Quick answer
When a bearing RTD indicates rising temperature, treat possible overheating as real until safely assessed. Correlate the indicated trend with load, speed, lubrication, vibration and other bearing measurements. Under an approved procedure, check the RTD type, two-/three-/four-wire configuration, lead resistance, terminations, transmitter scaling and calibration. Compare with an independent calibrated measurement at a documented accessible location, allowing for thermal gradients between the sensor element and bearing-housing surface. RTDs do not use thermocouple cold-junction compensation. Isolate hazardous energy before intrusive testing, and do not bypass or raise a protective alarm threshold to make the indication disappear.
Problem / job
An unexpected bearing-temperature indication may reflect actual bearing heating, a sensor or wiring fault, or an instrument configuration error. Investigate without assuming that an alarm, shutdown, temperature rise or maintenance history occurred at a particular plant. Use the installed bearing, sensor and protection-system documentation to determine the required safe response.
Evidence
- Record the actual RTD value, alarm threshold and trend; no measured temperature rise is assumed.
- Check available historical trends for drift, steps or load-dependent changes; do not assume a six-month trend exists.
- Review available vibration, lubrication and oil-analysis evidence; absence of an alarm does not rule out damage.
- Document any authorized housing inspection findings; no inspection outcome is assumed.
- If authorized testing is performed, record measured RTD and lead resistances against the correct sensor characteristic and wiring configuration; do not assume a test result.
Decision
Proceed with a systematic verification that isolates sensor integrity from actual bearing temperature. Use an independent temperature measurement method to cross‑check the RTD. Compare the independent measurement with the RTD only after accounting for sensor location, thermal gradient and instrument uncertainty. Confirm the cause before selecting bearing maintenance or instrument repair.
Workflow – 7 practical steps
- Plan observation and safe isolation. First review available operating trends and remote measurements without approaching moving machinery or opening energized enclosures. Follow the plant’s approved response to any suspected bearing overtemperature. Before opening terminals, removing a sensor, or making intrusive checks, stop the relevant equipment as required, isolate electrical, mechanical and stored energy sources, apply authorized lockout/tagout, and verify the isolation. A controlled operating measurement may be taken only from an approved safe location under the site’s procedure.
- Document the alarm condition. Capture the RTD reading, timestamp, alarm set‑point, and any related process variables (load, speed, lubrication flow). Export the trend curve for later comparison.
- Inspect the RTD assembly. Visually check the sensor for corrosion, physical damage, or loose mounting. Confirm sensor seating and thermal contact at the OEM-specified mounting location; do not assume that the element directly contacts the bearing hub or that thermal paste is specified.
- Test wiring integrity. After approved isolation from the transmitter and electronics, check sensor resistance and continuity with a suitable low-energy meter. Verify Pt100/Pt1000 type, two-/three-/four-wire compensation, terminations and lead resistance against manufacturer data. Never apply a high-voltage insulation tester across an RTD element or connected electronics.
- Verify RTD input configuration and lead compensation. RTDs have no cold junction. Confirm the transmitter is configured for the installed RTD type and wiring method; check excitation, lead-wire compensation, scaling and input diagnostics using the manufacturer’s approved calibration procedure.
- Obtain an independent temperature measurement. From a safely accessible location under an approved operating or isolated test condition, take an independent temperature reading on a documented housing surface. Record surface condition, emissivity if using infrared, ambient conditions and thermal gradients; an embedded RTD and an external surface probe need not agree numerically.
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Analyze and act. Compare the independent temperature with the RTD reading:
- If independent evidence supports genuine heating after allowing for different measurement locations and uncertainty, apply the OEM bearing-protection response and investigate lubrication, alignment, load and vibration.
- If readings differ, do not infer a sensor fault from surface temperature alone. Check thermal gradients, measurement uncertainty, RTD resistance, lead compensation and transmitter scaling; repair a confirmed fault without relaxing protection set-points.
Document the conclusion, corrective actions taken, and any changes to maintenance schedules.
