Faults in liquid thermometers, bimetallic thermometers, and pressure thermometers are usually quite obvious; most problems can be identified through observation, allowing for targeted replacement or repair. The mechanism structure of pressure thermometers and bimetallic thermometers is basically the same as that of Bourdon tube pressure gauges, and they can be repaired using the same methods. Common faults in thermocouples and resistance temperature detectors (RTDs) include: open circuit, short circuit, grounding, poor contact, poor insulation, and deterioration. These two types of thermometers can usually be checked with a multimeter and standard meters, which can often pinpoint the cause of the fault. Temperature parameters have a significant lag; temperature changes require a certain amount of time, a characteristic that helps in diagnosing faults.
A sudden increase or decrease in temperature readings is mostly caused by instrument malfunction, as temperature parameters cannot change abruptly. An open circuit in the RTD element or its wiring will cause the temperature display to suddenly show a low reading. A short circuit in the RTD element or its wiring will cause the temperature display to suddenly show a low reading. A malfunction in the amplifier of the display instrument or board will also cause a sudden increase or decrease in the temperature reading. If the temperature display curve remains flat for an extended period, check for false readings. For recorders, manually rotate the measuring slide rail to observe if the resistance increases during the up-and-down stroke, or if there is any mechanical jamming. Large fluctuations in the temperature display value may be due to process issues. If the process conditions have not changed, check the instrument. Switch the temperature control system to manual control and observe the temperature changes. If the fluctuations significantly decrease, the regulator or control valve may be faulty. Rapid changes in the temperature curve, or oscillations resembling an oscillation, are mostly due to instrument problems. In the temperature control system, this could be due to improper PID parameter tuning or interference. Observe nearby sources of interference, such as welding machines, frequency converters, or other electrical equipment. If the temperature curve does not change significantly, but the regulator's output current suddenly becomes very low or extremely low, the regulator should be the focus of inspection, such as whether the regulator's amplifier and output circuit are functioning properly.
