What are the common faults of surface-mounted gasket-type thermocouples

Low Thermoelectric Potential (Displayed temperature is lower than actual)

This is the most common issue, affecting control precision.


A weak gasket pre-tightening force

The bolts were not tightened after installation, or long-term high temperatures annealed the gasket metal, reducing its compression and rebound performance and clamping force. This generates an air gap between the temperature detecting end and measured surface, increasing contact thermal resistance.

Tighten with a torque wrench to the standard torque and examine and replace aged gaskets.

Bad temperature sensing surface contact

Oil stains, oxide coatings, carbon deposits, or uneven surfaces hamper heat conduction, causing temperature lag or lower readings.

Solution: Clean and polish the temperature sensing region before installation for metal-to-metal contact.

Intrusion of moisture or reduced thermocouple insulation

Water and conductive dust can seep between the thermoelectrodes and the casing in the junction box or protective tube due to poor sealing, reducing the output signal.

Solution: Dry the thermocouple core, inspect and replace the sealing ring, and verify junction box sealing.

Problems with compensation wire

Moisture, insulation degradation, reversed polarity, and compensation wire-thermocouple mismatches cause mistakes. Using an E-type compensation wire with a K-type thermocouple will incorrectly compensate cold junctions.

Solution: Replace with matching compensating wire, check polarity, and waterproof connectors.



II. High Thermoelectric Potential (Displayed temperature is higher than actual)

Though rare, this defect impacts system judgment.

Incorrect thermocouple-display instrument match

If the thermocouple is K-type but the instrument is S-type, the same thermoelectric potential will be misunderstood as a greater temperature due to calibration differences.

Solution: Match thermocouple and instrument calibration types. External heat source interference raises cold junction temperature.

The compensating wires or junction box are near a high-temperature location, raising the cold junction temperature over the specified value and creating faulty cold junction compensation and artificially high readings.

Solution: Avoid heat sources by optimizing wiring paths and installing insulation covers.



III. Unstable signal output (frequent value changes)

Variable instrument readings influence automatic control system stability.

Gaskets loosening or vibration producing intermittent contact

Long-term vibration on pumps and fans can loosen the fastening bolts, producing occasional temperature sensing end contact failure.

Anti-loosening gaskets or double-nut locking structures and vibration damping brackets may help.

Bad contact or oxidation of wiring

If the wiring terminals are not tightened or exposed to high temperatures, the contact surface oxidizes and signal variations occur.

Solution: Clean the contact surface, adjust the wiring, and apply anti-oxidation conductive paste if needed.

Intermittent thermocouple break

Fatigue or mild damage causes micro-cracks in the thermocouple wire, creating intermittent contact under vibration and signal interruption.

Solution: Use a multimeter to measure circuit resistance; replace the thermocouple promptly if it fluctuates.

Electricity interference

Signal line interference voltage may result from alternating magnetic fields near strong electromagnetic sources like inverters and motors.

Solution: Ground the shield at one end of shielded compensating wires and keep them away from power lines.



IV. No thermoelectric potential output (no display or error)

This usually signifies the signal circuit is broken.

Wire thermocouple broke

The thermocouple wire breaks due to installation bending, high-temperature embrittlement, or mechanical damage.

If the break position allows, cut and reweld; otherwise, replace with a new thermocouple.

Open circuit or improper compensating wire wiring

Broken circuits originate from wires being wrenched apart, crushed, or improperly wired (open circuit). To find and fix the break site, check circuit continuity in parts.



Preventive Maintenance Tips

The following steps can increase surface-mounted gasket-type thermocouple reliability:

Avoid loosening by quarterly checking bolt pre-tightening force and gasket condition.

Standardize installation: Keep the temperature sensing end in touch with a clean surface to avoid forced assembly damage.

Strengthen environmental protection: Seal the junction box and face the cable output downwards to prevent moisture and dust.

Set a calibration cycle: Every 6-12 months, depending on operating conditions, calibrate on-site or offline to assure measurement accuracy.