Monnit AVM Crest Factor Below 1.41: Check Accelerometer Range Before Accepting the Trend
A falling crest factor on a vibration dashboard can look reassuring. On a Monnit ALTA Advanced Vibration Meter, however, a value below 1.41 deserves a measurement check before anyone interprets it as an improvement in machine condition. The configured accelerometer range may be too small to capture the actual acceleration peaks.
The practical answer is to preserve the original readings, check the range and mounting, and compare fresh measurements under repeatable operating conditions. Do not change maintenance decisions solely because a single dimensionless number has fallen.
What crest factor tells you
Crest factor is peak acceleration divided by RMS acceleration. Both quantities describe acceleration from the vibration signal; their ratio has no unit. A change in that ratio can reflect a change in waveform shape, so it needs to be considered alongside vibration amplitude, operating conditions and the measurement settings.
Monnit’s Advanced Vibration Meter configuration and installation guide specifically lists a crest factor below 1.41 as a possible result of clipping when the accelerometer range is lower than the vibration signal. Its troubleshooting advice is to increase the accelerometer G-range. This is a diagnostic clue for this instrument, not a universal healthy-machine boundary or proof that clipping is the only possible cause.
The ALTA Advanced Vibration Meter user guide explains the trade-off: a smaller range provides finer resolution at low amplitudes, while a larger range captures higher acceleration with less resolution. Choosing the smallest available range without checking the actual peaks can therefore undermine the measurement.
The pictured Monnit Wireless Advanced Vibration Meter 2 with 10 Foot Lead is a specific catalogue configuration. Confirm the exact sensor, firmware, regional radio option and matching gateway before applying settings guidance to a supplied unit.
Why a clipped signal can look less peaky
An accelerometer measurement has a finite range. If the incoming motion exceeds that range, the measurement cannot faithfully preserve the highest peaks. The reported data then represent a distorted signal rather than the complete motion that occurred at the sensing point.
For a simple mathematical illustration, a pure sinusoidal acceleration has a crest factor of √2, approximately 1.414. A hypothetical measured peak of 1.8 acceleration units divided by an RMS value of 1.5 in the same units gives 1.20. That calculation explains the ratio only: it is not a test result, a machine acceptance limit or evidence that a particular bearing is healthy.
Nor should a low overall velocity figure be used by itself to dismiss an acceleration-range issue. Record which vibration mode and units are displayed, and investigate the accelerometer configuration that produced the data.
A controlled check that preserves useful evidence
Save the as-found configuration and several readings. Include sensor identity, timestamp, axis, vibration mode, units, accelerometer range, frequency limits, sample rate and report mode.
Record the machine condition for those readings: speed, load, process state and any start-up or stopping activity. A range comparison means little if the machine changes operating condition between observations.
Check the accelerometer cube’s mounting under the site’s safe access and isolation arrangements. Record location and axis orientation. Keep that arrangement consistent throughout the comparison.
Have the responsible person select a larger supported accelerometer range appropriate to the observed motion. Keep unrelated analysis settings unchanged so the comparison has a clear purpose.
Verify that the revised configuration has reached the sensor before labelling subsequent readings as results from the new range. Then collect several fresh readings at comparable speed and load.
Review amplitude, crest factor and repeatability together. If clipping remains suspected, or the readings are inconsistent, escalate to an appropriately equipped vibration specialist rather than repeatedly changing settings until the display looks comfortable.
Monnit’s installation guidance stresses a rigidly mounted cube. A loose mounting can introduce a separate measurement problem, so it should not be disguised by changing the range. Mounting work belongs in the site’s normal maintenance process; do not reach through guards or attach a sensor to moving equipment.
Keep the old baseline separate
A change in range creates a configuration boundary in the trend. Mark when the new setting became effective and retain the earlier records with their original settings. Do not overwrite the baseline or quietly merge both periods as though the measurement arrangement were identical.
In a hypothetical commissioning job, a pump may show persistently low crest factors on one axis while using a low accelerometer range. A controlled range increase is followed by more consistent readings at the same operating condition. That supports further assessment of the original range choice; it does not establish that the pump has improved or that mechanical inspection is unnecessary.
Before handover, document the final configuration, mounting point, comparison readings and reasons for the chosen range. Agree how future configuration changes will be recorded and who will assess abnormal trends. This monitoring workflow does not establish a certified machine-protection function or replace application-specific inspection and validation.
Need help with your application?
Send the sensor model, configuration and example readings to ProSense Instruments at sales@prosenseinstruments.com.au or call 1800 560 854.
For complete monitoring projects, contact ProSense Solutions at sales@prosense.com.au. Explore ProSight Software or email info@prosightsoftware.com for software enquiries. Sensor integration and application suitability must be confirmed separately.
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