9.7.6 Frequency warning flags
Frequency warning flags are triggered at the following times:
l
When a resonant frequency falls between the estimated- and actual-frequency ranges. This
may indicate a change in the output of a sensor before the output moves outside the
actual-frequency range. See
l
When the low-frequency boundary or high-frequency boundary is set outside the range
allowed by the selected sample rate.
l
When the Sample Rate setting is invalid for the resonant frequency. See
For example, at the VWIRE 305 sample rate of 100 Hz, the VWIRE 305 can measure frequencies
anywhere from 580 to 6000 Hz. A resonant frequency outside this range will trigger a flag.
Furthering this example, if signal frequencies output by the sensor always fall between 2000 and
3000 Hz, you can set the low-frequency boundary to 2000 Hz and the high-frequency boundary
to 3000 Hz. The VWIRE 305 will adjust the boundaries to 1907.3 Hz and 3051.68. If a measurement
falls between 1907.3 and 2000, a low-frequency warning flag is raised. If a measurement falls
between 3000 and 3051.68, a high frequency warning flag is raised.
See
(p. 36) for further discussion of warning flags and how they are tagged to a
measurement.
9.7.7 Sample rate and noise performance
Electrical noise limits the effective resolution, or precision, of a vibrating wire measurement. To
increase precision, decrease the measurement sample rate. The fifth column of
lists the noise level associated with each sample rate.
Other topics that discuss noise performance are found in the following sections:
l
l
l
l
Calculating vibrating wire measurement error
9.7.8 Standard deviation
The
CDM_VW300Static()
instruction has an option to output a 1 Hz calculation of the
standard deviation of the dynamic readings. If the standard deviation changes significantly,
spend time to figure out what is causing the change. Significance of change depends on the
application. For example, a significant increase from a baseline (be sure to determine and record
a baseline when first installing a station) may indicate a problem with the sensor or a significant
shift in the structure or phenomenon being measured.
VWIRE 305
41
