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A field troubleshooting guide for temperature measurement faults. Each fault class is listed in order of likelihood.
| Possible cause | Diagnostic test | Fix |
|---|---|---|
| 1. Thermocouple wire break | Measure the resistance at the controller end (open TC). A short sensor <10Ω and a long MI cable <50Ω; above 1kΩ or ∞ means a break. | Replace the thermocouple. Common break points: the furnace outlet (thermal cycling fatigue) and the compression ferrule (over-tightened). |
| 2. Loose terminals | Wiggle the wires at the controller and sensor ends; if “OPEN” appears intermittently, the connection is loose. | Tighten the terminal screws; if corroded/oxidised, cut back 10 mm and re-terminate, using ferrules for stranded wire. |
| 3. Extension wire break | Measure open-circuit from the controller to the sensor end; ∞ means a break; check along the routing for crushed/cut sections. | Replace the cable. Vulnerable points: sharp edges of cable trays, equipment pressing down, rodent damage. |
| 4. Thermocoupleburnout | If the furnace exceeds the TC upper limit (e.g. Type K at 1450°C), the wires melt/oxidise and break inside the sheath. | Replace with the correct type (>1350°C use R/S); check the controller log for overtemperature events. |
| 5. controllerinputfault | Swap the channel: if the fault follows the TC, the TC is bad; if it stays in place, the channel is bad. | Replace the input card/module — less common than field cable faults but it still happens. |
| Possible cause | Diagnostic test | Fix |
|---|---|---|
| 1. Wrong extension-wire type | Check the label and confirm the type matches; the error is constant (always +X or −X at all temperatures). | Replace with the correct type. See section 3 — the most common field error. |
| 2. Thermocoupledrift(High temperatureaging) | Compare against a known-good reference TC or calibration instrument; Type K drifts −5 to −15°C after 1000 h above 1000°C. | Replace the thermocouple; the drift is permanent (metallurgical change); Type N drifts less, so schedule replacements. |
| 3. Cold-junction compensation error | Check the controller’s cold-junction sensor; it should read room temperature (20–25°C); if not, the CJC is faulty. | Replace the controller or recalibrate the CJC; some controllers have a CJC offset adjustment in the setup menu. |
| 4. Out of calibration (contamination) | Noble-metal TC (R/S/B) contaminated by metal vapour, silicon or carbon; the wire discolours (Grey to Brown/Black) and is permanently damaged. | Replace the TC; prevention: high-purity alumina protection tubes, avoid reducing atmospheres, keep vacuum chambers clean. |
| 5. Sheath grounding (Type K ”green rot”) | Moisture + oxygen enter the sheath (porous ceramic insulation); Chromel oxidises to chromic oxide (green powder); an ohmmeter reading TC-to-sheath <100 Ohm indicates grounding. | Replace the TC; prevention: seal the sheath (welded tip), avoid thermal cycling below 500°C (condensation), use Type N (more resistant). |
| Possible cause | Diagnostic test | Fix |
|---|---|---|
| 1. Electrical noise pickup | Switch off nearby VFDs, welding machines and contactors; if the noise stops, it is EMI; TC cable running parallel to power cable = an antenna. | Use shielded TC cable; ground the shield layer at one end only; keep at least 300 mm away from power cables. See section 7. |
| 2. Intermittent connection | Tap the cable, wiggle the terminals and flex the cable near the furnace outlet; if the reading jumps, the connection is bad. | Re-terminate the cable ends; replace over-tightened compression ferrules (crushed wire = intermittent) and check for oxidation. |
| 3. Ground loop | Measure the TC to earth at both the sensor and controller ends; multiple grounding points mean current flows through the TC = false voltage. | Isolate the TC and sheath (insulating ferrule); ground the sheath at one point only, never at both ends. |
| 4. Water ingress | Check the terminal box for condensation; moisture creates a low-resistance path between the positive and negative wires (a virtual short). | Dry out the terminal box and seal the entry; if water has entered the MI cable sheath, replace it (cannot be dried, must be replaced). |
| Possible cause | Diagnostic test | Fix |
|---|---|---|
| 1. Large thermal mass (thick sheath) | Check the diameter: a 12 mm MI cable responds in 10–30 s, a 3 mm one in 1–3 s. | Use a finer TC or a bare junction (fastest but brittle), trading speed against durability. |
| 2. Protection-tube air gap | Poor TC-to-protection-tube contact; the air gap insulates and makes the sensor deviate from the process temperature. | Use a thermally conductive filler sleeve (graphite powder, thermal conductive paste), or a spring-loaded TC to maintain contact. |
| 3. Low flow velocity (poor convection) | Still air and stagnant liquid transfer heat extremely slowly; flowing fluids respond quickly. | Increase the flow velocity around the sensor, mount it in the flow path rather than a dead zone, and add baffles. |