Currently, thermocouples used internationally adhere to a standard specification. Internationally, thermocouples are categorized into eight different types: B, R, S, K, N, E, J, and T. Their temperature measurement range extends from as low as -270 degrees Celsius to as high as 1800 degrees Celsius. The B, R, and S types belong to the platinum series of thermocouples, which are also known as precious metal thermocouples due to platinum being a precious metal. The remaining types are referred to as inexpensive metal thermocouples.

There are two types of thermocouple structures: standard and armored.

Standard thermocouples are typically composed of thermoelectric wires, insulating tubes, protective sheaths, and junction boxes. In contrast, armored thermocouples are a robust assembly created by combining thermocouple wires, insulating materials, and metallic protective sheaths, which are then processed through stretching. However, the electrical signal of thermocouples requires a special type of wire for transmission, which we refer to as compensating wires.

Different thermocouples require different compensation wires, which primarily serve to connect with the thermocouples, keeping the reference junction away from the power source, thereby stabilizing the reference junction temperature.

Compensation wires are further categorized into two types: compensation and extension.

The chemical composition of the extension wire is identical to that of the thermocouple; however, in practice, the extended wire is not made from the same metal as the thermocouple. Generally, a wire with the same electron density as the thermocouple is used instead. The connection between the compensating wire and the thermocouple is usually clear: the positive pole of the thermocouple connects to the red wire of the compensating wire, while the negative pole is connected to the remaining color.

Most compensation conductors are made of copper-nickel alloy.

Thermocouples are widely used temperature measuring devices in various applications. Their main features include a broad temperature measurement range, stable performance, simple structure, and excellent dynamic response. Additionally, when paired with a transmitter, they can remotely transmit 4-20mA current signals, facilitating automatic and centralized control.

The principle of thermocouple temperature measurement is based on the thermoelectric effect. By connecting two different conductors or semiconductors into a closed loop, when the temperatures at the two junction points are different, a thermoelectric potential is generated in the loop. This phenomenon is known as the thermoelectric effect, also referred to as the Seebeck effect. The thermoelectric potential produced in the closed loop consists of two potentials: the thermoelectric potential and the contact potential.

Resistance thermometers, though widely used in industry, are limited by their temperature measurement range. The principle of resistance thermometers is based on the characteristic that the resistance of conductors or semiconductors changes with temperature. They offer numerous advantages, including the ability to transmit electrical signals remotely, high sensitivity, strong stability, good interchangeability, and accuracy. However, they require power supply and cannot measure temperature changes instantaneously.

Industrial thermistors used for measuring low temperatures do not require compensation wires and are also more affordable.