Using a Velocimeter 59
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2.7 Quality Assurance of Velocimeter Data
Tilt (Vector):
Since there are numerous possibilities for things that can go wrong during a deployment, we need to
use the tilt and heading sensors to qualify if the deployment itself went as planned.
The first step is to make sure that the instrument was oriented as intended. This is done by
checking the time series of the tilt (Roll and Pitch) and heading sensor data:
If the tilt is large (greater than 10 degrees) then a post-processing option such as “map to vertical”
is encouraged.
Tilt readings between 20° and 30° affect the data accuracy in a way that is likely to make the data
fail to meet the specifications.
Data acquired during tilts exceeding 30° are in general not reliable and should be discarded.
If the tilt or heading measurements change greatly from measurement to measurement (buoy
mounted), it is wise to ensure the instrument was configured with a suitable Compass update rate
(check the deployment file or the header file).
Pressure (Vector):
The next step is to check that the pressure sensor is reasonable. In case of a sudden change in
pressure or pressure reading indicating shallower water depth than expected, extra attention should
be shown. These events usually occur during the deployment, retrieval, and the occasional curious
mariner. Data collected during these events should be avoided in the post processing. If the Vector
sample volume is out of the water there is no way to recover velocity data. Data during these periods
should fail most data quality checks since the SNR will fall to (near) zero.
Velocity:
Current measurements that show large variations vertically or vertical structures should be flagged
suspicious unless the measurements are taken in an area where this is expected. The ones that
are most doubtful are those that are physically unrealistic (e.g. 1 m/s changes over a short period
of time). The measurements in a time series that are far from the mean are best to be carefully
inspected.
Raw wave data: The raw pressure signals give an immediate indication about the wave condition;
long waves (T>8 s) give a strong pressure signal and a variation of 1 m in the pressure signal mean
that the wave height is at least the same. On the other hand, a very weak signal in the pressure
sensor (variations of only a few cm) usually means that the waves are small. The raw wave
velocities will appear noisy even at moderate sea states. Only at times with large waves can you
expect a smooth signal. Even so, the noise should not be excessive and it should be possible to
see the wave signal when there are significant wave action on the surface. Watch out for large
jumps or discontinuities.
Vibrations: If velocity spectra show a very large spike at one or more frequencies, it is typically
caused by a vibration in the stem. 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. The
impact on measurements will depend on what flow parameters are of interest.
Probe Check:
For the Vector: A probe check is made before and after each burst sequence, and also at the very
beginning of the sampling when running Continuous mode. After bin to ASCII conversion it ends up in
the .pck file as explained in the .hdr file. For the others, probe check can be enabled or disabled.
The probe check is designed to act as a measurement quality assurance tool; it lets you inspect the
region where the instrument makes it measurements by showing how the signal varies with range.
SNR and Correlation:
SNR and correlation are reported in the data file because they are extremely useful when judging
data quality. It is therefore recommended assess overall data quality by looking at SNR and
correlation time series and histograms before attempting to interpret the data. Here are some tips:
It is important to note that SNR and correlation is tied to a specific beam velocity measurement,
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