Troubleshooting Thermocouples and RTDs

Temperature measurement faults are often due to wiring issues, poor installation, or sensor degradation rather than sensor failure itself. Effective troubleshooting requires a systematic approach—starting with simple checks and moving toward calibration and instrumentation.

The most common source of error in temperature measurement is the sensor not accurately reflecting the temperature of its environment. This can be due to:

• Poor thermal contact — between the sensor and the medium, or between the sensor and its housing (probe, sheath, or thermowell).
• Sensor degradation — resulting in drift from its original calibration.
• Wiring faults — which are more common than sensor or instrument failures.
• Instrument calibration errors — which can mimic sensor faults.

1. Start with Wiring

   Faulty wiring and poor connections often cause more issues than the sensors themselves. Always inspect wiring first. Ensure connections are tight, free of corrosion, and follow the correct wiring scheme.

2. Check Thermal Contact

   Poor installation can prevent the sensor from properly sensing the medium. If installation guidelines were followed, many of these issues should be avoided. Visually inspect for:

   • Clean and intact contact areas
   • Secure mechanical junctions
   • Adequate insertion depth and positioning in the process

   Poor contact, especially in closed control loops, can cause lag, error, and unstable process control.

3. Look for Sensor Degradation

   Sensor drift typically happens over time and differs between sensor types:

   • Thermocouples: May suffer from poor insulation, thermoelectric inhomogeneity, or junction deterioration.
   • RTDs: Prone to internal lead issues, mechanical shock, chemical poisoning, or lead wire imbalance.

   Use calibration checks and diagnostics to confirm degradation.

4. Perform Quick Tests
   • Thermocouple: Disconnect sensor and short input terminals. If ambient temperature displays, the sensor is at fault.
   • RTD: Disconnect sensor and connect a 100 Ω resistor. If 0°C displays, the sensor is faulty.

   Measure resistance:

   • Low resistance: May indicate shorts or wiring faults.
   • High resistance: Could suggest thinning wires (thermocouples) or open circuits.

   Be alert to:

   • Reversed polarity: Leads to downscale readings or large errors (up to 100°C).
   • Wrong cable type or thermocouple type: Can also cause significant error.
5. Recognize Common Indicators

   Symptom	Possible Cause
   Full-scale high reading	Open circuit, loose wire, sensor failure
   Persistent low reading	Wrong thermocouple/cable type, short circuit
   Gradual drift	Sensor aging or degradation
   Intermittent fluctuations	Loose wiring, poor joints, EMI/noise pickup
   High RTD reading	High circuit resistance, excessive current, self-heating
   Low RTD reading	Insulation leakage, lead shorts

6. Polarity and Cable Checks

   Thermocouples are particularly sensitive to connection polarity and cable alloys. Reversed polarity or mismatched extension cables can cause major errors. Always check:

   • Cable color codes and type
   • Correct polarity (positive to positive)
   • That all connections are secure and follow isothermal practices
7. Consider Instrumentation and Environment

   If the sensor and wiring are sound, turn to the rest of the system:

   • Inspect isolators, transmitters, multiplexers, and receiving instruments.
   • Use portable calibrators and simulators for diagnostics.
   • Confirm cold junction compensation is functioning correctly in thermocouple systems.

Summary

Sensor faults are rarely the root cause of measurement errors. Start with wiring and connection checks, then look at thermal contact and sensor degradation. Use resistance measurements and simulation tools to identify open or short circuits, reversed polarity, or incorrect cable types. Thermocouples are more prone to drift and wiring sensitivity, while RTDs are typically stable but affected by insulation leakage or self-heating. Systematic diagnostics and good installation practices are key to reliable operation.

Further Reading

RTD vs Thermocouple – Choosing the Right Sensor
Explore the features and characteristics of thermocouples and RTDs

RTD Output Tables
View Resistance versus Temperature tables for all Pt100 sensors.

What are the RTD colour codes?
Explore RTD colour codes and wiring configurations.