What are the common faults of fixed-thread mounted junction box type thermocouples

I. Low Thermoelectric Potential (Low Reading on the Instrument)

This is the most typical problem, which shows up as a recorded temperature that is lower than the real value.


Terminals that are loose or rusted in the junction box

The junction box is exposed to high temperatures for a long period, and the copper terminals are prone to oxidation. This makes the contact resistance higher and the signal transmission weaker.

Solution: Open the junction box often, clean the terminal surfaces, tighten them again, and use anti-oxidation conductive paste if needed.

Compensation wires that are broken or shorted out

High temperatures, mechanical wear, or age and cracks in the outer sheath of the compensation wire can cause a short circuit between the core wires or leaking to ground, which causes signal shunting.

Solution: Use high-quality, high-temperature-resistant compensating wires, including fluoroplastic sheathed wires, and protect them with metal flexible conduits.

The thermocouple material is getting worse or "green corrosion"

When K-type thermocouples are used for a long time at 900℃, the nickel-chromium positive electrode can oxidize selectively, creating a green corrosion layer. This lowers the thermoelectric potential output.

Solution: Cut off the part that is falling apart and re-weld it, or replace it with an N-type thermocouple to make it more stable.

Failure of the protection tube seal lets moisture in.

The silicone sealing ring on the junction box gets old, the bolts come loose, or the weld on the metal protection tube splits. This lets humid air in and makes the insulation less effective.

To keep rainwater from coming back in, replace the sealing ring, tighten the bolts, and make sure the junction box outlet is facing down.



II. High thermoelectric potential (high instrument reading)

This kind of mistake doesn't happen very often, but it does impair how the system works.

The calibration type of the display instrument and the thermocouple don't match.

If the thermocouple is K-type and the instrument is set to S-type, the system will think the temperature is higher than it really is since S-type produces a lower potential at the same temperature.

Solution: Check the calibration type settings on both the thermocouple and the instrument and make sure they are the same.

DC interference signal on top of each other

If there are high-power DC devices nearby or the ground is not well-grounded, the ground loop current may mix with the thermoelectric potential, giving erroneous high readings. How to Fix It: Use single-point grounding. The shielding layer should only be grounded at one end to stop a loop from forming.



III. Unstable Signal Output (Value on Display Changes Often)

Big changes in sensor data make the automatic control system less stable.

Bad or loose contact in the terminal box

Vibration from equipment might cause wires to come loose, or bad termination crimping can cause contact to be intermittent.

Troubleshooting Method: Use spring washers to keep things from coming loose, and check the tightness often.Thermocouple that works sometimes or is poorly welded

The thermocouple wire may break from time to time because of fatigue, microcracks, or bad welding. This might cause the signal to stop working when it vibrates.

Use a multimeter to measure the circuit's resistance. If the value changes, you need to replace the thermocouple right once.

Interference from electromagnetic waves (EMI)

When the alternating magnetic field is close to strong electromagnetic sources like inverters and motors, it might cause interference voltage in the signal line.

Troubleshooting Method: Use insulated compensation wires, stay away from power cables, and don't put them in the same conduit.



IV. No Thermoelectric Potential Output (Instrument Doesn't Show Anything or Shows an Error)

This usually signifies that the signal circuit is fully broken.

Thermocouple wire is broken, or the compensation wire is open.

The thermocouple wire can break if it is bent too much during installation, if it becomes too hot and becomes brittle, or if it is damaged in some other way.

Troubleshooting Method: Check the circuit's continuity in parts, find the break point, and fix or replace it.

Short circuit or burnout inside the box

When the air is wet or there is too much voltage, an arc discharge happens, which melts the terminals and causes irreparable damage.

How to fix it: Replace the terminal box assembly. Use an explosion-proof terminal box with an IP67 protection certification first.



V. Suggestions for Preventive Maintenance

The following steps are suggested to make fixed-thread mounted terminal box type thermocouples more reliable:

Check the Sealing of the Terminal Box Regularly: Every three months, check the condition of the silicone ring and the tightness of the bolts to make sure that moisture doesn't get in.

Standardize Wiring Process: Use both crimping and welding to hold the wires in place, and don't just use screws to tighten them. Make environmental protection stronger by putting protective coverings in places with a lot of dust and humidity, and make sure the wire box outlet holes face down.

Set up a calibration system: Make sure measurements are accurate by calibrating on-site or offline every 6 to 12 months.