Comprehensive Manual50
© 2018 Nortek AS
ranges give you better measurements regarding uncertainty. An example of the problem with phase
wrapping is discussed in “A Practical Primer for Pulse Coherent Instruments”.
If you have collected data that are subject to phase wrapping, there is a solution. Take a look in our
Troubleshooting Guide for the procedure.
For the Vectrino Profiler, the Extended Velocity Range (EVR) may be an option to increase the
velocity range.
2.4.4 Extended Velocity Range
Extended Velocity Range (EVR) utilizes an additional pulse pair to create a set of pulses with two
different lags and velocity ranges. With this technique, two alternating ping intervals are used to
collect a single profile. We use the first lag to get a course estimate of the velocity and then we use
the second and larger lag to improve the estimate. The first lag defines the ambiguity range of the
setting and the second is the one that is actually reported after we have used the first estimate to
determine what ambiguity branch we are on. The actual lags change with the velocity setting. The
velocity calculated using this technique is:
= 4 ( 2 - 1)( 2 - 1)
The ambiguity velocity then becomes:
= 4 ( 2 - 1)
This technique has the advantage of producing an extended velocity measurement for all cells in the
profile (unlike the coarse / fine pulse-pairs technique). Signal noise effectively limits the minimum
usable time difference. In practice, the gain is limited to a factor of 3-5. Note that EVR does not
eliminate the possibility of ambiguity problems if velocities exceed the specified velocity range in the
Deployment Planning dialog.
2.4.5 Weak Spots
The Vector, Vectrino and Vectrino Profiler transmits two pings, and as such, they are susceptible to
pulse interference when measuring near boundaries. This especially applies to Vectrino Profiler, as
this is a profiling instrument and the pulse is in the measurement volume for a longer period of time.
Pulse interference for acoustic Velocimeters is called a “weak spot” and will show in data as low
SNR (Signal-to-Noise Ratio) and correlation values, as well as noisy velocity traces.
The table below summarizes the distances where weak spots occur for the Vector and Vectrino
indexed to the Nominal Velocity Range setting. Changing to the next higher or lower velocity range
as appropriate will eliminate the problem. Alternatively, moving the instrument (measurement volume)
up or down away from the boundary by just a few centimeters can reduce the weak spot effects.
Note that the distances to boundaries specified are approximate and will depend on the speed of
sound and the boundary surface.
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