Accelerometers are simply a piezoelectric crystal with a mass/spring pre-load that creates a voltage proportional to acceleration when subjected to acceleration. This signal is very faint, measured in pico-volts and requires amplification by an internal electrical circuit embedded in the accelerometer case.

Because of the piezoelectric accelerometers design, there is usually insignificant internal mechanical motion, making accelerometers virtually immune to mechanical wear. However, excessive mechanical shock from hard impacts or dropping on hard surfaces can destroy the internal electronic components or fracture the piezoelectric crystal.

Accelerometers generally have the widest frequency range and certainly the highest. However, vibratory response and electrical signal levels are limited at lower frequencies. Accelerometer outputs are usually tens or hundreds of mV-per-g, peak (1g = 386.1 in/sec2 = 9.8 meter/sec2). The most common output is 100mV/g.

Velocity transducers, or velometers, consist of a movable fine-wire coil in a magnetic field which generates a voltage by the Hall Effect principal of physics. The voltage generated is proportional to the velocity of the coil motion. Velometers do not usually require external power as signal levels are usually great enough to be used without pre-amplification. Since there are moving parts, velocity transducers are more prone to wear and mechanical damage than the other vibration transducers.

Velocity transducers have a frequency range lower than accelerometers, and the signal levels cover a range generally useful in machinery applications. Sensitivities are usually mV-per-inches-per-second (ips.), peak, with the standard being 100mV/in/sec.

Proximity probes, also known as eddy probes, are constructed of a small, flat "pancake" fine wire coil covered by a thin, nonconductive, protective sheath such as fiberglass or Ryton. A high frequency energizing current creates an electromagnetic field around the coil areas which is sensitive to position and vibration of metals near it. To interpret the signal generated by metals entering these fields, an oscillator/ demodulator and energizing power source (together called a ‘driver’) is required, as well as precisely matched extension cable between the probe and driver.

The proximity probe is the only commonly used non-contact vibration transducer. Non-conductive materials such as air, gas, and fluid between the probe and metal surface have no effect on the signal. However, surface conditions (finish, type of alloy, etc.), non-symmetry (run-out) and other characteristics of the metal surface being sensed can influence the signal for the driver. Wear is not normally a consideration in proximity probes, but the coil sheath is very thin and may be damaged by accidental impact.

Proximity probes have a very wide frequency range and also the lowest of the three transducer types. Sensitivities are usually on the order of 100 – 200 mV per mil (1 mil = 0.001 inch) peak-to-peak displacement. The gap distance between the probe coil and sensed surface is usually limited to about 0.1 – 0.2 inch or less, but systems with much wider probes can sense much greater distances.

Another important criteria in selecting a vibration transducer is the type of machinery being monitored. Machinery with blade or vane elements; such as pumps, fans, compressors, and turbines, generate vibration signals at frequencies equal to the number of blades/vanes times the running speed. For example a 4 blade fan will have a peak in the spectrum at 4X running speed. Accelerometers are often required in these instances because they can pick up these higher frequencies that are out of range for a velometer. The amplitude and utility of these signals are strongly influenced by the relative mass of the blade compared to the shaft. For example fans and pumps with relatively massive blades and vanes may yield much data in this frequency range. But, turbine and compressor blades with relatively small mass with respect to their large shaft mass often yield vibration signals which may be less significant and difficult to sense and measure.

The most useful and practical source of vibration signals is from bearing areas. Journal bearings are excellent applications for proximity probes since it is difficult to measure shaft vibration when the shaft is not in contact with the journal bearing, the layer of lubricant dampens many of the vibration signals. The proximity probe is able to detect shaft vibration directly from the shaft. Antifriction bearings, roller bearings,  generate useful vibration signals at running speed and much higher frequencies associated with ball-pass frequencies, which is the number of ball/roller elements times the running speed, and inner/outer race and cage related frequencies. These signals indicate antifriction bearing conditions as well as other vibration signals transmitted from other machine related sources. Accelerometers and sometimes velocity transducers are the usual choice with anti-friction bearings due to frequency response and difficulty in mounting proximity probes.

Vibration transducers are rated for the frequency range over they provide accurate data. This is often expressed as +/- some percentage or ‘dB’ (decibel) figure. The transducers response should be relatively linear from about 1/2 to a few times the machinery running speed frequency. Also check that the transducer is not resonant in this range.

Vibration transducers are also rated as to output. This is expressed as mV per vibration unit (mil, ips, or g). When the vibration levels of interest and the corresponding transducer voltage output levels result in signals under 5 mV, electrical noise and grounding of the entire electrical circuit must be carefully reviewed. Often it is more practical to select a transducer that will provide a higher output to avoid the noise problems.