The information provided is for help with Commtest Instruments products:
QUESTION
What is the accuracy of the low frequency (slow speed) response of the instrument?
ANSWER
We specify the vb instrument accuracy down to 3 Hz. The +/- 0.5 dB spec equates to about +/- 5%. You can take readings below that frequency but they will not be calibrated. This region includes the corner frequencies of both the AC coupling filter and the velocity integration filter, and hence is subject to variation between instruments and with temperature. As a guideline for this sub 3 Hz region, we have tested the velocity readings of standard vb instruments at room temperature with small signals. Typical results are indicated in the table:
|
Frequency |
Velocity |
vb Reads |
% of Correct |
|
1 Hz |
1.56 mm/s |
1.06 mm/s |
68% |
|
2 Hz |
0.78 mm/s |
0.72 mm/s |
92% |
|
3 Hz |
0.52 mm/s |
0.516 mm/s |
99% |
|
|
|
|
|
Please note that this is an indication of the response, not a calibration.
At these frequencies it is better to use the velocity or displacement readings, and not acceleration. The CTC AC102-1A accelerometer is specified as having a +/- 3 dB response between 0.5 Hz and 15 kHz.
At 0.5 Hz (30 CPM) its response could be anything from 71% to 141% of the correct (calibrated) value.
At 1 Hz (60 CPM) its response is within +/- 10% of correct.
At 1.7 Hz (102 CPM) its response is within +/- 5% of correct.
See http://www.ctconline.com for more information.
If you are analyzing a bearing for faults, then bearing frequencies etc,. will be at higher frequencies than the running speed, so they are more likely to be within the calibrated region. Demodulation is also particularly useful for detecting bearing faults of low speed machines. On slow speed machines some people advocate the use of time domain measurements where you look at the time differences between impact events in the time record and compare that with fault frequencies.
It is possible to balance very slow speed machines using the instrument (e.g. 30 RPM), even though the reading may not be absolutely accurate. This is because all balance runs are made at the same speed and the balancing algorithm analyzes the effect of fitting the trial weight. A low sensitivity at the running speed will affect all of the Initial, Trial and Balance runs to the same extent, so is effectively self compensating.
FAQ ID: 11598 Last Reviewed: 28/07/2003