What is the correct measurement method for thermocouple calibration?

The thermocouple calibration regulation requires that the reference terminals of the standard thermocouple and the tested thermocouple be introduced into 0 ℃ during calibration. For precious metal thermocouples (S, R, B scale marks) used in calibration work, the wires of the tested thermocouples are soft and easy to wire; However, for low-cost metal thermocouples used in calibration work, due to the relatively large diameter of most thermocouples, it is difficult to directly introduce the reference end into 0 ℃. Therefore, compensation wires are often used to introduce the free end of the thermocouple into the freezing point, and then copper wires are used to lead it out to the voltage range of a high-precision digital multimeter for measurement.

The correct measurement method for thermocouple calibration process is shown in Figure 1:

Figure 1 Correct measurement method during thermocouple calibration process

1. Common incorrect connections of compensating wires

① It is believed that with compensation wires, the freezing point can be eliminated, as shown in Figure 2.

Figure 2 Error measurement method of not using freezing point during thermocouple calibration process

② It is believed that connecting compensation wires is better, not only between the thermocouple and the freezing point, but also between the freezing point 0 ℃ and the high-precision digital multimeter, as shown in Figure 3.

When checking the thermocouple in Figure 3, use a compensating wire between 0 ℃ and the digital function table

Below is an analysis of the errors generated by these two wiring forms (assuming no compensation errors in the compensating wires):

When connected correctly (see Figure 1), if the type of thermocouple is Y and the compensating wire is YX, applying the law of intermediate conductor, the voltage accepted by the high-precision digital multimeter can be obtained as:

EZ=EY (T1, T3)+EYX (T3, T0)=EY (T1, T0) ------- Formula 1

The temperature T1 at the working end of the thermocouple is exactly the thermoelectric potential relative to 0 ℃, and the reading can be directly compared with the scale to determine whether it meets the requirements.

For the situation in Figure 2, applying the law of intermediate conductor, the voltage accepted by the high-precision digital multimeter is:

EZ=EY (T1, T3)+EYX (T3, T2)=EY (T1, T2) ------- Formula 2

Compared with equation (1), the input environmental temperature T2 of the electrical measuring instrument is exactly different from the potential at 0 ℃:

EY (T1, T0) - EY (T1, T2)=EY (T2, T0) --------------- Formula 3

If the verification result is lower than the actual potential value of the thermocouple according to the situation in Figure 2, the difference is the potential of the thermocouple's ambient temperature relative to 0 ℃.

For the situation in Figure 3, applying the law of intermediate conductor, the voltage accepted by the high-precision digital multimeter is:

EZ=EY (T1, T3)+EYX (T3, T0) - EYX (T0, T2)=EY (T1, T0) - EYX (T0, T2) ------- Formula 4

From the results of the working hours, although the freezing point was added, the high-precision digital multimeter was compensated in reverse by connecting a compensation wire from the freezing point. The final result is the same as Figure 2, where the measured value is lower than the actual potential value of the thermocouple, and the difference is still the potential of the thermocouple's ambient temperature relative to 0 ℃.

2. Suggest using thermocouple instead of compensating wire

In thermocouple calibration, compensating wires are often used for the connecting wires of low-cost metal thermocouples, but the accuracy of thermocouple compensating wires is generally not high. According to relevant national standards, the tolerance of compensating wires for precision grade thermocouples is ± 2.5 ℃ for S-couples and ± 1.5 ℃ for K-couples. This introduces too much error for thermocouple calibration and may result in unqualified results for qualified thermocouples.

The purpose of using thermocouple compensation wires in calibration is to compensate the temperature T3 at the thermocouple output terminal to the required T0 (0 ℃) for calibration (as shown in Figure 1). According to practice, for the calibration of low-cost metal thermocouples, a working grade I Φ 0.5mm thermocouple wire connection is used, with a flexible wire diameter, easy wiring, and low cost. Grade I thermocouples, as compensation wires, must undergo calibration and introduce correction values, otherwise they will introduce errors of (0.4-1.5) ℃; For the calibration of precious metal thermocouples (especially S and R type thermocouples with a length of 200-700mm, which cannot be directly placed in the reference end thermostat due to their short wires), a Φ 0.3mm same polarity thermocouple wire (with a tolerance of ± 1 ℃) that meets the accuracy level of first-class standard thermocouples can be used to connect the thermocouple wires to the positive and negative poles of the tested thermocouple, and the effect is very ideal. The use of thermocouple wires for the verification of precious metal thermocouples is indeed ideal, but the cost is not low. Corresponding compensating wires can also be used, but they must be calibrated and only used after correction has practical significance.

In theory, according to the homogeneous conductor law of thermocouples, the thermoelectric potential of a homogeneous conductor is only related to the temperature at the junction of the two ends, and is independent of the temperature distribution along the hot electrode. So the thermocouple is directly connected with a thermocouple wire, and the two electrodes of the thermocouple extend directly from the thermocouple wire to the freezing point, without considering the intermediate temperature T3 (see Figure 1).

Using compensating wires for connection, it is required that the temperature of the two connection ends of the thermocouple be consistent at T3. However, in actual verification, the free end of the thermocouple is located at the end of the verification furnace, and the temperature at the end is very high. The influence of radiation and convective heat transfer on the surrounding area is significant, making it difficult to achieve consistent temperature at the free end of the verified thermocouple. So in the verification process, the error caused by inconsistent temperature at the ends is incorporated into the verification results of the thermocouple. By connecting with thermocouple wires, even if the temperature at the free end of the thermocouple is not consistent, it does not affect the calibration results.

3. Selection and use of copper wires

Introduce the free end of the thermocouple into the freezing point, and connect it to the measuring end of the high-precision digital multimeter using copper wires from the freezing point. Some thermocouple calibration regulations have such provisions, while others do not. According to the verification regulations of "Low cost Metal Thermocouples for Work", it is explicitly required that the lead wires of the thermocouple reference end should be connected using copper wires of the same material, and the contact should be good; Although the verification regulations for "Precious Metal Thermocouples for Work" stipulate in the schematic diagram of the verification principle that the reference end of the thermocouple needs to be connected to the conversion switch (directional switch), they do not specify what type of lead wire to use. Therefore, some personnel have added unnecessary details and used compensation wires (the errors caused have been analyzed above).

The requirements for the lead wires of the thermocouple reference terminal are quite strict. The wires connected to the two poles of the thermocouple must be cut into two sections using the same copper wire and connected to the reference terminals of the two poles of the thermocouple respectively (the resistivity of the copper wire at 20 ℃ should be less than 0.01724 μ Ω· m). Because they are not the same copper wire, although their resistivity meets the requirements, there are also differences between them. Therefore, the pair composed of two copper wires will generate additional thermoelectric potential in the thermocouple circuit under the effect of temperature difference. The magnitude of this electromotive force is related to the difference in material between the two copper wires and the temperature difference between the two ends of the copper wires, which will introduce certain errors into the thermocouple being tested.

Furthermore, the connection between the copper wire and the reference terminal of the thermocouple electrode must have good contact. It has been proven that the method of using a fish mouth to clamp is still widely adopted by many calibration personnel. However, this method will introduce an error of 0.5-1.0 ℃ in the detection circuit, and this error is not a fixed value, sometimes enough to affect the judgment of the calibration results.