{"id":203,"date":"2025-10-31T00:25:51","date_gmt":"2025-10-31T00:25:51","guid":{"rendered":"https:\/\/vibromera.eu\/glossary\/proximity-probe\/"},"modified":"2026-05-23T22:30:43","modified_gmt":"2026-05-23T22:30:43","slug":"proximity-probe","status":"publish","type":"glossary","link":"https:\/\/vibromera.eu\/hy\/glossary\/proximity-probe\/","title":{"rendered":"What is a Proximity Probe (Eddy Current Sensor)?"},"content":{"rendered":"

Understanding Proximity Probes (Eddy Current Sensors)<\/h1>\n\t\t
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\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tPortable balancer & Vibration analyzer Balanset-1A<\/a>\n\n\t\t\t\t\t\t\t\t<\/h4>\n\n\t\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t\t
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€<\/span>1,975.00<\/span> + VAT (if applicable)<\/h4><\/div>\n\t\t\t\t\t\t\t\t\t
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\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tVibration sensor<\/a>\n\n\t\t\t\t\t\t\t\t<\/h4>\n\n\t\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t\t
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\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tOptical Sensor (Laser Tachometer)<\/a>\n\n\t\t\t\t\t\t\t\t<\/h4>\n\n\t\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t\t
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€<\/span>124.00<\/span> + VAT (if applicable)<\/h4><\/div>\n\t\t\t\t\t\t\t\t\t
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€<\/span>6,803.00<\/span> + VAT (if applicable)<\/h4><\/div>\n\t\t\t\t\t\t\t\t\t
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€<\/span>1,735.00<\/span> + VAT (if applicable)<\/h4><\/div>\n\t\t\t\t\t\t\t\t\t
\n\t\t\t\t\t\t\t\t\t\t<\/i><\/a>\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/a>Add to cart<\/a>\t\n\t\t\t<\/span>\n\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/section>\n\t\t\n

A proximity probe<\/strong> \u2014 also called an eddy current probe<\/strong> or a displacement transducer \u2014 is a non-contact sensor that measures the gap between its tip and a conductive target, almost always a rotating shaft. Where an accelerometer<\/a> bolts to the casing and senses how the structure shakes, a proximity probe looks through<\/em> the bearing housing and reports the actual motion of the shaft relative to its bearing. That distinction makes it the primary sensor for protecting and monitoring high-speed, critical machinery that runs in fluid-film bearings, and it is the foundation of shaft-relative vibration monitoring<\/a> on turbomachinery worldwide.<\/p>\n

1. Definition: What is a Proximity Probe?<\/h2>\n

The defining characteristic of a proximity probe is that it measures relative displacement<\/strong> \u2014 the position of the shaft surface with respect to the probe mount \u2014 directly, in micrometres or mils. This is fundamentally different from a seismic sensor such as a velocity transducer<\/a> or accelerometer, which measures absolute motion of the part it is fixed to. On a large machine with a heavy, stiff casing and a comparatively light shaft riding on an oil film, the casing barely moves while the shaft can orbit substantially inside its journal bearing<\/a>. In that situation only a shaft-observing sensor sees the real story, which is why proximity probes dominate machinery protection<\/a> on turbines and compressors.<\/p>\n

2. The Proximity Probe System: Three Matched Components<\/h2>\n

A complete proximity probe measurement chain is built from three precisely matched parts, calibrated together as a set:<\/p>\n

    \n
  1. Probe:<\/strong> a threaded-body sensor with a sealed tip enclosing a flat coil of wire. It is installed with a specific physical gap to the shaft and locked in place.<\/li>\n
  2. Extension cable:<\/strong> a coaxial cable of a defined length that links the probe to the driver. Its length is part of the electronic tuning of the system, not an arbitrary lead.<\/li>\n
  3. Proximitor \/ driver:<\/strong> an electronic module that generates a high-frequency radio-frequency (RF) signal, drives it into the probe coil, and demodulates the returning signal to produce an output voltage proportional to the gap.<\/li>\n<\/ol>\n

    Because the three elements are tuned as a unit \u2014 typically to the industry-standard scale factor of 200 mV per mil (about 7.87 mV\/\u00b5m) \u2014 they are not<\/strong> interchangeable with components from another system. Mixing a probe from one set with a driver or cable of a different length corrupts the calibration and the readings. The total electrical length error is corrected by cable compensation<\/a>, and the assembled chain should arrive with a calibration certificate<\/a> documenting its traceable scale factor.<\/p>\n

    3. How It Works: The Eddy Current Principle<\/h2>\n

    The proximitor sends its RF signal into the tip coil, which radiates a small magnetic field. When the tip is brought close to a conductive shaft, that field induces tiny circulating currents \u2014 eddy currents<\/strong> \u2014 in the surface layer of the shaft material. The eddy currents generate their own opposing magnetic field, and the energy they absorb loads the coil. The amount of energy lost depends on how close the conductive surface is: the nearer the shaft, the stronger the eddy currents and the greater the loading.<\/p>\n

    The proximitor measures this loading and produces two superimposed outputs: a DC voltage<\/strong> proportional to the average<\/em> gap, and an AC voltage<\/strong> proportional to the dynamic<\/em> motion of the shaft as it vibrates.<\/p><\/blockquote>\n

    Because the technique works on induced currents in the metal rather than mechanical contact or light, it is immune to oil, dirt and pressure in the bearing cavity, but it is sensitive to the electrical and magnetic uniformity of the shaft surface \u2014 a point that returns under runout below. The same physics underpins the broader family of eddy current probes<\/a> used for non-contact displacement sensing.<\/p>\n

    4. What Proximity Probes Measure<\/h2>\n

    A single probe \u2014 and especially a pair \u2014 yields a remarkable amount of information about rotor health and behaviour:<\/p>\n