Accelerometer vs Eddy Current Displacement Probe: Four Ways to Compare Measurement, Frequency Range, Mounting and Contact, Target, Environment and Cost

An accelerometer mounted on the casing or bearing housing measures vibration acceleration; an eddy current displacement probe mounted beside the shaft, without touching it, measures the shaft’s displacement relative to the probe. Sources say the former looks at high frequencies and casing vibration, the latter at low frequencies and the shaft’s position and motion. The difference is whether you measure how the casing moves, or how the shaft moves inside its bearings.

In this article, “accelerometer” means what the sources call an accelerometer, and “eddy current displacement probe” means an eddy current probe (also called a proximity probe or displacement probe). Velocity sensors are outside its scope, and this is not about proximity switches that detect whether an object is present. Sources are listed in the notes at the end; this article does not replace sensor makers’ data sheets. For overall selection, see How to choose a vibration sensor, whose stage 1 covers measured quantity and sensing principle; this article uses a different set of sources.

Aspect 1 | What they measure: the probe measures relative shaft displacement, the accelerometer casing vibration

How to look at it

• First write down: do you want to see how the shaft moves in its bearings, or the vibration transmitted by the casing and bearing housing?
• Source 5 covers only the probe side; source 3 discusses non-contact displacement sensors in general, not only on machinery.

What public sources say

• A vibration monitoring instrument company’s website article (undated, unsigned, promotional; source 1) says probes measure relative shaft vibration and displacement without contact; accelerometers are good at detecting vibration of the casing itself over a wide frequency range; probe data directly show the shaft’s position and motion; accelerometer data often need integrating and provide full spectrum analysis.
• A university course handout (undated, individually signed, educational; source 2) says probes measure static and dynamic displacement.
• A displacement sensor company’s application note (May 2013, unsigned, promotional; source 3) says displacement sensors output a displacement measurement; to see “inertial vibration” involving kinetic energy and force — for example monitoring motors, especially bearings, in critical applications — acceleration is what to measure; accelerometers are a good choice for large objects and higher-frequency vibration.
• A plant equipment magazine article (1 March 2008, editorial byline, with the author at the end being a sensor company’s application engineer; promotional; older; source 4) says an eddy current probe indicates the shaft’s position relative to the casing, which is not the same as the casing vibration measured by an accelerometer mounted on the casing.
• A turbomachinery magazine article (7 November 2019, from an engineer’s conference paper; technical, not promotional; probes only; source 5) says shaft displacement probes measure rotor or shaft displacement, which is distinctly different from bearing or casing vibration measurement.
• An instrumentation website article (undated, unsigned, naming specific product series; source 6) says eddy current sensors measure displacement (shaft movement), and accelerometers measure acceleration (high-frequency casing faults) (notes, table 1).

Aspect 2 | Frequency range and low frequency: sources say probes are accurate at low frequency with an upper limit; accelerometers are good at high frequency and struggle at low

How to look at it

• First write down: the machine’s speed, and whether you want to catch slow position changes or high-frequency local faults.
• The sources do not say how low “low frequency” is.
• On the probe’s upper frequency limit, sources 1 and 3 cover different scopes.

What public sources say

• Source 1 says probes typically go up to about 10 kHz and perform well at low frequencies; accelerometers may struggle with low-frequency measurement.
• Source 2 says probes respond accurately at low frequency but are limited at high frequency; accelerometers respond well at high frequency, but piezoelectric accelerometers are unreliable at very low frequencies; its table also lists accelerometers as sensitive to high-frequency noise.
• Source 3 says displacement sensors have frequency response above 10 kHz and up to 80 kHz, flat from DC to near the rated frequency; accelerometer signals are small at low frequency; for higher-frequency vibration, acceleration is larger than at low frequency.
• Source 6 says eddy current sensors are used for low to medium frequencies, speed and large slow shaft movements; accelerometers for high frequencies, such as early faults (notes, table 2).

Aspect 3 | Mounting and contact: the probe keeps a gap to the shaft and is harder to mount; the accelerometer mounts on a surface with more flexible placement

How to look at it

• First write down: can the probe be mounted beside the shaft; is the accelerometer’s mounting position secure?
• Accelerometers must be mounted securely (source 1), and mounting one changes the object’s mass (source 3).

What public sources say

• Source 1 says probes don’t touch, so sensor and shaft wear less; they need careful installation to keep a constant gap to the shaft; accelerometers can be placed more flexibly but must be mounted securely to measure accurately.
• Source 2 lists probes as “difficult to install” and “no wear”, accelerometers as “easy to install”, and both as needing external power.
• Source 3 says displacement sensors leave a small gap to the surface and don’t change the object’s mass or resonance; accelerometers mount directly on the surface and change the object’s mass.
• Source 4 says probes are precisely installed through holes in the machine casing.
• Source 5 says the probe tip should be at least three tip diameters from a side wall.
• Source 6 says eddy current sensors face the rotating shaft directly, without contact; accelerometers mount on the machine casing or bearing housing (notes, table 3).

Aspect 4 | Target, environment and cost: the probe needs a clean conductive surface; views on cost cover different scopes

How to look at it

• First write down: shaft material and surface condition, site environment and budget.
• On probe cost, source 1 discusses the system and source 2 only the probe, without stating what it is compared with; they cover different scopes.
• Only sources 1, 2, 3 and 5 discuss the target.
• On high temperature, sources 1 and 2 cover different scopes.

