How to Determine if a Sheathed Platinum Resistance Temperature Detector (PTT) Has Common Faults

To find out if a double-conduit-mounted PTT box has frequent problems, you need to undertake a series of tests on-site that look at both its electrical and physical status. First, unplug the power supply and look at the junction box. Make sure the cover is face down, check for any water buildup, and look for any sealing rings that are hard, cracked, or stuck. If you see water stains, mold, or loose glands, it means that the protection has failed and moisture has gotten inside. This is a sign that the insulation is starting to break down. Next, use a 500V megohmmeter to check how well the insulation works between the case and the terminals. If the value is less than 10MΩ, and especially if it stays below 1MΩ in wet places, it is clear that surface leakage or electrochemical migration is happening. In this situation, the signal will always drift or change, even if the resistance value is normal.




The next step is to measure the resistance value of the PTT box itself if the insulating resistance is good. Use a multimeter with high accuracy to check the resistance between R12, R13, and R23. At 0°C, the nominal resistance of Pt100 is 100Ω. If the measured value is off by more than ±0.5Ω, or if repeated tests at the same temperature show big changes, it means that the platinum wire has aged because it was exposed to high temperatures, sulfide contamination, or lattice deformation. If the measured resistance value is substantially lower than the nominal value (for example, below 95Ω), it is quite possible that the platinum wire touched the protective tube because the inner tube has corroded, which generated a ground short circuit. In this situation, the temperature reading will stay low, and it will get worse as the temperature around it rises. If the resistance value is infinite, the lead wire has broken due to fatigue. The break is normally within 50mm of the junction box intake. To check for metal fatigue cracks, the lead wire bends must be taken apart and looked at.







For further judgment, you need to think about how temperature affects things. Put the sensor in a bath with a consistent temperature that you know and watch how long it takes for the instrument to respond. If the response takes more than 30 seconds, or if there is a nonlinear jump during a temperature step change, it means that the airtightness between the tubes is broken and the dry or inert gas between the inner and outer tubes has escaped. This has caused an unusual path for heat to flow. Even if the insulation and resistance levels seem reasonable, the system will nonetheless send out an unstable signal in this instance. In very corrosive places like pyrolysis furnaces and saltwater desalination facilities, a "double failure chain" happens when water gets into the junction box and pitting corrosion happens on the outside of the tube at the same time. At the same time, the integrity of the protective tube needs to be verified; just replacing the junction box won't fix the problem.







Finally, you need to get rid of any outside interference. Check to see if the leads are straightened, bundled, or under mechanical stress, and make sure that the terminals are free of oxidation and looseness to avoid misdiagnosing sensor failure because of poor contact. If all the electrical characteristics are normal but the system still shows problems, compare the data with that from other sensors that are working in the same way. If this channel is the only one that drifts, the platinum resistance thermometer is probably broken. The whole diagnostic process should follow the rules of "external before internal, electrical before thermal, static before dynamic." This means that you shouldn't take things apart without knowing what you're doing, and you should make sure that the diagnosis is correct and quick.