What are the common faults of threaded thermocouples

I. Low Thermoelectric Potential (The instrument reading is low)

This is the most common sign of a problem: the measured value is lower than the actual temperature.


Water or a leak inside the thermocouple

If the threaded connection isn't sealed well, moisture or corrosive gasses could get into the protective tube. This would lower the insulation between the thermoelectrodes and the protective tube, which would cause leakage current and lower the output potential.

Take out the thermoelectrodes, dry them and the insulating ceramic tube, and look for fractures or leaks in the protective tube. Fix or replace it if you need to.

The working end of the thermocouple electrodes may corrode or break down.

Using a thermocouple for a long time at high temperatures or corrosive environments can induce "green corrosion" (like K-type thermocouples used at temperatures over 900°C) or oxidation of the thermocouple material. This can modify the thermoelectric properties and lower the output potential.

If the length allows, cut off the bad part and re-weld the working end. If the damage is too bad, replace the thermocouple with a new one.

Dust buildup or a short circuit in the junction box

If the junction box isn't sealed correctly, dust, metal shavings, or other conductive particles could get in and cause a short circuit or grounding between the terminals, which would impede signal transmission.

Solution: Open the junction box, clean out the dust and metal, seal it back up, and make sure the outlet hole is facing down so that moisture can't get in.

Problems with the compensation wire

If the insulation is damaged, the compensating wire doesn't match the thermocouple type, or there is too much moisture, the measurements will be wrong. For instance, utilizing regular copper wire instead of compensating wire will add more thermoelectric potential.

Solution: Replace with a compensating wire of the same model, make sure the polarity is right, and improve the waterproofing.



II. High Thermoelectric Potential (The reading on the instrument is high)

This kind of problem doesn't happen very often, but it does impair how accurate measurements are.

The thermocouple and the display instrument don't match.

If the thermocouple type (such K-type or S-type) doesn't match the secondary instrument, it will cause the wrong cold junction correction, which will make the displayed value higher or lower.

Solution: Check that the thermocouple and instrument types match and replace them with models that do. There are interference signals.

In industrial settings, strong electromagnetic fields can cause extra voltage to flow through the wires. This voltage is added to the thermoelectric potential, which makes the readings greater.

Check the grounding, use shielded compensation wires, and stay away from high-power motors, frequency converters, and other things that can cause interference.



III. Signal that is unstable or changes

The value on the instrument display changes, which is normal when there is vibration or a bad connection.

Wiring that isn't tight or bad contact

Threaded connections are safe, but if the terminals in the junction box aren't tight or the thermocouple isn't making good contact with the terminal posts, the signals may not always be clear.

Solution: Make sure all connections are tight, clean the contact surfaces, and make sure they are good conductors.

Thermocouple not properly installed or shaken up

If the equipment moves a lot and the thermocouple isn't completely tight or the threads are worn, the probe could come loose, which would make temperature measurements less stable.

Solution: Tighten again, and if needed, add washers or devices that stop things from loosening or vibrating.

Partially Broken Thermocouple Wire

Mechanical damage or fatigue can cause the thermocouple wire to develop micro-cracks, which can cause the signal to be intermittent.

Use a multimeter to check the resistance. If the value is unstable, you should replace the thermocouple.



IV. No Output of Thermoelectric Potential

The instrument doesn't show anything or indicates the highest value, which usually means that the signal circuit has broken.

Wire for a broken thermocouple

The thermocouple wire might break because of mechanical damage, high temperatures that make it brittle, or bad installation.

If the break is close to the end, it can be cut and re-welded. If the damage is serious, it needs to be replaced.

Compensation wire that is broken or turned around

If the polarity is reversed, there won't be a signal, but there will be big problems. If the circuit is broken, it will stop working right away.

To fix the problem, make sure the circuit is still working and that the connections are correct and there are no breaks.