In other articles, we've introduced what a thermocouple is. Now, we'll discuss how to determine the quality of a thermocouple during its use.

To determine if a thermocouple is damaged, the following systematic steps can be taken, covering aspects such as appearance, electrical performance, temperature response, and environmental adaptability.

I. Visual Inspection

• Physical Damage

• Check for cracks, deformation, or corrosion on the thermocouple’s protective sheath (e.g., metal oxidation, insulation layer damage).

• Inspect connecting wires for breaks, loose connectors, or oxidation (e.g., copper wires turning black, insulation aging).

• Verify if solder joints are detached or have cold soldering (especially at the connection between the thermocouple and compensation wires).

• Environmental Adaptability

• For thermocouples used in high-temperature, corrosive, or vibrating environments, check for material degradation (e.g., ceramic insulator cracking) due to prolonged use.

II. Electrical Performance Testing

• Resistance Measurement

• Method: Use a multimeter to measure the resistance across the thermocouple’s terminals.

• Standard Values:

• Short thermocouples (e.g., K-type, length <1m): Resistance typically ranges from a few ohms to tens of ohms.

• Long thermocouples (e.g., length >10m): Resistance may reach hundreds of ohms.

• Judgment: If resistance is significantly higher than standard or infinite, a (open circuit) may exist; if near zero, a (short circuit) may occur.

• Open/Short Circuit Detection

• Open Circuit: Use a multimeter’s diode or resistance mode. If it displays "OL" or infinite resistance, the circuit is broken.

• Short Circuit: If resistance is near zero, internal insulation damage may have caused a short between the positive and negative leads.

• Thermoelectric Voltage Verification

• Method: Immerse the thermocouple in a medium of known temperature (e.g., ice-water mixture or boiling water) and measure the output voltage with a voltmeter.

• Standard Values (for K-type):

• 0°C: ~0mV

• 100°C: ~4.095mV

• Judgment: If the measured value deviates by more than ±5% from the standard, the thermocouple may be aged or contaminated.

III. Temperature Response Testing

• Dynamic Response

• Rapidly ｉｎｓｅｒｔ the thermocouple into a high-temperature medium (e.g., boiling water) and observe if the temperature reading rises quickly and stabilizes.

• Abnormal Behavior: Slow response, temperature fluctuations, or failure to reach the expected value may indicate excessive thermal capacity or damage.

• Stability Testing

• Continuously monitor the temperature reading in a constant-temperature environment (e.g., a thermostatic water bath).

• Abnormal Behavior: Persistent drift or erratic readings may suggest poor contact or internal structural damage.

IV. Compensation Wire and Connection Checks

• Compensation Wire Compatibility

• Ensure the compensation wire material matches the thermocouple type (e.g., K-type compensation wire for K-type thermocouples).

• Consequences of Mismatch: Measurement errors or abnormal voltage.

• Connection Point Temperature

• Verify that the temperature at the connection between the compensation wire and thermocouple is uniform (avoid temperature gradients introducing errors).

• Inspection Method: Use an infrared thermometer to measure the connection point temperature and compare it with the displayed value.

V. Environmental Factor Troubleshooting

• Electromagnetic Interference

• If the thermocouple is near strong electromagnetic fields (e.g., motors, inverters), interference may cause abnormal readings.

• Solution: Shield the thermocouple wiring or relocate it away from interference sources.

• Cold Junction Temperature Compensation

• If using an uncompensated thermocouple, ensure the cold junction temperature is stable (e.g., via ice-point method or compensation circuits).

• Abnormal Behavior: Temperature drift due to cold junction fluctuations.

VI. Substitution Verification Method

• Replacement Test

• ｒｅｐｌａｃｅ the suspected faulty thermocouple with a known good one of the same model and observe if the temperature reading normalizes.

• Applicable Scenarios: When direct electrical parameter measurement is impractical (e.g., embedded devices).

• Comparative Testing

• Simultaneously use multiple thermocouples to measure the same temperature point. If one reading significantly deviates from the others, it may be faulty.

VII. Professional Testing Tools

• Thermocouple Calibrator

• Specialized equipment simulates a standard temperature source and directly outputs the thermocouple’s voltage for quick qualification.

• Oscilloscope

• Observe the waveform of the thermocouple’s output signal. Noise or distortion may indicate wiring interference or damage.

Summary of Judgment Criteria

If the above steps confirm the thermocouple is faulty, ｒｅｐｌａｃｅ it with an original or compatible part and recalibrate the system to ensure measurement accuracy.