A proximity probe is a non-contacting eddy-current sensor that measures the tiny gap between its tip and a rotating shaft, and from that gap it reports the shaft's vibration and its position. On machines that ride on fluid-film bearings, where the shaft floats in an oil film and never touches a casing-mounted sensor, the proximity probe is the primary way to see what the rotor is actually doing. This guide explains how the eddy-current probe and its proximitor work, what gap voltage means, and how the signal feeds a machinery protection system built to the API 670 standard.
Proximity Probe in one line: A proximity probe is an eddy-current displacement sensor that measures shaft position and vibration without touching the shaft, by sensing how a small radio-frequency field is absorbed as the conductive shaft moves nearer or farther from the probe tip. Paired with a signal conditioner called a proximitor, it outputs a DC gap voltage that tracks average distance and an AC component that is the shaft's vibration. It is the standard sensor for radial vibration, axial position, and shaft-speed reference on fluid-film-bearing machines.
A proximity probe is a coil in a small tip that a driver called a proximitor energizes with a high-frequency signal, creating a radio-frequency field that radiates a short distance out of the tip. When the conductive metal of the shaft comes into that field, it develops swirling eddy currents that draw energy out of the field. The nearer the shaft, the more energy is absorbed, so the strength of the field, and the voltage the proximitor derives from it, varies directly with the gap between tip and shaft.
The proximitor turns that into a voltage that has two useful parts. The steady DC level is the gap voltage, proportional to the average distance from tip to shaft, and it tells you the shaft's position, for example how far the rotor has moved axially. The fluctuating AC component riding on it is the shaft's vibration, the rapid movement of the surface toward and away from the probe as the shaft orbits in its bearing. Reading position and vibration from the same sensor is a large part of why these probes are so central to machine protection.
Because the probe is non-contacting and observes the shaft directly, it sees rotor motion that a casing-mounted sensor cannot, which is exactly what is needed on fluid-film-bearing machines where the shaft floats and moves relative to the housing. Each probe is calibrated to the specific shaft material, and installers verify its sensitivity and set a nominal gap so the probe operates in the linear middle of its range, leaving room to see the shaft move in either direction.
Proximity probes are deployed in a few distinct roles. For radial vibration, two probes are mounted at each bearing, usually ninety degrees apart, so their combined signal describes the shaft's orbit in the bearing clearance and reveals imbalance, misalignment, rubs, and instabilities like oil whirl. For axial position, one or two probes watch the end of the shaft or the thrust collar to catch axial movement that would signal a failing thrust bearing.
A third role is the shaft-speed and phase reference, commonly called a keyphasor. Here a probe watches a once-per-revolution feature on the shaft, a notch or a raised key, producing one pulse per turn. That pulse gives exact shaft speed and, just as importantly, a timing reference that lets analysis software tie vibration to a specific angular position on the rotor, which is what makes balancing and phase-based fault diagnosis possible.
Read together, these signals build a full picture of the rotor. Orbit and spectrum from the radial probes, average position from the axial probes, and the keyphasor tying it all to shaft angle let an analyst distinguish an unbalance from a misalignment from a bearing instability. That diagnostic depth is why fluid-film machines are instrumented with proximity probes rather than the seismic sensors used on rolling-element machines.
Proximity probes rarely stand alone. They are the front end of a machinery protection system, and the recognized reference for how such a system is specified, installed, and wired is API 670, the industry standard for machinery protection systems. It governs probe types and placement, proximitor arrangement, cabling, monitor logic, voting for trips, and the whole chain from shaft to shutdown, so that a critical machine's vibration and position monitoring is consistent and dependable across vendors.
The protection monitor turns probe signals into overall vibration and position values, compares them against alarm and trip setpoints, and, on a confirmed exceedance, votes to shut the machine down before damage occurs. That fast protective action is local and self-contained, exactly where it needs to be for a millisecond-critical trip, and it is intentionally kept independent of any supervisory system so that monitoring integrity does not depend on a network link.
Alongside that, the same values are worth trending over the life of the asset, and that is where cloud SCADA fits. Overall vibration, gap and axial-position readings, and alarm and trip status leave the protection system, and a platform like Merobix historizes them across the fleet and alarms operators on a trip. A shaft position that drifts or a vibration level that creeps up shows against its own baseline weeks before a trip, turning what proximity probes see on one machine into fleet-wide reliability insight.
A proximity probe is a non-contacting eddy-current sensor that measures shaft displacement, the actual movement and position of the shaft relative to the bearing, which is what you need on fluid-film-bearing machines where the shaft floats. An accelerometer is a contacting seismic sensor mounted on the casing that measures the housing's vibration, which suits rolling-element machines where casing motion tracks bearing condition. Many critical machines carry both, one watching the shaft and one watching the case.
Gap voltage is the steady DC output from the proximitor, and it is proportional to the average distance between the probe tip and the shaft surface. Technicians set a nominal gap during installation so the probe sits in the linear middle of its measurement range, giving room to see the shaft move toward or away from the tip. Changes in the DC gap voltage indicate the shaft has changed position, for example moved axially, while the AC component riding on it is the vibration.
A keyphasor is a proximity probe aimed at a once-per-revolution feature on the shaft, such as a notch or key, producing a single pulse each turn. That pulse gives precise shaft speed and a timing reference tied to a known angular position on the rotor. Analysis software uses it to relate vibration to shaft angle, which is what makes rotor balancing and phase-based fault diagnosis possible.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.