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What is the difference between thermocouple and thermal resistance?


At present, the thermocouples used internationally have a standard specification. The international regulations stipulate that thermocouples are divided into eight different divisions, namely B, R, S, K, N, E, J and T, and the measured temperature is lower. It can measure minus 270 degrees Celsius and up to 1800 degrees Celsius. Among them, B, R, and S belong to the platinum series of thermocouples. Since platinum is a precious metal, they are also called precious metal thermocouples and the remaining ones are called cheap Metal thermocouple.

There are two types of thermocouples, common type and armored type.

Ordinary thermocouples are generally composed of thermode, insulating tube, protective sleeve and junction box, while armored thermocouple is a combination of thermocouple wire, insulating material and metal protective sleeve. A solid combination formed by stretching. But the electrical signal of the thermocouple needs a special wire to transmit, this kind of wire is called compensation wire.
Different thermocouples require different compensating wires, and their main function is to connect with 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 two types: compensation type and extension type
The chemical composition of the extension wire is the same as that of the thermocouple being compensated, but in practice, the extension wire is not made of the same material as the thermocouple. Generally, it is replaced by a wire with the same electron density as the thermocouple. The connection between the compensation wire and the thermocouple is generally 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.

Most of the general compensation wires are made of copper-nickel alloy.
Thermocouple is the most widely used temperature device in temperature measurement. Its main characteristics are wide temperature measurement range, relatively stable performance, simple structure, good dynamic response, and the conversion transmitter can transmit 4-20mA current signals remotely. , It is convenient for automatic control and centralized control.

The principle of thermocouple temperature measurement 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, 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 is composed of two kinds of electric potentials; the temperature difference electric potential and the contact electric potential.

Although thermal resistance is also widely used in industry, its application is limited due to its temperature measurement range. The temperature measurement principle of thermal resistance is based on the resistance value of conductor or semiconductor changing with temperature. characteristic. It also has many advantages. It can also transmit electrical signals remotely. It has high sensitivity, strong stability, interchangeability and accuracy. However, it needs power supply and cannot instantaneously measure temperature changes.

The temperature measured by the thermal resistance used in industry is relatively low, and the temperature measurement does not require a compensation wire, and the price is relatively cheap.