Diagnostic matrix
| Evidence | Likely direction | Next check |
|---|---|---|
| Indicated RTD temperature drifts while independent condition evidence remains stable | Possible sensor, wiring or transmitter drift; genuine heating remains possible | Compare the isolated RTD element resistance with the correct sensor resistance–temperature characteristic at a known reference temperature; evaluate lead and cable resistance separately. |
| Independent surface measurement trends upward alongside the embedded RTD | Corroborating evidence of heating, not proof of the embedded bearing temperature | Account for measurement location, thermal gradients and uncertainty; assess lubrication, vibration and load |
| RTD input is configured for the wrong sensor type or wiring method | Input scaling or lead-wire compensation error | Verify Pt100/Pt1000 characteristic, wiring configuration, lead resistance and transmitter settings |
| Visible corrosion or loose connector on RTD leads | Signal loss or offset | Under approved isolation, inspect and repair connections according to the manufacturer’s terminal and torque requirements; then verify the loop. |
| Approved testing of an isolated cable identifies insulation damage or intermittent continuity | Possible cable-related signal error | Assess and repair the isolated cable using the manufacturer’s approved test method; never apply high-voltage insulation testing to an RTD or connected electronics |
| Vibration alarm active while temperature normal | Possible mechanical fault despite a normal temperature indication; investigate independently | Follow the approved vibration investigation: review overall levels and, where appropriate, spectrum data, lubrication condition and oil-analysis evidence against equipment-specific limits. |
| Temperature spike coincides with load increase | Possible genuine load-related heating, transient thermal response or a measurement-chain artifact; timing alone does not establish causation | Correlate load, speed, vibration and lubrication trends with an independent safe temperature check; evaluate actual operating limits and RTD input integrity before diagnosing the cause. |
Acceptance criteria
Field verification record — capture before closeout
| Check | Record on the work order | Acceptance basis |
|---|---|---|
| RTD identification | Sensor type, element characteristic, 2-/3-/4-wire connection, tag and drawing revision | Installed OEM instrument documentation |
| Electrical verification | Reference temperature, isolated element resistance, cable/lead observations and calibrated test instrument ID | Applicable resistance–temperature table and test uncertainty; not a universal fixed ohm limit |
| Transmitter and protection | Input configuration, scaling, diagnostic state, alarm/trip settings and functional-test boundaries | Approved instrument datasheet and protection cause-and-effect matrix |
| Independent thermal evidence | Measurement point, surface/emissivity condition, instrument ID, operating load and trend | Documented location gradient and measurement uncertainty, not direct numerical equality with embedded RTD |
| Return to service | Fault disposition, corrective action, authorizing person, controlled-restart trend and any outstanding limitations | Plant/OEM operating limits and signed release procedure |
Verification is complete when all of the following measurable conditions are satisfied:
- Under approved isolation, check the RTD element resistance against the installed sensor type’s resistance–temperature characteristic at a documented reference temperature; record measurement uncertainty and separately assess lead resistance and wiring compensation. A single ambient-temperature resistance check detects some faults but does not prove accuracy across the operating range. Where required, use an approved multi-point calibration or substitution test with traceable references and manufacturer-defined tolerances.
- RTD sensor type, transmitter configuration and lead-wire compensation are checked against manufacturer data; RTDs require no cold-junction compensation.
- Independent temperature evidence is reconciled with the RTD after documenting measurement locations, thermal gradients, surface emissivity where relevant, and each instrument’s stated uncertainty. No universal ±1 °C agreement criterion applies between an embedded RTD and an external housing measurement.
- Before return to service following confirmed heating, investigate and correct the cause, and verify applicable vibration, lubrication, oil-condition and load indicators against approved OEM or plant limits for the operating condition. Document unavailable or nonapplicable measurements rather than inventing results.
- Document all corrective actions and verify temperature-protection set-points and alarm/trip functions against the approved plant configuration using an authorized functional test. A simulated transmitter or controller input checks only the portions of the protection chain included in that test; it does not by itself prove RTD-element accuracy, field wiring integrity or final shutdown-device operation. Record the test boundaries, results and any independently verified functions. After a controlled restart, record the operating trend; a second temperature reading alone does not validate a protective set-point.
Complete intrusive checks, restore guards and protective functions, and obtain authorization before releasing isolation. Where acceptance requires operating-temperature trends, perform a controlled restart under the plant’s approved procedure and verify the trends before declaring normal service restored. Never defeat temperature protection to perform the test.
Common mistakes and repeat‑failure traps
- Assuming the RTD is correct without cross‑checking. Omitting a suitable independent cross-check can cause a false diagnosis, unnecessary bearing replacement or missed overheating.
- Confusing RTD and thermocouple compensation. An RTD uses a resistance–temperature characteristic, not cold-junction compensation. Check the transmitter’s RTD type, lead-wire compensation and termination integrity.
- Overlooking lead resistance and poor connections. Uncompensated lead resistance can bias an RTD indication, especially in two-wire circuits. Quantify the error using the installed sensor’s resistance–temperature characteristic rather than assuming that milliohms cause several degrees of error.
- Failing to isolate hazardous energy before intrusive inspection. Accidental energization while probing the sensor can cause personal injury or damage to instrumentation.
- Replacing the RTD without verifying the root cause. If the underlying wiring or compensation issue remains, the new sensor will drift in the same manner.
- Using an infrared thermometer without accounting for emissivity. Incorrect emissivity settings produce temperature readings that appear higher or lower than the actual metal temperature.
- Resetting the alarm set‑point without confirming sensor accuracy. This masks the symptom and allows a genuine bearing overheating condition to persist.
Closeout record: Record the RTD element type and wiring, measured resistance and reference temperature, transmitter input configuration, independent measurement location and uncertainty, bearing condition evidence, corrective action, alarm/trip functional-test result, and controlled-restart trend. Mark any unavailable measurements explicitly. Release the equipment only when the responsible plant authority accepts the documented evidence against the applicable OEM and site criteria.
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