Thermocouple Trouble Shooting
When a thermocouple is suspected of giving incorrect readings, the following steps may be taken to isolate the source of error.
- Circuit- The first step is to check the polarity of thermocouple circuit and all connections. The positive leg (wire) should be properly connected to the positive side of the instrument. The negative leg (wire) should be properly connected to the negative extension wire which should be connected to the negative side of the instrument. A check at these points will save delays in production.
- Instrument – If the circuit checks out, the next step is to check the instrument. If a millivolt type meter is used, it should first be checked as to room temperature setting (cold junction compensation). This is done by removing one of the extension wires, either position or negative, from the meter and observing the meter reading. It should coincide with the room temperature.
If further testing is required, or if the instrument in question is a potentiometer, the working pyrometer set up should be checked by comparing its reading against those obtained with a test thermocouple of known accuracy. In making checks this way, it is important that the test thermocouple be inserted alongside the working thermocouple with the hot junction weld beads of bother as close as possible. It is also essential that the temperature of both the working and the test meter be the same.
If the test meter reading agrees with that indicated by the working meter the source of the trouble is not in the pyrometry circuit but the furnace itself. If the test meter reading does not agree with the working meter reading, the following checks should be made to isolate the trouble. - Thermocouple – Severely corroded or oxidized thermocouples are always a possibility of trouble. Changes in wire composition can result from corrosion and contamination by foreign elements. Impurities such as sulfur and iron plus other factors picked up from furnace refractories, oxide scale, brazing alloys and fluxes constitute possible sources of drift away from initial calibration.
To check the working thermocouple, hook it to the test meter of known accuracy and observe the reading. If the reading is the same as that previously obtained from the test thermocouple of known accuracy, then the working thermocouple is not the problem - Meter and Extension Wires – To check the working meter and extension wires, connect the extension wires to the test thermocouple of known accuracy and observe the temperature reading. If the reading is different from that obtained with the test meter, the trouble is either in the extension wires or in the working meter.
- Extension Wire Resistance – If the working meter is the millivolt type, it will have been calibrated for a certain external resistance. Accordingly, the extension wire loop should be checked to make sure that it meets the established external resistance required.
The above checks are intended only as elementary guides in trying to pinpoint the possible cause of erroneous readings. If the cause of erroneous readings can be definitely localized in the thermocouple itself, it should be removed and inspected. A visual inspection, plus a few tests that can be made with hand instruments, will often reveal the condition which caused the thermocouple wires to be out of calibration. Severely corroded or oxidized thermocouples should be replaced. It is usually more economical to replace the thermocouple element than to risk loss of productivity, product, or equipment through inaccurate temperature measurement.
