Campbell CR6 Series, Operator'S Manual

Page: 451 / 690

Section 8. Operation
451
Decay Ratio = Ending Amplitude / Beginning Amplitude
Some sensors will decay very rapidly. A good practice is to characterize sensor
decay and amplitude when a sensor is new, so the health of the sensor can be
monitored over time.
Signal-to-Noise Ratio Diagnostic
The spectrum data shown in the figure VSPECT Data
(p. 450)
are derived from the
time-series data through an FFT
(p. 583).
These data emphasize the amplitude of
the natural-resonant frequency, labeled Response Amplitude , and the amplitude
of noise frequencies, labeled Noise Amplitude . The signal-to-noise ratio is
calculated as follows:
Signal-to- Noise Ratio = Response Amplitude / Noise Amplitude
Low Signal Strength Amplitude Warning
When the response amplitude is measured as less than 0.01 mV RMS, the
Resonant Frequency value reports NAN indicating that low signal strength
amplitudes have occurred. The 0.01 mV threshold can be modified in the
VibratingWire() instruction.
Invalid Voltage Supply Warning
A Resonant Frequency value of -555555 is an error code indicating an invalid
voltage supply in the hardware of the CR6, which is an internal problem requiring
factory repair.
Vibrating Wire Quality
The following may improve measurement quality:
• Match frequency ranges to expected frequencies
• Reject noise
•
Minimize resonant decay
• Prevent spectral leakage
Match Measurement Ranges to Expected Frequencies
Measurements are best when the frequency ranges of the swept excitation and of
the response analysis match the range of resonant frequencies expected from the
sensor. The swept and analysis ranges for specific sensors are determined using
the tools in Device Configuration Utility
(p. 148)
software CR6 VW Diagnostics
tab. Once determined, the ranges are then programmed into the CRBasic program
by adjusting the BeginFreq and EndFreq parameters in the
VibratingWire()
instruction.