Understanding Accelerometer Specifications - The Basics
Accelerometer datasheets include a tremendous amount of information. Here is a quick guide to understanding the basics. For the purposes of this guide, CTC’s AC292 Series compact, premium accelerometer specifications will be used:
Sensitivity
Sensitivity is the nominal output value of the sensor, generally specified in mV/g (millivolts/g). It is usually accompanied by a tolerance value. In the example of the AC292, the sensitivity is nominally 100 mV/g. The tolerance value is expressed as a percentage of the overall nominal sensitivity. The sensitivity tolerance of the AC292 is +/- 5%. This means that the sensor's actual output when sensing a vibration of 1 g can be anywhere from 95 millivolts to 105 millivolts. All CTC sensors are sent out with a calibration sticker showing the actual tested output from the sensor when shaken at 1 g at 100 Hz.
Frequency Response
On a premium sensor like the AC292 Series, with a sensitivity tolerance of +/-5%, the frequency response range may be specified in three different steps: 5%, 10%, and 3 dB. As industrial vibration analysis depends on trending of values to determine when a machine condition is worsening, exact measurements are typically not required. The frequency response values are normally acceptable for use even at 3 dB. The frequency response of our AC292 sensors is +/-5% from 10 Hz to 5000 Hz (600 to 30000 CPM). This is the “sweet spot” of the sensor where the readings are most accurate; it is also the frequency band where the majority of all vibration readings are taken in the industrial world. The +/-10% value allows the analyst to understand when the data may be a little less precise than the 5% range. As mentioned above the +/-3 dB response is still acceptable for trending analysis, but some do not quite understand the logarithmic scale, to clarify +/-3dB can be interpreted as a +29%/-41% range.
Dynamic Range
Dynamic range is the maximum vibration the accelerometer can accurately sense before the electronics of the sensor are saturated with too much signal for the amplifier board to process. When the sensor becomes saturated by the excessive vibration, the amplifier board effectively turns itself off and on and resets itself if the vibration has dropped below the saturation level. Analysts should ensure that they determine the Maximum frequency response required before selecting their sensors.
