PT100 (100 Ω platinum, α = 0.00385 Ω/Ω/°C) resistance temperature detectors are the default element for anti-friction bearing alarm loops because they combine IEC 60751-defined interchangeability, Class A accuracy of ±(0.15 + 0.002·|t|) °C per [S4] (JUMO, 2024), and a measurement range that fully covers the standard bearing operating window of 0–120 °C per [S7].
Selection, however, is not a single-part decision. The element class, the lead configuration (2/3/4-wire), the stem construction (mineral-insulated rigid, spring-loaded, or thin-film chip), the connection head, and the alarm trip logic all interact — and a misstep in any one of them shows up as nuisance trips or, worse, missed failures on a pressure sensor-style monitoring channel feeding the PLC cabinet.
PT100 Class and Tolerance Bands per IEC 60751
IEC 60751 splits Pt100 elements into wire-wound (W 0.1, W 0.15, W 0.3, W 0.6) and thin-film (F 0.1 to F 0.6) tolerance classes, with Class A corresponding to ±(0.15 + 0.002·|t|) °C across the applicable range, per [S4] (JUMO, 2024); Class B is roughly twice that, and the AA grade tightens further for calibration work.
Wire-wound elements give better long-term stability and are typically specified where the probe tip sits in a thermowell subject to mechanical vibration; thin-film elements are smaller, faster-responding, and cheaper, but they trade drift performance for thermal mass, per the [S4] class summary. For a single-pass qualification on a critical fan, gearbox, or motor bearing, request the manufacturer's IEC 60751 conformance certificate and confirm the lot was tested at 0 °C and 100 °C — that is the range that actually decides whether the alarm threshold is met cleanly. Where the bearing is on a servo motor or VFD-driven machine, remember that high dV/dt switching noise from the drive can couple into the RTD leads; in that case, a Class A element on a 4-wire run, terminated at a dedicated analog card, is the configuration with the fewest field callbacks.
Alarm and Rate-of-Rise Thresholds for Bearing Applications
Industry practice for grease-lubricated anti-friction bearings is to set the PRE-ALARM at 80 °C and the TRIP at 100 °C, with oil-bath designs shifting the PRE-ALARM up to 95 °C and the TRIP at 100 °C, per [S7] (Industrial Monitor Direct, 2024).
Static setpoints alone are not enough. A second tier of rate-of-rise limits catches developing failures before the absolute temperature crosses the trip line, per the [S1] Muller Beltex / SKF PT100V3C installation guide: the STOP-ALARM fires at a sustained rise of 2.5 °C/min (with 10 °C/min as the maximum allowed slope) or 1 °C in 5 s (with 5 °C/5 s as the maximum), while the PRE-ALARM trips at 0.2 °C/min (10 °C/min max) or 0.7 °C in 5 s (5 °C/5 s max). These slopes are the ones that actually flag a lubrication breakdown, because the absolute temperature of a healthy 70 °C bearing does not change measurably on a 10-minute scan interval — the derivative does. Rate-of-rise logic is implemented either in the PLC fast counter block or in a dedicated safety relay, and the alarm relays should be wired fail-safe (de-energize to trip) so that a broken sensor or severed cable produces a trip, not a clear.
2-Wire vs 3-Wire vs 4-Wire Lead Configurations
A Pt100 is a resistance device, and any lead resistance adds directly to the measured value — a 100 m round-trip of AWG 24 copper is roughly 0.84 Ω, equivalent to about 2.2 °C of error, which is why the wiring topology is part of the sensor specification, not an installation afterthought, per [S3] (Beamex, 2023).
2-wire hookups are acceptable only for short pigtail leads (a few metres at most) where the lead resistance is well below the element's 0.385 Ω/°C sensitivity, per [S3]. 3-wire is the workhorse configuration for bearing RTDs because it compensates for lead resistance using a single return leg, and it is the default build for general-purpose industrial probes, per [S6] (SOR Controls, 2023 catalog). 4-wire (Kelvin) connection is the only topology that fully cancels lead and contact resistance on both legs, and it becomes mandatory once the cable run exceeds 15 m on a Class A channel, per [S7]. The TI PT100 measurement guide [S2] (Texas Instruments SBAA275, 2018 — older reference, cited for measurement principle only) reinforces this: the RTD occupies roughly 20 Ω to 400 Ω across −200 °C to +850 °C, the reference resistor in the front-end must be larger than the maximum RTD resistance, and any uncompensated lead resistance on a 2-wire hookup can consume a significant fraction of the ADC's positive full-scale range. A practical decision rule: stay on 3-wire up to 15 m with AWG 24 or heavier; switch to 4-wire beyond 15 m, on any channel resolving below 1 °C, or on redundant / SIL-rated loops.
Sensor Construction, Stem Rating, and Mechanical Fit
Mineral-insulated (MI) rigid-stem Pt100 probes are the default for bearing housings, with a typical 480 °F (≈249 °C) stem rating on catalog units per [S5] (TC Inc), and 100 Ω platinum / 0.00385 α construction standard per [S6] (SOR); spring-loaded designs are required wherever the probe sits in a thermowell, to keep the sensing tip in firm contact with the well bottom despite thermal expansion or vibration.
High-temperature (PH/PM-series) probes rated to 900 °F (482 °C) per [S6] are over-spec for normal bearing service but useful for bearing housings adjacent to process hot sections — for example, the bearing frame of a pump downstream of a flow meter on a hot oil line, or a fan bearing next to an industrial valve body in a steam header — where ambient soak can approach 200 °C. Bendable MI probes with a stripped-and-banded hot junction allow the tip to be peened into a tapped housing boss for direct measurement of the outer-race housing wall, which is preferred over grease-port immersion when the bearing is a sealed-for-life unit.
Mounting, Cabling, and Incoming Inspection
Incoming inspection on a lot of bearing Pt100s starts with a continuity check on the lowest ohms range — for a Pt100 at room temperature, the reading should land at 100.0 Ω ± the class tolerance, per [S8] (Minco Engineering Instruction 1148, Rev C).
Three additional checks belong in the receiving SOP: (a) insulation resistance at 50 V DC between each lead and the sheath, typically >100 MΩ at room temperature, per [S8]; (b) 0 °C ice-bath verification on a sampled basis — Class A probes read 100.00 Ω ± 0.06 Ω at 0.000 °C, and any reading outside ±0.15 °C equivalent should be rejected; (c) lead configuration verification — ring out each conductor against the wiring diagram so a 3-wire element is not mistakenly landed on a 2-wire transmitter channel (the symptom is a consistent +0.5 to +2 °C offset, not a flat failure). Once installed, route the RTD cable in its own conduit or at least 300 mm away from VFD power cables, terminate with shielded cable and a single-point ground at the PLC cabinet end, and avoid junction boxes in the field — every additional splice adds lead resistance that erodes the 3-wire compensation. The signal then feeds a dedicated RTD input card, or a pressure transmitter-style 4–20 mA head-mount transmitter if the run to the marshalling cabinet is long, and from there into the alarm and rate-of-rise logic above.
Two trackable signals to watch over the next procurement cycle: IEC 60751 amendment activity that may tighten Class A tolerance at the 0 °C anchor, and the growing use of IO-Link PT100 heads that move the linearization and rate-of-rise math into the sensor itself, simplifying the PLC side. Both are flagged in vendor product roadmaps through 2026 and would change the cost trade-off between a discrete RTD plus analog input card and a smart sensor plus bus interface.