Determination of the graduation number and polarity of the compensation wire

Determination of the graduation number and polarity of the compensation wire

We know that the compensation wire only extends the reference end of the thermocouple to move the reference end position. The temperature of the extended reference end should be constant or equipped with a device that has automatic reference end temperature compensation. Otherwise, the measurement error may still be caused by the change of the new reference end temperature.

Determination of the graduation number and polarity of the compensation wire

The reliable and commonly used method is the test method, which is to strip the insulation layer of both ends of the compensation wire, twist the two wires together to make the hot end of the thermocouple, put it in boiling water, and connect the other ends of the two wires to the DC potentiometer (it should not be connected to the moving coil direct reading mV meter because the reading is low when the current is taken during measurement). Compare the measured thermoelectric potential with Table 1. The one that is closer to it is the graduation number of the compensation wire. The polarity of the compensation wire can be determined according to the positive and negative poles of the potentiometer. Since the reference end temperature of the thermocouple composed of the compensation wire is not necessarily 0℃ during the test, for example, it is 20℃, the measured thermoelectric potential is lower than the thermoelectric potential value of the reference end at 0℃. Take a compensation wire with unknown graduation number as an example. If the reference end temperature is about 20℃ and the measured value is within the range of 3.928±0.150mV, it can be judged that the graduation number of this compensation wire is K. 3.928 is the difference in thermoelectric potential of the thermocouple with graduation number K at 100℃ and 20℃, and 0.150 is the tolerance of the ordinary compensation wire with graduation number K.

Position of the connection point of the compensation wire instrument panel

For the early moving coil instruments equipped with thermocouples, there are two line resistance requirements of 5Ω and 15Ω. When the thermocouple is installed far away from the moving coil meter, or when using compensation wires containing copper-nickel materials such as graduation numbers K, N, E, J, T, etc., the line resistance is relatively large. When selecting, pay attention to selecting compensation wires with larger cross-sections. For example, when a moving coil instrument with an external 15 Ω line resistance and E graduation is selected, the cross-section of the compensation wire used is 1.0 mm2 and 2.5 mm2, and the corresponding unit length line resistance is 1.25Ω/m and 0.5Ω/m respectively. The maximum allowable length of the compensation wire is only 12 m and 30 m. If you are not careful during design, this length can easily be exceeded, causing measurement errors.

When the compensation wire of the thermocouple is introduced into the digital display, if it is connected to the terminal of the digital display, and the terminal of the digital display and the terminal of the instrument are connected with copper wire, the above-mentioned temperature difference will cause measurement errors. Therefore, it is best to connect the compensation wire directly to the terminal of the instrument across the terminal of the instrument panel. For example, when wiring in the digital display, the temperature at the terminal of the commonly used panel display and recorder itself is higher than the temperature at the terminal of the digital display because it is heated by power.