The thermocouple compensation wire is a pair of metal wires with different chemical compositions, which have the same temperature-thermoelectric potential relationship with the thermocouple it is connected to within a certain temperature range. The thermocouple and the secondary instrument are connected by a compensation wire. If the polarity is connected correctly, it is equivalent to extending the hot electrode, so that the cold end of the thermocouple is extended to a lower temperature (the ideal temperature is 0°C) and stable Occasions, in order to perform cold junction temperature compensation, so as to achieve the purpose of accurate temperature measurement. The price of the compensation wire is much cheaper than the corresponding hot electrode.
The following introduces the related issues of whether the compensation wire and the thermocouple match.
1. Compensation wire and thermocouple reverse connection
If the positive and negative poles of the compensation wire are connected to the positive and negative poles of the thermocouple, and the positive and negative poles of the thermocouple are connected to the positive pole of the meter, it is correct. Take the K-type couple as an example, see the figure below. This kind of error is more common in applications, because after connection, the temperature change trend of the controlled object is consistent with the display instrument. In addition, many current thermocouple compensation wire products are not standardized and difficult to identify; some manufacturers even mark the color incorrectly. The following analyzes the error caused by this situation.
The temperature near the general industrial furnace is at least 8°C higher than the temperature in the control room. Then the resulting error is exactly twice the compensation value of the compensation wire. For the K-type couple, the differential potential value is basically around 40°C/(μV), and the measured temperature is about 16°C lower than the actual temperature. If the control temperature is set at 600°C, the actual temperature should be around 616°C.
From the above analysis, it can be seen that when the positive and negative poles of the thermocouple compensation wire are connected reversely, not only does it not have a compensation effect, but the error is doubled compared with not connecting the compensation wire. Therefore, the polarity of the compensation wire must be paid attention to when connecting.
If you cannot determine the polarity of the thermocouple compensation wire, you can take a section of the compensation wire, remove the insulation at one end, screw it together, and place it in a hot water cup. Use an ordinary multimeter to measure the 2 wires at the other end with the lowest DC voltage range. The positive and negative of the measured voltage will be displayed on the top, and the positive of the signal is the positive of the compensation wire.
2, the selection of compensation wire does not meet the requirements of thermocouple
Some temperature measurement sites use nickel-chromium-nickel-silicon thermocouples, but they are equipped with nickel-chromium-copper or platinum rhodium-platinum thermocouple compensation wires. None of this can exert its correct compensation function. The former leads to over-compensation and the actual furnace temperature is lower than the indicated value of the instrument, while the latter leads to under-compensation and the actual furnace temperature is higher than the indicated value of the instrument. Suppose the temperature tn at the thermocouple junction box is 100℃, and the temperature t0 at the terminal of the compensation wire and the meter is 0℃. In this case, the thermocouple required
The compensation potential is eab(100,0)=4.095mv, the compensation potential of the former compensation wire should be ea′b′(100,0)=6.317mv, then the overcompensation is 2.222mv, and the compensation potential of the latter is ea′b′ (100, 0)=0.645mv, the undercompensation is 3.450mv, the resulting compensation difference is quite large.
4. The temperature of the thermocouple junction box is too high
Some salt-bath furnaces use straight thermocouple junction boxes, and the temperature is as high as 200℃. Although compensation wires are also used, they have exceeded the specified temperature range. The compensation effect is seriously affected. The resulting compensation difference is difficult to estimate. The method of on-site proofreading can be used for comparison. In this case, the right-angle thermocouple needs to be replaced, and the junction box can be protected from the direct heat radiation of the salt bath liquid surface.
①Selection of compensation wire
The compensation wire must be selected correctly according to the type of thermocouple used and the occasion used. For example, K-type couple should choose the compensation wire of K-type couple, and choose the operating temperature range according to the application. Usually the working temperature of KX is (-20~100℃, and the wide range is (25200℃. The error of ordinary grade is ±25℃, and the precision grade is ±15℃.
The two contacts of the thermocouple wiring station should be as close as possible to keep the temperature of the contacts as consistent as possible. The connection point with the instrument terminal should be as warm as possible. Where there is a fan in the instrument cabinet, the contact point should be protected from blowing the fan directly to the contact point.
Because the signal of the thermocouple is very low, it is μV. If the distance used is too long, the signal attenuation will be coupled with the interference of the strong electricity in the environment, which can cause the signal of the thermocouple to be distorted, resulting in inaccurate temperature measurement and control. Will produce temperature fluctuations.
It is usually better to control the length of thermocouple compensation wire within 15m. If it exceeds 15m, it is recommended to use a temperature transmitter to transmit the signal. The temperature transmitter converts the electric heating value corresponding to the temperature into a direct current for transmission, which has strong anti-interference.
Compensation wire wiring must be far away from power lines and interference sources. Where crossing is unavoidable, crossovers should be used as much as possible instead of leveling.
⑤Shield compensation wire
In order to improve the anti-interference of the thermocouple connecting wire, shielded compensation wire can be used. For occasions with many on-site interference sources, the effect is better. However, the shielding layer must be strictly grounded, otherwise the shielding layer will not only not play a shielding role, but will increase interference.
At present, the thermocouple is a temperature measurement device widely used in industrial temperature measurement, but it must be equipped with corresponding compensation wires, and the positive and negative poles must be connected correctly, so that the expected compensation effect can be obtained. Otherwise, the gain will not be worth the loss, which will cause An irreparable loss. In addition, the application has a long enough compensation wire.