What are the common malfunctions of thermocouples with movable threaded connection junction boxes

I. Unstable Thermoelectric Potential Output (Value on the Display Changes Often)

This is the most common problem with movable threaded constructions, and it happens mostly because the mechanical connections aren't reliable.


Loose threaded connections that don't make good contact:

Because of equipment vibration or thermal expansion and contraction, threaded connections can move slightly, which changes the contact resistance between the thermocouple and the junction box.

Solution: Check and tighten threaded connections on a regular basis. If needed, use anti-loosening glue or add lock nuts.

Connections at the terminals that are loose or rusty:

Taking things apart and putting them back together a lot causes wear on the terminals. Oxidation of the copper surface makes contact resistance worse, which makes the signal transmission go in and out.

To remedy this, clean the terminal surface, use anti-oxidation conductive paste, and employ both crimping and welding to hold it in place.

Vibration from outside or electromagnetic interference:

Signal lines can be affected by interference when they are situated near sources of vibration, including fans and pumps. Unshielded lines can also pick up noise from powerful electromagnetic fields.

Solution: Put in vibration-damping pads, utilize shielded compensating wires, and ground the shield at one end.



II. Thermoelectric Potential Is Less Than the Real Value (Instrument Shows Low)

This shows up as the recorded temperature being lower than the real value, which makes process control less accurate.

Thermocouple short circuit or partial short circuit of the compensation wire:

If the insulation at the moveable joint gets damaged, dust builds up, or moisture gets in, it can cause the electrodes to leak or short circuit, which can cause signal shunting.

Solution: Look for water and dust in the junction box, dry it off right away, and replace any insulation that is getting old.

Not enough depth for the thermocouple to fit in or the wrong place to put it:

Because the movable structure can move, the temperature sensing end may not be deeply immersed in the medium and may be impacted by the temperature of the surrounding area.

To fix the problem, make sure that the insertion depth is at least 8 to 10 times the outer diameter of the protective tube, and set the installation position.

Compensation for an abnormal cold junction temperature:

The junction box is exposed to a high-temperature environment, causing the cold junction temperature to rise without effective compensation, resulting in a lower calculated temperature.

Fix: Move the junction box to a place with a lower temperature or use an automatic cold junction adjustment module. III. The thermoelectric potential is higher than the actual value (the instrument reading is higher).

This kind of mistake is less common, but it can still lead to wrong decisions.

The calibration of the thermocouple and the display instrument do not agree.

If you don't change the settings on the instrument after replacing the thermocouple, the thermoelectric potential conversion will be wrong.

Check to make sure that the thermocouple model (for example, K-type or S-type) matches the parameters on the instrument.

Superposition of DC interference signals

Ground loop currents can be added to the thermoelectric potential when there is a lot of high-power DC equipment nearby or when the grounding is not good. This can make readings look higher than they really are.

Solution: Use single-point grounding to prevent the formation of a loop by grounding both ends of the shielding layer.



IV. Decreased or Complete Loss of Insulation Performance

The movable structure has poor sealing and is easily affected by the environment.

Aging or not putting the junction box sealing ring in the right place

Moisture and dust intrusion lead to a decrease in insulation resistance, and in severe cases, cause short circuits.

To keep rainwater from getting in, use silicone sealing rings that can handle high temperatures and make sure that the outlet holes are facing down.

Moisture inside the protective tube or electrode leakage

During long-term disuse or in high-humidity environments, moisture can seep into the protective tube along the wires.

Solution: Remove the thermocouple and dry it; replace the insulating ceramic tube if necessary.



V. Suggestions for Preventive Maintenance

To improve the reliability of movable threaded junction box type thermocouples, the following measures are recommended:

Make the connection more stable: After each installation, check the tightness of the threads, and use a torque wrench to make sure they are all the same if you need to.

Standardize wiring process: Avoid simply tightening the wires with screws; prioritize using crimp terminals and reinforce with soldering.

To make the environment more adaptable, put protective covers on the junction box outlet holes and make sure they face down.

Check the insulation resistance often: Use a 500V megohmmeter once a year to check the insulation resistance between the thermoelectrode and the protective tube. It should be more than 100MΩ.

Set up a way to check things: every three months, check the sealing, wiring, and stability of the instrument readings.