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9.7.5.1 Harmonics
Some sensors have very strong harmonics that can exceed the amplitude of the fundamental
resonant frequency; this results in measurement error. You can eliminate harmonics by adjusting
the frequency boundaries. By putting the harmonic outside the range of allowed frequencies, it is
eliminated as a source of error. Also, spurious noise sources can be excluded from the
measurement range by selecting appropriate range boundaries.
If adjusting frequency boundaries does not reduce error, force the VWIRE 305 to re-assess the
primary frequency by either disabling then re-enabling the input channel or by resending the
CRBasic program to the data logger.
9.7.5.2 Estimated frequency range
The VWIRE 305 filters unwanted frequencies, such as harmonics, from vibrating wire output using
the operator-entered low-frequency boundary and high-frequency boundary. The range
bounded by these estimates is the estimated-frequency range. Enter boundary estimates into
DVW Toolbox low-frequency boundary and high-frequency boundary columns or CRBasic
CDM_
VW300()
instruction
LowFreq
and
HighFreq
parameters. The best source for boundary
estimates is the sensor manufacturer — perhaps in a calibration report.
9.7.5.3 Actual frequency range
Due to the mathematical constraints of the algorithms, the VWIRE 305 cannot usually comply
with the exact boundary values specified by the user. It establishes the actual-frequency range,
which is bounded by the actual-low frequency and actual-high frequency. Boundary values are
excluded from the frequency ranges.
(p. 40) shows how boundaries and ranges relate to each other:
VWIRE 305
39