The principle and function of thermocouple compensation wire

Principle of thermocouple compensation wire
A few days ago, a customer called to inquire and reported that the thermocouple temperature measurement was inaccurate. After analysis by an engineer from Shaoxing Zhongyi, it was found that the customer's factory used K-type thermocouples for temperature measurement, but the wire connecting the thermocouple to the ZYW330RG forty channel paperless recorder was made of ordinary 2-core copper wire. Later, the customer replaced it with K-type thermocouple compensation wire, and the fault was resolved, resulting in accurate temperature measurement. In fact, engineers from Shaoxing Zhongyi Electronics Co., Ltd. found on the construction site that many customers had many issues due to incorrect use of thermocouple compensation wires, such as inaccurate temperature measurements or unstable fluctuations.

Here, the editor will analyze the principle, function, and usage of thermocouple compensation wires for you.

The difference between thermocouple compensation wire and general wire
Thermocouple compensation wires are used for extending thermocouples, i.e. for transmitting thermocouple signals, to provide signal compensation and ensure accurate temperature measurement. Generally, wires can only serve the purpose of transmitting voltage and current.

Thermocouples have become the most widely used temperature sensors in industrial fields due to their low cost, wide temperature measurement range (especially for high temperature measurements, such as K thermocouples that can measure temperatures above 1000), and diverse choices. Thermocouples and corresponding compensation wires, control instruments, PLC systems, or DCS systems constitute thermocouple temperature measurement systems. After using compensation wires in the thermocouple circuit, the thermocouple potential value is only related to the temperature at the measurement end and the temperature at the connection between the compensation wire and the instrument. The function of thermocouple compensation wires is to extend the thermoelectric electrode (i.e., move the thermocouple reference end) and save high cost thermocouple materials.

Schematic diagram of a simple thermocouple temperature measurement system

Thermocouple compensation wire is a wire that has a nominal value of thermoelectric potential in communication with the matched thermocouple within a certain temperature range. The following is a brief list of common cases of incorrect use of compensating wires in industrial sites, hoping that everyone will gain some insights after reading.

By first understanding the principle, function, method of action, and details of thermocouple compensation wires, we can fully utilize their function. Otherwise, it will backfire. To use thermocouple compensation wires correctly, practitioners need to understand the working principles of thermocouples and compensation wires, and have a strong sense of responsibility and a serious and down-to-earth work attitude. In industrial production, although thermocouples have been widely used as temperature sensors for temperature measurement and control, people are also familiar with this. However, if the correct usage method is not used in detail, it will cause significant deviation in temperature measurement and control, and in severe cases, it will directly cause economic losses. Therefore, it should be taken seriously in order to avoid the above-mentioned errors and ensure the accuracy of thermocouple temperature measurement, The above introduces the incorrect use of thermocouple compensation wires. Below are some details to be taken into account during use.

The function of thermocouple compensation wire
If a thermocouple uses ordinary wires instead of compensating wires, unexpected situations may occur.

1. Using ordinary two core wire as thermocouple signal extension wire, without compensating wire

For example, in a heat treatment factory in Paojiang, Zhejiang, when using thermocouples for temperature measurement, the thermocouple signal was directly connected to the control room instrument through a two core copper wire. During use, there were frequent occurrences of non-conforming heat-treated workpieces. Later, a technician from Shaoxing Zhongyi inspected the site and found that the reason for the defective workpieces was due to quenching temperature deviation. The inaccurate measurement of quenching temperature was due to the use of ordinary wires instead of compensating wires with the corresponding scale marks in the thermocouple temperature measurement system, resulting in inaccurate temperature measurement.

