Universal Thermocouple Cold Junction Compensation Method

A pair of wires with an insulating layer that have the same nominal value of the thermoelectric electromotive force as the matched thermocouple within a certain temperature range (including room temperature) are used to connect the thermocouple and the measuring device to compensate for the error caused by the temperature change at the connection between them and the thermocouple.
Notes on compensation wires:
1. Selection of compensation wires
The compensation wire must be correctly selected according to the type of thermocouple used and the occasion where it is used. For example, a k-type couple should choose a compensation wire of a k-type couple, 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. Connection point 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 as much as possible. 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 contact should be protected so that the fan does not blow directly to the contact.
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 coupling of strong electricity in the environment can distort the signal of the thermocouple, causing inaccurate measurement and control of temperature, and temperature fluctuations in serious 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 away from the power line and the interference source. In places where crossing is unavoidable, cross-type methods should be used as much as possible, not parallel.
5. Shielded compensation wire
In order to improve the anti-interference of the thermocouple connection line, shielded compensation wire can be used. For occasions with more interference sources on site, the effect is better. However, the shielding layer must be strictly grounded, otherwise the shielding layer will not only fail to play a shielding role, but will enhance the interference.
The material of the compensation wire has excellent acid, alkali, wear resistance and non-flammable properties, and can be immersed in oil and water for long-term use. The operating temperature is -60～205～260℃, model and name:
Compensation wire models are divided into: SC, KC, KX, EX, JX, TX according to product types. Among them:
a) The first letter of the model corresponds to the thermocouple graduation number;
b) The letter "X" indicates the extended compensation wire (type)
c) The letter "C" indicates the compensation compensation wire (type)
Note: SC type compensation wire can be equipped with R type thermocouple graduation number.
Compensation cables are divided into: SC BC KC KX JX TX EA according to product types, among which the letter "×" indicates the extended compensation wire, and the letter
"C" indicates that the thermoelectric potential and resistance value of the compensation cable meet the requirements of the following table:
1. The insulation resistance of the compensation cable moisture-proof test is not less than (10 meters) 25MΩ after 24 hours in 40℃ water.
2. After 24 hours of heat aging, the compensation cable is tested for 5 times the outer diameter, and it is not broken down after the voltage test of 5000V/1min.
The function of the compensation wire is to extend the cold end of the thermocouple, that is, the moving thermocouple, and connect it with the display instrument to form a temperature measurement system. The equivalent national standard IEC 584-3 "Thermocouple Part 3 - Compensation Wire" is adopted. The product is mainly used in various temperature measurement devices and has been widely used in nuclear power, petroleum, chemical industry, metallurgy, power and other departments.
When selecting thermocouple compensation wires, you must know the ambient temperature and on-site industrial and mining conditions of the thermocouple compensation wires. According to the on-site ambient temperature, select the appropriate compensation wire sheath. Generally, when the ambient temperature is -25 to 105℃, select polyvinyl fluoride sheath, when the ambient temperature is -60 to 205℃, select polytetrafluoroethylene as the sheath of the compensation wire, and when the ambient temperature is -60 to 260℃, select polytetrafluoroethylene as the sheath of the thermocouple compensation wire. Therefore, you must pay attention to the on-site industrial and mining conditions when selecting.
Thermocouple compensation wire temperature measurement principle:
The function of the thermocouple compensation wire is to extend the cold end of the thermocouple, that is, the moving thermocouple, and connect it with the display instrument to form a temperature measurement system. The product is mainly used in various temperature measuring devices and has been widely used in petroleum, chemical, metallurgy, electric power and other departments.
Generally speaking, the thermocouple may be tens of meters away from the temperature meter, and the temperature of the cold end (outlet end) of the thermocouple is different from the ambient temperature of the temperature meter (even up to tens of degrees).
If ordinary copper wire is used, according to the principle of thermocouple, the connection point will generate a temperature difference potential, which will cause measurement errors.
The voltage drop problem of long-distance transmission wires, because the input impedance of the temperature meter is high, the temperature difference potential (millivolt level) generated by the thermocouple and the transmission current (micro-A level) are very small, and the voltage drop loss on the wire is very small. Under normal circumstances, it is within the error range. Therefore, there is a thermocouple transmitter, which inputs the thermocouple signal and outputs 4-20ma, so that the compensation wire can be omitted and long-distance transmission can be performed.
If a compensation wire is used (it must match the thermocouple graduation number), the metal material selected can generate the smallest possible temperature difference potential at the connection point and minimize the temperature measurement error. In other words, move the cold end of the thermocouple to the temperature meter.