Could Accelerometer Drift be Masking Increased Vibration?
Adjusting Analyzer Input Settings to Match Transducer Output
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No matter how sophisticated the portable vibration analyzer, each is reliant upon a piezoelectric accelerometer. The same way tires are the only aspect of an automobile design that actually touches the road, the accelerometer is the only component in route-based vibration monitoring that touches the machinery. The accelerometer is also the only mechanical part of the measurement chain. The piezoelectric crystal inside is subject to dings, drops, slams, shock and sometimes temperature variation. Hundreds, if not thousands of vibration data points rely upon the consistent output of the accelerometer.
Inside the sealed structure of the accelerometer is a quartz or ceramic piezoelectric element from which charge is produced when vibration is present. No matter the sensor design the piezoelectric element is always compressed by a pre-load. Loosening of the pre-load causes sensor sensitivity drift in the downward direction, i.e. reduced output. Quartz accelerometers are naturally piezoelectric and more stable over time. However most industrial accelerometers are actually made of a ceramic element.
Using a NIST-traceable portable shaker, one can ensure accuracy of vibration trend data collected with a portable analyzer and prevent sensor drift from hiding rising vibration levels. In this video we explain how to use a portable shaker to first test accelerometer sensitivity and then adjust the analyzer to match. Finally the accuracy of the analyzer is confirmed with a system test.
The test in the aforementioned video is only done at 1800 CPM (30 Hz). But if the test were conducted at six frequencies instead of just one it would meet the requirements of ISO 16063-21 “Vibration Calibration by Comparison Method.”