How to determine if there is a fault in the thermocouple input

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After proper wiring according to the instrument’s wiring diagram and power is applied, the instrument first displays the thermocouple calibration number; then it shows the measurement range of the instrument. The digital display at the bottom of the instrument indicates the set temperature, while the digital display at the top shows the actual measured temperature. If the digital display on the instrument shows not the temperature of the heating element, but instead values such as “OVER”, “0000” or “000”, it indicates a fault in the input section of the instrument. The following tests should be carried out: 1) Remove the thermocouple from the instrument’s thermocouple input terminal, and then short-circuit that input terminal using any wire. When power is applied and the value displayed on the upper digital display of the instrument is approximately that of the room temperature, it indicates an open circuit in the internal wiring of the thermocouple; in such a case, a thermocouple of the same type should be replaced. If the situation remains as described above, it indicates that the input terminal of the instrument was damaged during transportation, and the instrument needs to be replaced. 　　2) Remove the thermocouple from the faulty instrument mentioned above and replace it with one from a nearby instrument of the same type that is working properly. After powering on the system, if the digital display on the faulty instrument shows the temperature of the heating element, it indicates that there is an open circuit in the thermocouple wiring; in such a case, replace the thermocouple with one of the same type. 　　3) Remove the faulty thermocouple from the instrument. Set the multimeter to the ohm (R) setting, and use its test leads to measure the two ends of the thermocouple. If a high resistance value is displayed on the multimeter, it indicates an open circuit within the thermocouple; in such case, replace it with a thermocouple of the same type. Otherwise, if there is a certain resistance value, it indicates a problem with the instrument’s input terminal, and the instrument should be replaced. 　　4) Connect the wires according to the instrument wiring diagram correctly. If, after the instrument is powered on, negative values appear on the upper digital display of the instrument, it is due to the wrong connection of the “+” and “—” terminals of the thermocouple connected to the instrument. Just swap them around again. 　　5) When the wiring is correct and the instrument is in operation, the temperature displayed on the digital display of the instrument differs from the actually measured temperature by 40 degrees to 70 degrees. The difference is even greater, indicating that the scale number of the gauge and that of the thermocouple have been mixed up. Looking at the relationship between the temperature of thermocouples of types B, S, K, E, etc. and their corresponding millivolt (MV) values, at the same temperature, the millivolt value generated is lowest for thermocouples of type B, slightly higher for type S, higher still for type K, and highest for type E. Judgments can be made based on this principle.