What are the common malfunctions of simple probe-type thermocouples

I. Low Thermoelectric Potential (Low Instrument Reading)

This is the most typical problem, and it shows up as a measured temperature that is far lower than the actual value.


Thermoelectric electrode material can break down or "green corrosion."

When K-type thermocouples are used for a long time in high-temperature situations (>800℃), the nickel-chromium positive electrode can selectively oxidize, creating a green corrosion coating that lowers the thermoelectric potential output.

To make it more stable, cut off the bad part and re-weld it, or replace it with an N-type thermocouple.

Contamination or fouling of the insulation tube

Dust, oil, or slag in the furnace sticks to the inside wall of the ceramic insulation tube. This causes leakage between electrodes or conduction to ground, which makes the signal output less effective.

Solution: Take out the thermocouple core on a regular basis, clean it with anhydrous alcohol, dry it, and see if the insulation resistance has gone back up (it should be >100MΩ).

Damage to or leaking of the protective sheath

When the sheath gets too hot, it can fracture or get broken by workpieces hitting it. This can let media in, speed up electrode corrosion, or cause short circuits.

To make it more impact-resistant, replace it with a model that has a thicker protective tube or use a metal-ceramic composite sheath.

Compensation wires that are not the same or have their polarity switched

Using compensating wires that don't match (like E-type instead of K-type) or switching the positive and negative poles during installation may cause the cold junction compensation to be wrong, which will make the readings consistently low.

Check the calibration type to make sure the compensation wire matches the thermocouple, and make sure the polarity is accurate.



II. High thermoelectric potential (high instrument reading)

This kind of mistake doesn't happen very often, but it does alter how people make decisions.

Thermocouple and Display Instrument Calibration Type are not the same.

If the thermocouple is K-type and the instrument is set to S-type, the system will think the temperature is higher since S-type outputs a smaller potential at the same temperature.

Solution: Check and set the calibration type for both the thermocouple and the instrument. Superposition of DC Interference Signals

Ground loop currents can be added to the thermoelectric potential when near high-power DC equipment or because of bad grounding. This can make readings seem higher than they are.

Solution: Use a single-point grounding method, which means grounding the shielding layer at only one end so that a loop doesn't form.



III. Unstable Signal Output (Value on the Display Changes a Lot)

The stability of the automatic control system is affected by big changes in instrument readings.Intermittent or Bad Contact of Thermoelectric Electrode

The thermocouple wire may connect and disengage from time to time because of fatigue, microcracks, or bad welding when it vibrates, which can cause the signal to be interrupted.

Use a multimeter to check the circuit's resistance as a solution. If the resistance changes, you should replace the thermocouple right away.

Terminal Blocks that are Loose or Oxidized

The terminals in the junction box aren't tight, or the contact surface gets oxidized after being in a high-temperature environment for a long time, which alters the contact resistance and makes the signal fluctuate.

Solution: Clean the terminal surface, tighten it again, and use conductive paste that prevents oxidation.

Interference from electromagnetic fields (EMI)

The alternating magnetic field can cause interference voltage in the signal line when it is close to strong electromagnetic sources like frequency converters and motors.

To fix the problem, use shielded compensating wires, stay away from power cables, and don't put them in the same conduit.



IV. No Thermoelectric Potential Output (Instrument No Display or Error)

This usually signifies that the signal circuit is fully broken.

Thermocouple Wire That Is Broken

The thermocouple wire breaks when it bends too much during installation, gets too hot and becomes brittle, or gets damaged mechanically.

If the break is in a place where it can be cut and re-welded, do that. If not, get a new thermocouple.

A short circuit or an open circuit in the measurement line

A short circuit happens when water gets into the junction box or dust builds up there. An open circuit happens when the compensation wire is pushed apart.

Solution: Test the circuit in parts to see if it is still connected, find the problem area, and fix it.



V. Suggestions for Preventive Maintenance

To make simple probe-type thermocouples more reliable, several steps are suggested:

Regularly check and clean: Every three months, clean the temperature measuring end and junction box to keep dust and moisture from building up. Standardized installation procedure: Don't bend the wires too much, and make sure the insertion depth is at least 8 to 10 times the diameter of the protective tube.

Better protection for the environment: To keep moisture out, the wire box outlet should face down. In places where there is a lot of vibration, you should also add vibration-dampening measures.

Set up a calibration system: To make sure the measurements are accurate, calibrate the equipment on-site every 6 to 12 months, depending on how it is used.