in industry, but its temperature measurement range is limited to a certain extent, the temperature measurement principle of RTD is based on the characteristics of the resistance value of conductors or semiconductors with temperature. It also has many advantages, it can also transmit electrical signals remotely, has high sensitivity, strong stability, good interchangeability and accuracy, but it requires power excitation and cannot measure temperature changes instantaneously. In industry, Pt100, Pt10, Cu50, Cu100 are generally used, and the temperature range of platinum RTD is minus 200-800 degrees Celsius, and copper RTD is between minus 40-140 degrees Celsius. Thermal resistance is the same type as a thermocouple, but it does not require wire compensation and is less expensive than a hot couple.
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