⚠️ Disclaimer: These tasks are guidelines only. They do not include lockout/tagout (LOTO), energy isolation, or other safety requirements. Review and verify suitability for your specific equipment and application. Add all required safety procedures per your company's policies and regulatory requirements before use. You are responsible for the safe and appropriate execution of all maintenance activities.
Temperature sensors don't announce when they start lying. They drift. They corrode. They develop a loose terminal connection that introduces just enough resistance to shift the reading by a few degrees — and the control system adjusts accordingly, because it trusts the number. Nobody questions a reading that's plausible. That's how bad product gets made, how processes run out of tolerance for weeks, and how failures get traced back to a sensor that was never verified.
Thermocouples and RTDs are low on most PM lists because they have no moving parts. That's the logic that gets you. This checklist is for maintenance teams building or auditing a PM program for temperature sensing instrumentation.
Start with the full sensor and instrumentation framework: industrial sensor and instrumentation PM program
How to Use This Checklist
Record findings with specificity. "Terminal connections inspected" tells you nothing six months from now. "Terminal TS-003 block at J-box 14 — two screws re-torqued, light surface oxidation cleaned, no discoloration" tells you something. Trend your calibration data over time. A sensor that reads 2°F low at one annual calibration and 4°F low at the next isn't passing — it's drifting, and the next reading will tell you where it ends up. A good finding looks like this: "As-found deviation: +3°F at 300°F reference. Adjusted to within ±1°F. As-left recorded." A bad finding looks like this: "Calibration checked — OK."
Field Checklist — Critical Tasks
Visual Inspection
| Task | Freq | Type |
|---|---|---|
| Inspect sensor body and mounting hardware for physical damage, corrosion, or loose fittings. Tighten or flag for replacement as needed. | Every PM | MEC |
| Check sensor wiring and conduit entry for chafing, cuts, or moisture intrusion. Ensure cable is properly supported and protected. | Every PM | ELE |
| Inspect weather protection, sun shield, or enclosure for integrity if sensor is installed outdoors or in a harsh environment. | Semi-Annually | MEC |
| Check sensor well or thermowell (if installed) for corrosion, scale buildup, or mechanical damage. Clean if accessible and condition allows. | Semi-Annually | MEC |
Operational Checks
| Task | Freq | Type |
|---|---|---|
| Verify sensor reading matches the process temperature using a calibrated reference thermometer or known process condition. Note and flag any deviation greater than ±5°F (or per site standard). | Every PM | ELE |
Electrical Inspection
| Task | Freq | Type |
|---|---|---|
| Inspect all terminal connections at the sensor head and junction box for tightness, corrosion, and proper seating. Clean and re-torque as needed. | Quarterly | ELE |
| Verify sensor calibration against a traceable reference standard. Document as-found and as-left readings. Adjust or replace if deviation exceeds site acceptance criteria. | Annually | ELE |
| Confirm sensor type, range, and output signal match the process requirements and control system configuration (thermocouple type, RTD wiring, mA range, etc.). | Annually | ELE |
Reference Checklist — Full Task Library
Visual Inspection
| Task | Freq | Type |
|---|---|---|
| Inspect sensor body, mounting hardware, and process connection for physical damage, corrosion, leaks, or looseness. Tighten fittings or flag for replacement. | Every PM | MEC |
| Check sensor wiring, conduit entry, and cable support for chafing, cuts, pinching, or moisture intrusion. Verify drip loops are in place where required. | Every PM | ELE |
| Check transmitter display or local indicator (if present) for readability and accurate reflection of the measured value. Clean display face. | Every PM | MEC |
| Inspect process isolation fitting or compression fitting for leaks or seeping process fluid around the sensor entry point. Re-seal or replace fitting as required. | Semi-Annually | MEC |
| Inspect thermowell or sensor well for corrosion, pitting, erosion, or scale buildup. Clean interior with appropriate brush or solvent if accessible. Replace if wall thickness is questionable. | Semi-Annually | MEC |
| Inspect outdoor or harsh-environment sensors for weather protection integrity (sun shields, enclosures, purge connections). Confirm enclosure seals and drainage are intact. | Semi-Annually | MEC |
Operational Checks
| Task | Freq | Type |
|---|---|---|
| Verify sensor output reading against process temperature using a calibrated reference thermometer or stable known process condition. Document any deviation. | Every PM | ELE |
| Review historical trend data for the sensor in the DCS/historian. Identify any spikes, drift, or erratic behavior that may indicate a degrading sensor before it causes a process event. | Quarterly | ELE |
| Verify sensor response time by simulating a known temperature change (heat gun or ice bath) if process conditions allow. Sluggish response may indicate thermowell fouling or a failing sensor element. | Annually | ELE |
Electrical Inspection
| Task | Freq | Type |
|---|---|---|
| Inspect terminal connections at the sensor head, transmitter, and junction box for tightness, corrosion, discoloration, or signs of arcing. Re-torque and clean as needed. | Quarterly | ELE |
| Measure loop resistance (RTD) or check millivolt output (thermocouple) and compare against expected values for the measured temperature. Flag if out of range. | Semi-Annually | ELE |
| Verify insulation resistance on RTD leads using a low-voltage megohmmeter. Resistance should be >1 MΩ to ground. Investigate and repair any low readings. | Annually | ELE |
| Perform full calibration verification against a NIST-traceable reference standard. Record as-found and as-left readings with technician and date. Adjust transmitter or replace sensor if deviation exceeds acceptance criteria. | Annually | ELE |
| Confirm sensor type (thermocouple: J, K, T, E; RTD: Pt100, Pt1000), wiring configuration (2-wire, 3-wire, 4-wire), and output range match the transmitter and DCS/PLC configuration. | Annually | ELE |
Parts and Spares
| Task | Freq | Type |
|---|---|---|
| Confirm spare sensors of the correct type, range, and configuration are stocked. Note any low-stock or obsolete-part concerns for procurement follow-up. | Annually | ALL |
Failure Modes This Checklist Targets
Calibration drift — The sensor's output shifts gradually from its true value over time, driven by thermal cycling, contamination, or aging of the sensing element. It doesn't fail. It just stops being accurate. By the time someone notices, the process has been running on bad numbers for months.
Terminal connection corrosion — Oxidation or moisture at terminal blocks increases resistance in the measurement circuit, introducing a temperature offset that looks like a real reading. Thermocouples and RTDs are low-level signal devices. A corroded connection doesn't have to be obvious to corrupt the output.
Thermowell fouling and plugging — Scale, process deposits, or corrosion products inside the thermowell create a thermal barrier between the process fluid and the sensor element. The sensor responds more slowly, reads lower than actual temperature, or in severe cases stops tracking process changes entirely.
Moisture intrusion — Water in a junction box or conduit entry introduces leakage paths that compromise insulation resistance on RTD leads and corrupt millivolt signals on thermocouples. The result is noisy, erratic, or offset readings that are hard to diagnose without a systematic inspection.
Sensor type or wiring mismatch — A thermocouple of the wrong type installed after a field repair, or an RTD wired as 2-wire when the transmitter expects 3-wire, produces a systematic error that reads as a plausible process temperature. Nobody questions it because the number makes sense. This one ends careers.
Physical damage and vibration fatigue — Sensors installed on vibrating equipment, in high-flow pipes, or in accessible locations subject to mechanical contact will develop cracks in the sensing element, loose fittings, or broken leads that produce intermittent or wandering readings.