Which Bently Nevada Vibration Sensor to Choose: 3300 XL Proximity Probe, Velomitor, Accelerometer or 177230 Transmitter

Bently Nevada makes dozens of sensor types, and "which vibration sensor should we use" is one of the most common questions we get. The answer depends on three things: what bearings the machine has, what you need (protection, diagnostics or just a signal into the control system) and which frequencies matter.

Below is how the choice works and which typical solutions work in practice.

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The main question: fluid-film or rolling element bearings

Fluid-film bearings (turbines, large compressors, generators, large pumps). The oil film damps rotor vibration, and only a small part reaches the casing. You need to measure shaft vibration relative to the bearing, which is the job of a proximity probe looking at the shaft without contact. The standard for such machines is the 3300 XL 8 mm system.

Rolling element bearings (general-purpose pumps, fans, motors, gearboxes). Rotor vibration transmits well to the casing, so casing vibration is measured with a velocity sensor or accelerometer.

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Selection table

Sensor	Measures	Frequencies	Typical task
3300 XL 8 mm	Shaft displacement, gap	Low and medium	Fluid-film bearings, thrust position, phase reference
3300 XL 25 / 50 mm	Large axial movement	Low	Turbine differential expansion
Velomitor 330500	Casing velocity	4.5 Hz – 5 kHz	General vibration of machines on rolling element bearings
330400 / 330425	Casing acceleration	10 Hz – 15 kHz	Gearboxes, high-frequency defects, API 670
200150 / 200155 / 200157	Casing acceleration	2 Hz to 10 kHz (by model)	Online fleet monitoring with Trendmaster and 1900
177230	Velocity → 4–20 mA	10 Hz – 1 kHz (RMS)	Vibration straight into a PLC/DCS without a monitor

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Velocity or acceleration

A velocity sensor (Velomitor) is a good choice for overall condition: most machine vibration standards are in mm/s, and the 4.5 Hz – 5 kHz range covers unbalance, misalignment and looseness. The Velomitor is a piezoelectric sensor with embedded electronics and no moving parts that mounts in any orientation. Operating temperature −55 to +121 °C.

An accelerometer is needed when high frequencies matter: gear mesh and early rolling element bearing damage. The 330400 is designed to address API 670 requirements for accelerometers: 100 mV/g, 50 g peak, up to 15 kHz. For machines with high amplitudes there is the 330425 (25 mV/g, 75 g).

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Monitor or straight into the control system

With a monitor (3500, Orbit 60, 1900/65A) the sensor provides the full dynamic signal: the monitor calculates parameters, applies setpoints, drives protection relays and passes data to System 1 for diagnostics.

Without a monitor, the 177230 seismic transmitter outputs 4–20 mA proportional to RMS vibration velocity and connects to a PLC analogue input like any other transmitter. Its dynamic acceleration output lets you connect a portable analyser when needed. This is a sensible route for dozens of non-critical pumps and fans.

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Typical solutions

Machine	Solution
Steam turbine	3300 XL 8 mm (X–Y at each bearing, thrust position, phase reference), 3300 XL 25/50 mm for DE, 3500 or Orbit 60 monitor
Centrifugal compressor with speed increaser	3300 XL 8 mm on shafts, 330400 on the gearbox casing
General-purpose pump or fan	Velomitor or 200150 with a 1900/65A monitor, or 177230 into a PLC
Fleet of hundreds of auxiliary machines	200150 / 200157 with Trendmaster ProTIM and System 1

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Selection mistakes

• A casing sensor on a fluid-film bearing machine. Shaft vibration may rise several-fold while the casing shows almost nothing.
• A velocity sensor for a gearbox. Mesh frequencies are above its working range.
• Choosing by name rather than order code. Thread, cable length and hazardous area approval are set by options.

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Summary

Sensor selection starts with the bearings: fluid-film means a 3300 XL proximity probe, rolling element means a Velomitor or accelerometer. High frequencies and gearboxes need the 330400 accelerometer. If you just need a signal into the control system, use the 177230 transmitter. For a fleet of auxiliary machines, the 200150 with Trendmaster.

KEG TRK is an official business partner of Bently Nevada in Kazakhstan. Official PDF datasheets are published on the sensor pages. We will help select a sensor by order code for your machine; more in the article on working with an official partner.