Using a Velocimeter 57
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adjusted upwards if waves are expected to be exceptionally large.
2.5.6 Near Boundary Measurements
The middle of the sampling volume is positioned approximately 5 cm (or 10 cm on the field probe)
below the transmit transducer located at the center of the probe for the Vectrino. For the Vector it is
15 cm, and the measurement volume for the Vectrino Profiler spans from 4-7 cm from the probe
(user selectable).
The distance to the boundary shown on the computer display is the distance from the middle of the
sampling volume to the nearest boundary. The sampling volume is of finite extent and the minimum
distance to the boundary that still permits data collection is 4-6 mm for a standard set-up. Some
experimentation is required because the exact limit depends both on the condition of the boundary
as well as the exact software configuration.
One thing to note when operating in a flume is that the bottom material can have a huge effect on the
data quality. Some tips about what to do in case the bottom material is reflecting too much of the
acoustics is described here: Interference from Highly Reflective Bottoms.
Sidelooking Vectrino probe:
There is no minimum distance to the bed with the sidelooking probe - it can physically be sitting
on the bed.
Sidelooking probes do not have the problem of "weak spots" so they are a little more robust to
operate, especially in high flow environments.
They have higher instrument noise in the vertical than in the horizontal.
They are well suited for use in wave flumes.
They have an awful response to flow coming "from behind" the probe.
If only the two lower beams are submerged: It will work in about 2-cm water at low flow rates and
in this case it will give you just the two horizontal velocity components.
When all four beams are covered with water (around 3-4 cm) you can measure all three velocity
components.
2.6 Common Configuration Scenarios
The signal-to-noise ratio (SNR) is very low.
Vector and Vectrino: SNR should be >15 dB
Vectrino Profiler: SNR should be around 30 at the center of the profile and above 20 at the ends, with
a parabolic shape peaking at the sweet spot of 0.05 m range. SNR at ranges greater than 0.06 m will
drop sharply so attention should be paid to this region of the profile if used.
If low SNR is encountered, try adding scatterers to the water column such as the seeding material
provided with the instrument. Add a little at a time and see if SNR improves.
If there are enough scatterers in the water, check the Power Level in the Configuration dialog and
make sure it is on high.
If the Power Level is on High and scatterer concentration is high enough, but SNR is still low,
please contact support for further guidance.
The velocity time series look spiky and correlation is low or varies widely.
Correlation should be very high, not widely scattered. For the Vectrino Profiler: Correlation profiles
should have the same parabolic shape as SNR. Try and achieve correlations greater than 90%
throughout the profile. The highest correlations will be at the sweet spot at a range of 0.05 m, with
correlations at ranges greater than 0.06 m dropping off sharply.
To improve correlation, first make sure SNR is high (see points above). Then, check the Velocity
Range the Configuration dialog. The actual Horizontal range and Vertical range are reported in the
configuration dialog just below where the Velocity Range is set. These need to be appropriate for
your flow. If your velocity range is not what you expected, check which ping algorithm is selected.
Reported velocity ranges will depend on the algorithm.
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