Q&A on common problems with platinum rhodium thermocouples

Platinum rhodium thermocouple is a traditional temperature measuring element with stable thermoelectric performance and strong oxidation resistance, suitable for continuous use in oxidizing and inert atmospheres. The long-term usage temperature is 1600 ℃, and the short-term usage temperature is 1800 ℃.
1, How to identify faults generated by platinum rhodium thermocouple inputs?
1. According to the instrument wiring diagram, the wiring is correct. If the digital display on the upper row of the instrument shows negative values after the instrument is powered on, it indicates that the thermocouple "+" and "-" connected to the instrument are connected incorrectly. Just exchange it again.
2. The wiring is correct. When the instrument is running, the temperature displayed on the digital tube on the upper row of the instrument differs from the actual measured temperature by 30 º C to 60 º C. Even greater differences indicate that the calibration number of the instrument is incorrect compared to the calibration number of the thermocouple. According to the correspondence between the temperature (º C) and millivolt (MV) values of thermocouples with division numbers B, S, K, E, etc., under the same temperature (º C), the resulting millivolt value (MV) is determined by the principle that B division number is the smallest, S division number is the second smallest, K division number is the largest, and E division number is the largest.
3. After correctly wiring and powering on according to the instrument wiring diagram, the instrument first displays the thermocouple graduation number of the instrument, then displays the range of the instrument, and then measures the set temperature on the digital tube in the lower row of the instrument, and the measured temperature on the digital tube in the upper row of the instrument. If the digital display on the upper row of the instrument panel does not show the temperature of the heating element, but instead shows conditions such as "Over", "0000", or "000", it indicates a fault in the input part of the instrument. The following tests should be conducted:
a) Remove the thermocouple from the input end of the instrument thermocouple and short circuit the input end of the instrument thermocouple with any wire. When powered on, when the display value of the digital tube on the instrument panel is about room temperature, it indicates that the internal wiring of the thermocouple is open and should be replaced with a thermocouple of the same type. If the above situation persists, it indicates that the input terminal of the instrument has been damaged during transportation and needs to be replaced.
b) Remove the thermocouple from the faulty instrument mentioned above and replace it with a thermocouple connected to the same type of calibration instrument that is running normally next to it. After being powered on, when the digital tube on the upper row of the original faulty instrument displays the temperature of the heating, it indicates that the internal wiring of the thermocouple is open circuit. Replace it with a thermocouple of the same type. If the above situation persists, it indicates that the input terminal of the instrument has been damaged during transportation and needs to be replaced.

c) Remove the faulty thermocouple from the instrument, place a multimeter in the measurement ohm (R) * 1 range, and use two multimeter rods to measure both ends of the thermocouple. If the resistance value displayed on the multimeter is large, it indicates an open circuit in the internal connection of the thermocouple. Replace the thermocouple with a similar type. Otherwise, if there is a certain resistance value, it indicates that there is a problem with the input terminal of the instrument and the instrument should be replaced.
2, How to prevent interference during temperature measurement?
When using the reference terminal grounding method, one end of the output end of a platinum rhodium thermocouple (or compensation wire) is grounded through a sufficiently large capacitor (the larger the capacitor, the better if conditions permit). The measurement end grounding method is to ground the measurement end of a thermocouple, which is to introduce a metal wire from the measurement end of the thermocouple to ground. This method has a good preventive effect on high-temperature leakage interference. When selecting metal wires, they should be heat-resistant and harmless to thermocouple electrodes.
During the use of thermocouples, there are always many situations that interfere with our measurement accuracy, which is a very tricky problem. When using thermocouples, we need to be prepared to prevent interference in such situations. With the improvement of current industrial production level, the usage rate of thermocouples is very high, so the measurement accuracy of thermocouples is a very important thing to pay attention to!
3, What should be done when a thermocouple encounters interference during use?
Shielding method: Thread the compensating wire of the thermocouple into an iron tube or other metal shielding material for shielding. This can prevent electromagnetic interference and interference from high-voltage electric fields. When using this method, the iron pipe and shielding material should be well grounded, and the compensating wire should be twisted together.
Isolation method: It is to install the thermocouple in the air so that it does not come into contact with the refractory bricks on the furnace wall. Insulation is also used to isolate the thermocouple from the bracket. This method can effectively prevent high-temperature leakage interference.
Grounding method: This method involves grounding the measurement circuit to introduce interference into the ground and ensure the accuracy of the instrument's measurement. This method has two grounding forms: the first is the grounding of the thermocouple reference end, and the second is the grounding of the thermocouple measurement end.
Based on different temperature measurement environments and usage environments, the specific methods used for prevention are all aimed at minimizing interference and achieving the most ideal accuracy of temperature measurement. Therefore, in the process of prevention, it is important to pay attention to how to best deal with related interference.