Using a Velocimeter 61
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as needed based on data quality). An outlier filter to remove data which is statistically different than
expectations will also help improve the quality of a dataset. Histograms can be quite helpful in
determining the effectiveness of any quality control procedures implemented.
Velocity time series are really spiky
This can be caused by many problems, e.g. weak spots, a too small Velocity Range, low SNR. The
velocity spikes can be removed in post processing using an outlier filter. In this case, a simple hard
threshold discarding velocities, for example > 1 m/s and < 0 m/s, will work.
Velocity changes sign when it should not
This is caused by aliasing in beam velocities when the maximum measurable velocity is exceeded.
Velocities larger than the ambiguity velocity wrap around to the opposite sign.
To avoid this during data collection, the Velocity Range needs to be set larger than the expected
velocities (see Configuration Issues). To aid users in selecting an appropriate Velocity Range,
the Configuration dialog reports the horizontal and vertical velocity ranges.
Correcting phase wraps is possible. It involves identification of wrapped velocities and correcting the
measured velocity using the reported ambiguity velocity. For details on how to unwrap velocities,
check out the relevant section in the Troubleshooting Guide.
Velocity spectra show a very large spike at one or more frequencies
This is typically caused by a vibration in the stem for fixed stem probe heads or the mount a cabled
probe head is attached to. Wave impact, high flow rates, loose mountings, or vibrations from nearby
equipment will generate vibrations in the probe head.
Typical vibrations from instrument mountings will have a very sharp peak and occur for a very narrow
frequency band. Vector fixed stem probes will for example often see a peak around 20 Hz. The
impact on measurements will depend on what flow parameters are of interest.
The Vectrino Profiler software provides real time velocity spectra which can help diagnose and
correct vibrations. The real time spectra are accessed through the Display time series data. Note
real time spectra and imaging can not be used at the same time.
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