Precautions for the Thermocouple Cold Junction Temperature Compensation Wire Method

In order to keep the cold end temperature of the thermocouple constant, the thermocouple can be made very long, the cold end is far away from the working end, and placed together with the measuring instrument in a constant temperature or a place with relatively small temperature fluctuation. However, this method makes installation and use inconvenient, and may consume a lot of precious metal materials. Therefore, a connecting wire called a compensation wire is generally used to extend the cold end of the thermocouple. This wire has the same thermoelectric properties as the connected thermocouple within a certain temperature range (0-150℃). If the thermocouple is made of cheap metal, its own material can be used as a compensation wire to extend the cold end to a place with a constant temperature.

Precautions for using compensation wire:

(1) Selection of compensation wire

The compensation wire must be correctly selected according to the type of thermocouple used and the occasion of use. For example, a k-type thermocouple should choose a k-type thermocouple compensation wire, and select the working temperature range according to the occasion of use. Usually the kx working temperature is -20-100℃, and the wide range is -25-200℃. The error of the ordinary grade is ±2.5℃, and the precision grade is ±1.5℃.

(2) Contact connection

The two contacts of the thermocouple terminal should be as close as possible, and the temperature of the two contacts should be kept consistent. The temperature of the connection with the instrument terminal should be kept consistent as much as possible. Where there is a fan in the instrument cabinet, the contacts should be protected so that the fan does not blow directly onto the contacts.

(3) Use length

Because the signal of the thermocouple is very low, at the microvolt level, if the distance used is too long, the attenuation of the signal and the interference of strong electricity in the environment can cause the signal of the thermocouple to be distorted, resulting in inaccurate measurement and control of temperature. In serious cases, temperature fluctuations will occur during control.

According to our experience, it is usually better to control the length of the thermocouple compensation wire within 15 meters. If it exceeds 15 meters, it is recommended to use a temperature transmitter to transmit the signal. The temperature transmitter converts the potential value corresponding to the temperature into a DC current for transmission, and has strong anti-interference.

(4) Wiring

The wiring of the compensation wire must be kept away from power lines and interference sources. In places where crossing is unavoidable, cross-type wiring should be used as much as possible, and parallel wiring should not be used.

(5) Shielded compensation wire

To improve the anti-interference performance of the thermocouple connection wire, a shielded compensation wire can be used. This is more effective for situations where there are many interference sources on site. However, the shielding layer must be strictly grounded, otherwise the shielding layer will not only fail to play a shielding role, but will increase interference.