Thermocouples used internationally have a standard specification. The international standard divides thermocouples into eight different calibrations: B, R, S, K, N, E, J, and T. Their measurable temperature ranges from a minimum of minus 270 degrees Celsius to a maximum of 1800 degrees Celsius. Among them, B, R, and S belong to the platinum series thermocouples. Since platinum is a precious metal, they are also called precious metal thermocouples, while the remaining ones are called base metal thermocouples. There are two types of thermocouple structures: ordinary type and sheathed type. Ordinary thermocouples generally consist of thermoelectrodes, insulating tubes, protective tubes, and junction boxes, while sheathed thermocouples are a solid combination formed by assembling thermocouple wires, insulating materials, and metal protective tubes together and then drawing them. However, the electrical signal of a thermocouple requires a special wire for transmission, which we call compensation wire. Different thermocouples require different compensation wires. Their main function is to connect with the thermocouple, moving the reference end of the thermocouple away from the power source, thereby stabilizing the reference end temperature. Compensation wires are divided into two types: compensating type and extension type. The chemical composition of extension wires is the same as that of the compensated thermocouple, but in practice, extension wires are not made of the same metal as the thermocouple; they are generally replaced by wires with the same electron density as the thermocouple. The connection between the compensation wire and the thermocouple is usually clear: the positive pole of the thermocouple connects to the red wire of the compensation wire, while the negative pole connects to the remaining color. Most compensation wires are made of copper-nickel alloy.
Sheathed thermocouples, like industrial assembled thermocouples, are used as temperature sensors and are typically used in conjunction with display instruments, recording instruments, and electronic regulators. They can also serve as temperature-sensing elements for assembled thermocouples. They can directly measure the temperature of liquids, steam, other gas media, and solid surfaces within the range of 0°C to 800°C in various production processes. Compared with assembled thermocouples, sheathed thermocouples have the advantages of being bendable, high-pressure resistant, having a short thermal response time, and being robust and durable.
Since the materials of sheathed thermocouples are generally relatively expensive (especially when using precious metals), and the distance from the temperature measurement point to the instrument is often long, to save sheathed thermocouple materials and reduce costs, compensation wires are usually used to extend the cold end (free end) of the sheathed thermocouple to a control room with a more stable temperature, connecting it to the instrument terminals. It must be noted that the function of the thermocouple compensation wire is only to extend the thermoelectrode, moving the cold end of the sheathed thermocouple to the instrument terminals in the control room. It itself cannot eliminate the influence of cold end temperature changes on temperature measurement and does not play a compensating role. Therefore, other correction methods must be used to compensate for the influence of cold end temperature t0≠0°C on temperature measurement. When using sheathed thermocouple compensation wires, care must be taken to match the model, ensure the polarity is not reversed, and the temperature at the connection end between the compensation wire and the sheathed thermocouple must not exceed 100°C.