According to the principle of thermocouple temperature measurement, the thermoelectric potential of the thermocouple circuit is related to the measured temperature and the temperature of the thermocouple reference terminal. The temperature of the thermocouple reference terminal (referring to the temperature at the thermocouple junction box) installed on the site of use varies with the ambient temperature and cannot be constant. In the case of temperature fluctuations at the reference end of the thermocouple, compensation wires are used to extend the reference end to an environment with relatively stable temperature or away from the heat source to compensate for the errors caused by temperature changes at the reference end of the thermocouple. Ordinary wires can transmit the mV signal value generated during thermocouple temperature measurement, but cannot compensate for extending the reference temperature of the thermocouple to the instrument control room, resulting in inaccurate temperature compensation in the thermocouple temperature measurement system. Correct method: Thermocouple signal transmission must use compensating wires of the corresponding model of thermocouple, and it is prohibited to use ordinary cables instead of compensating wires.

2. Mixing thermocouples with different division numbers and compensating wires introduces measurement errors

A certain unit used S-type thermocouples to measure furnace temperature. Although the on-site staff knew that thermocouples must use compensation wires, they did not choose the correct and suitable compensation wires. Instead, they used K-type thermocouple compensation wires in stock to connect the platinum rhodium 10 platinum thermocouple signal to the instrument. During use, they found that the actual furnace temperature deviated greatly from the measured value. They called Shaoxing Zhongyi, and after technical analysis by Shaoxing Zhongyi Company, the compensation wires were replaced with SC. The temperature measurement returned to normal.

According to the regulations of the State Administration for Quality Supervision, Inspection and Quarantine, the thermoelectric potential and allowable error of thermocouple compensation wires should comply with the provisions of JJG 351-1996 Verification Regulations for Low Cost Metal Thermocouples for Work and relevant standards. The thermoelectric potential generated by thermocouple compensation wires corresponding to different division numbers is different at the same ambient temperature. Mixing thermocouples with different division numbers and thermocouple compensation wires will inevitably introduce thermocouple reference temperature compensation errors into the thermocouple measurement system. Correct method: All kinds of thermocouple compensation wires must be matched with thermocouples with corresponding scale marks.

Thermocouple and compensation wire types do not match

3. The insulation layer of the thermocouple compensation wire is damaged

During the wiring and installation of thermocouples, occasional wear of the insulation layer may occur at the outlet of the thermocouple junction box and other parts of the compensating wire. The fault phenomenon is manifested as the temperature readings of the instrument or DCS system being generally lower.

Correct method: Find the damaged insulation layer of the thermocouple compensation wire, perform insulation treatment again, and restore the normal reflection value of the control instrument or paperless recorder.

4. The positive and negative polarities of the thermocouple compensation wire are reversed, which introduces measurement errors

After replacing the on-site thermocouple, a certain chemical plant experienced a significant deviation between the temperature measurement value and the actual temperature, which was unintentionally high or low. After careful inspection, it was found that the polarity of the thermocouple and the compensation wire was reversed. After adjusting the compensation wire according to the polarity, the fault was eliminated.

There is also a newly introduced spheroidization furnace device in a steel pipe production enterprise. Due to the installation personnel reversing the positive and negative poles of thermocouples and the lack of knowledge among the equipment users, more than 20 temperature measurement points of the device were scrapped in the furnace due to incorrect furnace temperature during work, resulting in direct economic losses of over one million yuan. The lessons learned are profound.

Wiring method of thermocouple compensation wire

Thermocouples and thermocouple compensation wires both have positive and negative poles. When the compensation wire is reversed, there is a significant change in the displayed value of the paperless temperature recording instrument

A, After reversing the polarity of the compensation wire, when the temperature at the connection between the thermocouple and the compensation wire is higher than the temperature in the control room, the instrument reflects a temperature lower than the actual measured temperature.

B, After reversing the polarity of the compensation wire, when the temperature at the connection between the thermocouple and the compensation wire is lower than the temperature in the control room, the instrument reflects a temperature higher than the actual measured temperature.

C, After reversing the polarity of the compensation wire, when the temperature at the connection between the thermocouple and the compensation wire is in communication with the temperature in the control room, the instrument reflects the temperature in communication with the actual temperature.

It has been theoretically proven that the error caused by reversing the polarity of the thermocouple compensation wire during use is about twice that of not using the compensation wire.

The positive insulation layer of compensating wires for different models of thermocouples is red in color, while the negative insulation layer is different in color. The compensating wire model can be distinguished based on the insulation layer color.