Campbell Scientific AVW200-series Vibrating Wire Interfaces - Manual (Page 89)

Appendix E. Status Table
F.2 Good Sensors with Noise
The measurements graphed in FIGURE F-3 and FIGURE F-4 are made by the
same sensor used for FIGURE F-1 and FIGURE F-2. However, for FIGURE
F-3 and FIGURE F-4, a drill is running about ½ inch away from the sensor.
This shows the effects of narrowing the begin/end frequency to deal with noise
generated by an electric motor. The narrow frequency range in F.2-1 has
reduced the effects of the noise source and has yielded a signal-to-noise ratio of
4 times better than F.2.-2. Notice that if the begin frequency in FIGURE F-4
was much less than 450 Hz, the 60 Hz harmonic would have been the dominate
frequency.
Holding the drill ½ inch away from the sensor is an invasive noise source.
When the sensor is measured with the drill a few inches away, the harmonics
of the 60 Hz are a lot less and are not more dominate than the wire’s natural
frequency. Sensors with a frequency range that are below 450 Hz should work
fine even in the presence of a 50 or 60 Hz noise source, however they should
be characterized.
FIGURE F-3. Good Sensor with Noise (600 to 1800 Hz)
F-3