PT100 RTD temperature sensor — Failure Modes & Failure Rate

Platinum-resistance thermometer with 100 Ω resistance at 0 °C following the IEC 60751 curve. The most common process-temperature sensor — used for over-temperature trips, control loops, and machinery-protection skin-temperature monitoring. λ is dominated by lead and termination failures rather than the platinum element itself.

λ typical

3.0×10-7 / h

Range

1.0×10-7 – 1.0×10-6

Source

OREDA

Failure modes

Lead / connection failure

Root causes: Vibration fatigue at the head termination; corrosion of crimped or screw-terminal junctions; thermal cycling fatiguing the lead-to-element joint.
Detection: Open-circuit reading drives the transmitter to NAMUR alarm current; intermittent fluctuations on the trend chart.
Mitigation: 4-wire connection eliminates lead-resistance error and makes single-lead failure detectable; spring-loaded thermowell mounting reduces vibration coupling; head-mounted transmitter at the sensor reduces lead length.

Calibration drift

Root causes: Thermal hysteresis after exposure beyond calibration range; mechanical stress on the platinum film/wire; long-term annealing of the platinum at high service temperatures.
Detection: Comparison against a calibrated reference (dry-block or boiling-point bath); HART self-diagnostic on smart transmitters; redundant-sensor disagreement check.
Mitigation: Specify Class A (IEC 60751) for tighter accuracy class; periodic calibration check at proof-test; avoid sustained operation at the upper temperature limit.

Sheath / thermowell corrosion

Root causes: Process-medium attack on the thermowell or sheath; flow-induced vibration cracking the thermowell; chloride pitting in stainless thermowells.
Detection: Process leak through the thermowell; sudden temperature reading anomaly when the sensor is wetted by escaping process; periodic thermowell inspection.
Mitigation: Thermowell material match to process; vortex-induced-vibration analysis per ASME PTC 19.3 TW; periodic ultrasonic thickness measurement in corrosive service.

Element short-to-sheath

Root causes: Insulation breakdown from moisture ingress; mechanical damage to the element insulation; manufacturing defect surfacing under thermal cycling.
Detection: Insulation-resistance test below threshold; reading shifts in a pattern correlated to humidity or temperature.
Mitigation: Hermetically-sealed sheath construction; periodic insulation-resistance check; redundant-sensor cross-check.

Typical applications

Over-temperature shutdown trips on compressors, motors, transformers; process-control temperature loops; bearing-temperature monitoring on rotating machinery; reactor / vessel skin-temperature monitoring; flue-gas temperature in burner-management systems.

How to model in a fault tree

PT100s are usually configured to NAMUR NE 43 alarm currents at the head transmitter, so an open-circuit element fails to a safe state for a high-trip function but to a dangerous state for a low-trip function — the FTA needs to be explicit about which side is dangerous. For redundant 2oo3 thermocouple/RTD voting, model each sensor as its own basic event and apply Beta-factor CCF for the shared thermowell or shared cable run that defeats independence.