Troubleshooting Guides · Free Tool
Thermocouple and RTD Fault Finder
Choose the temperature measurement symptom and get likely causes for RTDs and thermocouples, with checks and fixes.
Open sensorReads high or lowSlow responseNoisy reading
SymptomReading goes upscale or downscale (sensor break)
Most likely causeOpen circuit sensor or broken lead
First checkMeasure resistance or continuity at the transmitter
Causes in order of likelihood
| Cause | How to check | Fix |
|---|---|---|
| Open circuit sensor or broken lead | Measure resistance or continuity at the transmitter | Repair the connection or replace the sensor |
| Loose terminal in head or junction box | Inspect and tighten terminals | Tighten; check for corrosion |
| Burnout direction setting | Check whether upscale or downscale burnout is configured | Set the burnout direction your process needs |
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Before You Start
- Work under a permit, inform the control room and put the loop in manual or bypass the trip before touching a live instrument.
- Check the transmitter or DCS sensor type matches the installed sensor (Pt100 vs type K etc.).
- Measure the sensor directly: resistance for RTDs, mV for thermocouples.
- Compare with a reference thermometer where possible.
Thermocouple and RTD Fault Finder: All Symptoms
Reading goes upscale or downscale (sensor break)
| Likely cause | How to check | Fix |
|---|---|---|
| Open circuit sensor or broken lead | Measure resistance or continuity at the transmitter | Repair the connection or replace the sensor |
| Loose terminal in head or junction box | Inspect and tighten terminals | Tighten; check for corrosion |
| Burnout direction setting | Check whether upscale or downscale burnout is configured | Set the burnout direction your process needs |
Reads ambient instead of process temperature
| Likely cause | How to check | Fix |
|---|---|---|
| Sensor not fully inserted in the thermowell | Check insertion length | Insert fully; use a spring loaded sensor |
| Thermocouple shorted in the head or cable | A short creates a new junction at the short location | Find and repair the short |
| Wrong sensor connected | Check tags and wiring | Correct the wiring |
Reads consistently high or low
| Likely cause | How to check | Fix |
|---|---|---|
| Wrong sensor type configured | Compare configuration with the sensor | Configure the correct type |
| Wrong or reversed thermocouple extension cable | Check cable type and polarity (ANSI negative is red) | Use the correct extension cable with correct polarity |
| 2 wire RTD lead resistance | Measure lead resistance | Use 3 or 4 wire connection or compensate |
Slow response to changes
| Likely cause | How to check | Fix |
|---|---|---|
| Air gap in thermowell | Check sensor fit in the bore | Use correct diameter sensor, add heat transfer paste where allowed |
| Heavy thermowell or deposits | Inspect thermowell | Clean or use a thinner tip thermowell |
| High damping in transmitter or DCS | Check damping settings | Reduce damping |
Noisy or jumping reading
| Likely cause | How to check | Fix |
|---|---|---|
| Electrical noise on thermocouple wiring | Check screen and segregation | Use twisted, screened cable and a head mounted transmitter |
| Intermittent loose connection | Wiggle test with the loop monitored | Tighten or replace terminals |
| Ground loop with grounded thermocouple | Check whether the junction is grounded | Use an isolated transmitter or ungrounded junction |
Key insight: a thermocouple always measures at its hottest junction; any short in the cable creates a new junction there, so the reading drops to the temperature at the short.
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Frequently Asked Questions
How do I test an RTD?
Measure its resistance: a Pt100 reads about 100 Ω at 0 °C and 138.5 Ω at 100 °C.
How do I test a thermocouple?
Measure the mV with the cold junction temperature known, and convert using the reference table.
Why does reversed thermocouple polarity read low?
The signal at the extension cable junction subtracts instead of adds, producing a large low error.
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