What should be noted in the application of thermocouple compensation wires?

When we use compensating wires with K division marks and thermocouples with N division marks, it will cause overcompensation and display a higher temperature; On the contrary, using a compensating wire with N division marks and a thermocouple with K division marks will result in undercompensation and lower display temperature.

2. Determination of compensation wire division number and polarity

Sometimes it can be judged based on the material, insulation layer, and sheath color of the compensating wire listed in the data, but due to differences in the regulations of new and old domestic standards and IEC standards, it is often difficult to accurately determine the division number and polarity of the compensating wire using this method.

The reliable and commonly used method is the testing method, which involves stripping the insulation layer from both ends of the compensation wire, twisting the two wires together to make the hot end of the thermocouple, and placing them in boiling water. The other end of the two wires is connected to a DC potentiometer (should not be connected to a moving coil direct reading mV meter, as the current reading is low during measurement). The measured thermoelectric potential is compared with Table 1, and the one that is close to it is the scale mark of the compensation wire. The polarity of the compensation wire can be determined based on the positive and negative poles of the potentiometer. Due to the fact that the reference temperature of the thermocouple composed of compensating wires during testing may not necessarily be 0 ℃, for example, 20 ℃, the measured thermoelectric potential is lower than the thermoelectric potential value at the reference end of 0 ℃. Taking a compensation wire with an unknown division number as an example, if the reference temperature is about 20 ℃ and the measured value is within the range of 3.928 ± 0.150mV, it can be determined that the division number of this compensation wire is K. 3.928 is the difference in thermoelectric potential between K scale thermocouple at 100 ℃ and 20 ℃, and 0.150 is the allowable deviation for K scale ordinary compensating wire.

3. Location of compensating wire instrument panel connection points

We know that compensating wires only extend the reference end of the thermocouple to move the position of the reference end. The extended reference end temperature should be constant or equipped with a device that automatically compensates for the reference end temperature, otherwise measurement errors may still occur due to changes in the temperature of the new reference end.

For example, when wiring inside the dashboard, the temperature at the wiring terminals of commonly used panel mounted displays and recorders is higher than that at the dashboard wiring terminals due to heating caused by power on. When the compensation wire of the thermocouple is introduced into the instrument panel, if it is connected to the wiring terminal of the instrument panel, and the wiring terminal of the instrument panel is connected to the wiring terminal of the instrument panel with copper wire, the temperature difference mentioned above will cause measurement errors. So the compensation wire is directly connected to the instrument panel's wiring terminal by crossing over the wiring terminal of the instrument panel.

4. Compensate for the line resistance of the wire

For early moving coil instruments equipped with thermocouples, there are two requirements for line resistance: 5 Ω and 15 Ω. When the installation location of the thermocouple is far away from the moving coil meter, or when using compensating wires containing copper nickel materials such as division marks K, N, E, J, T, etc., the line resistance is relatively high. When selecting, attention should be paid to choosing compensating wires with larger cross-sections. For example, when using a dynamic coil instrument with an external 15 Ω line resistance E scale, the compensation wire section used is 1.0 mm2 and 2.5 mm2, and the corresponding unit length line resistance is 1.25 Ω/m and 0.5 Ω/m, respectively. Therefore, the allowable length of the compensation wire is only 12 m and 30 m. If not careful during design, this length can easily exceed and cause measurement errors.

5. Compensation wires for R and S scale thermocouples

Thermocouples, also known as platinum rhodium platinum, have two calibration marks, R and S, representing platinum rhodium 13 platinum and platinum rhodium 10 platinum thermocouples, respectively. The former is less commonly used in China, but its thermoelectric potential is relatively high (at 1600 ℃, the thermoelectric potential of R and S calibration thermocouples are 18.849mV and 16.777 mV, respectively). At a low temperature of 100 ℃, the two are basically the same (the thermoelectric potential of R and S calibration marks are 0.647 mV and 0.646 mV, respectively), and there is a slight difference at 200 ℃ (the thermoelectric potential of R and S calibration marks are 1.467 mV and 1.441mV, respectively). Therefore, the compensation wires for R and S calibration marks are currently universal in the domestic market. If the compensation wire with S division mark commonly purchased in the market is used for the thermocouple with R division mark, there will be no error below 100 ℃. Even if the maximum temperature of the heat-resistant compensation wire is 200 ℃, when the hot end temperature of the thermocouple is 600 ℃, 1000 ℃, and 1300 ℃, the error caused is only 2.5 ℃, 2.2 ℃, and 2.0 ℃.

This can be considered as a special case in Section 1.

In commonly used thermocouples, the R and S scale compensation wires are correct, but from the perspective of temperature usage range, the error is very small within the range of 0-60 ℃, while the error is relatively large between 100-150 ℃. When the measurement error requirement is high, the temperature of the reference end must be kept below 100 ℃.

6. Comparison between compensation type and extension type compensation wires

The compensation wires for the K division mark include the compensation type KC compensation wire and the extension type KX compensation wire. The following performance comparison table 2 can be used as a reference for practical selection.

7. Double platinum rhodium thermocouples do not require compensation wires

As mentioned earlier, it is necessary to use compensation wires to compensate for the temperature at the reference end of the thermocouple. However, among commonly used thermocouples, the double platinum rhodium (platinum rhodium 30-platinum rhodium 6) thermocouple with division number B is an exception as it does not have a dedicated compensation wire. In other words, in practical applications, it is generally not necessary to use compensation wires.

Double platinum rhodium thermocouples are commonly used for temperature measurement in the range of 1300~1600 ℃ (platinum rhodium platinum thermocouples are usually used for temperatures ≤ 1300 ℃). The thermoelectric potential in the low-temperature range is surprisingly low, such as only 0.033mV at 100 ℃ and 0.178mV at 200 ℃. Compared with the average thermoelectric potential of 0.700mV per 100 ℃ in the entire temperature measurement range (0~1800 ℃), the difference is significant, so even without compensation, the error caused is very small. For example, when the hot end temperature is 1300 ℃ and 1600 ℃, such as when the reference end temperature t1=100 ℃, the error caused is ± 3.0 ℃, and when t1=120 ℃, the error caused is ± 5.0 ℃, both of which meet the requirement of ± 5 ℃ for using ordinary compensating wires. However, it is worth noting that when t1=200 ℃, it may cause an error of ± 16.3 ℃. Therefore, for dual platinum rhodium thermocouples, although compensation wires are usually not used, the limiting condition is that the reference terminal temperature t1 ≤ 120 ℃, otherwise it will cause significant errors.

In uncommon thermocouples, nickel cobalt nickel aluminum thermocouples have almost zero thermoelectric potential below 200 ℃ and do not require compensation wires, while nickel iron nickel copper thermocouples have negligible thermoelectric potential below 50 ℃ and do not require compensation wires within this temperature range.