What public sources say

• Source 1 says probes are particular about the measured material, preferring clean, conductive surfaces; probe systems usually have a higher initial cost; accelerometers and velocity sensors are more budget-friendly and versatile; piezoelectric sensors are usually used on machines with casing temperatures below 120 °C, and for higher temperatures (up to 375 °C) moving-coil velocity sensors are usually used instead.
• Source 2 lists probes as “not calibrated for unknown metal materials” and “small and low cost”, and accelerometers as “withstand high temperature”; another section says charge-output (high-impedance) piezoelectric accelerometers are also used at high temperatures above 120 °C, while low-impedance types cannot be.
• Source 3 says an advantage of eddy current types is use in fairly harsh environments; non-conductive materials are invisible to eddy current sensors.
• Source 5 says scratches, corrosion and other flaws on the target surface are read as displacement; residual magnetism causes similar errors; materials other than 4000-series steel need separate planning at the design stage (notes, table 4).

Comparison table

How to choose

Based on public sources, the choice can be divided like this (grouping the sources’ statements into four situations is this article’s summary):

• Look at an accelerometer when: you need casing or bearing housing vibration (sources 1, 4, 6); you want to catch high-frequency faults (source 6); you want easy mounting and flexible placement (sources 1, 2); the budget is limited (source 1).
• Look at an eddy current displacement probe when: you need the shaft’s position and displacement (sources 1, 2, 4, 5, 6); low frequency, static position, speed and slow movement (sources 1, 2, 3, 6); no contact with the shaft and no added mass (sources 1, 2, 3); the shaft is a conductive metal with a clean surface (sources 1, 2, 3, 5).
• Check carefully: the two do not measure the same thing (sources 4, 5); for hot sites, source 2 says accelerometers withstand high temperature (charge-output types above 120 °C) while source 1 says piezoelectric types usually run below 120 °C casing temperature with velocity sensors above — different scopes; probe cost and upper frequency limit are covered differently by the sources; source 2 is a university handout and source 5 a conference paper reprinted in a magazine, while the rest are makers or maker staff; source 5 covers only probes; sources 2 and 6 are undated; source 4 dates from 2008.
• Near the boundary: when you need both shaft position and casing vibration, or both low and high frequency, the sources give no dividing line (such as a speed or a frequency below which counts as low); follow the sensor maker’s data sheets and measurements on the machine.

These points summarize public sources; they are not a conclusion about your machine. Which to choose depends on whether you need the shaft or the casing, the speed and the installation space.

How JIAHON can help

Accelerometers:

• Vibration accelerometer — the page is labelled “vibration accelerometer”; please ask about availability.

Eddy current displacement probes:

• Vibration displacement probe — the page is labelled “vibration displacement probe” and “eddy current”; please ask about availability.

To match a type, please prepare six items: whether you need the shaft or the casing; the machine speed and the faults to catch; mounting position and available space (whether a probe can face the shaft); shaft material and surface condition; site temperature and environment; and your current sensor and the problem you are seeing. Then contact JIAHON or email [email protected].

Notes: sources and limitations

These notes are for readers who want to check the sources. Where sensor makers’ written specifications, or your machine, speed and environment, differ from this article, follow them.

Limitations

• Of the six sources, only source 2 (handout) and source 5 (reprinted conference paper) are not by makers or maker staff; no neutral direct comparison of accelerometers and eddy current probes was found, nor any Taiwanese price, supply or vendor data, nor any side-by-side measurement on the same machine. Items not listed in the notes tables were not seen in what was read, which does not mean the originals omit them.
• Figures used: probes typically up to about 10 kHz (source 1); displacement sensors above 10 kHz and up to 80 kHz (source 3); probe tip at least three tip diameters from a side wall, and 4000-series steel (source 5); piezoelectric types usually below 120 °C casing temperature, higher temperatures up to 375 °C (source 1); dates of each source. Units are as given; nothing was converted.
• Sources differ: on probe cost, source 1 says probe systems usually have a higher initial cost, while source 2’s table lists “small and low cost” without a comparison; on probe upper frequency, source 1 says about 10 kHz and source 3 above 10 kHz up to 80 kHz, source 3 covering non-contact displacement sensors including capacitive; on accelerometer high temperature, source 1 says piezoelectric types are “usually” below 120 °C, while source 2 lists “withstands high temperature” and elsewhere says charge-output types are used above 120 °C. These cover different scopes and do not directly conflict; they are listed side by side, not combined into one rule.
• Single-source points: side wall distance, target flaws, residual magnetism and non-4000-series steel (source 5); high temperature, high-frequency noise and charge-output types above 120 °C (source 2); piezoelectric below 120 °C and moving-coil velocity sensors above (source 1); accelerometers adding mass and displacement sensors not changing mass or resonance (source 3); probes installed through holes in the casing (source 4).
• Older material: source 4 is from 2008 and source 3 from 2013, so follow current product data; sources 2 and 6 are undated; source 5 is from 2019.
• Positions: sources 1, 3, 4 and 6 are makers or maker staff; source 2 is a university handout and source 5 a reprinted conference paper; none is a standard or neutral test report, and sources 1, 3 and 4 are promotional. Sources are in English.
• Scope: velocity sensors and presence-detecting proximity switches are out of scope, as are model cross-references, prices and lead times. This is a selection overview, not a sensor design document, and does not say which type is “better”.
• “How JIAHON can help” uses only names and wording from our product pages, not their specifications.
• This article’s summary: “How to choose” groups the sources’ statements and states that the sources give no speed or frequency boundary; the “How to look at it” points are practical steps, not the sources’ statements; the opening and the comparison table are summarized from the aspects and sources as noted.

This is a general overview, not a restatement of any standard, and not an endorsement of any maker, equipment builder or service provider, nor an assurance about your machine. Follow sensor makers’ written specifications, your machine and site conditions, and your own verification; individual cases vary.