Comprehensive Manual40
© 2018 Nortek AS
Nominal Velocity Range
The velocity range is set before data collection is started. The range is a nominal value and the exact
maximum velocity is different along the vertical axis (i.e., toward the transmitter) and in the horizontal
plane. The horizontal and vertical ranges are presented in the planning dialog. Setting the velocity
range to an appropriate value for the flow being measured is the most important instrument
parameter in order to get good data quality. Too large a velocity range will result in noisy data
because the detected phase shift is very small relative to the ambiguity velocity. Too low a range will
result in de-correlation of the return signals or phase wrapping. The appropriate Nominal Velocity
Range will generate good quality data minimizing data quality problems.
For example, if the velocity range is set to +/-0.3 m/s, the maximum velocity Vz is 0.27 m/s
toward the transducer and 0.94 m/s in the plane perpendicular to the transmit axis (Vx and Vy).
The ratio between the software adjustable (nominal) velocity range and the maximum value for Vx,
Vy, and Vz does not remain constant but is different for each range setting.
As a general rule, the velocity range should always be set as small as possible, but still cover the
expected velocities.
For example, if the maximum expected velocity is 4 cm/s, the velocity range should be set to +/-
0.1 m/s. The reason for this is that the noise in the data increases with increasing velocity range. If
we set the velocity range to +/- 4 m/s and the actual velocity is only 2 cm/s the data will appear
excessively noisy.
If the conditions are not known in advance, the velocity range must be set high enough to cover the
whole deployment period. Be aware that the velocity range must cover the full range of wave motion
on top of the mean current. If, for example, the mean currents are expected to be around 0.1 m/s
and the wave orbital velocity may reach 0.4 m/s, the velocity range should be set to 1 m/s.
Adjusting sampling volume and/or transmit length
Transmit length is the length of the acoustic pulses transmitted by the instrument. In theory, sample
volume size is entirely independent and set by the amount of time spent listening. In reality, the
sampling volume size has been optimized for a given transmit length for the best performance.
Theoretically, a shorter transmit length is better because it results in a higher signal bandwidth. In
reality, the SNR will decrease simply because there is less energy in the return signal. There are
tradeoffs to be made, but usually there is not a need to change the transmit length to try and further
optimizes performance. The exception is if the user is trying to get closer to boundaries.
Power Level
There are four different power levels:
Low-
Low+
High-
and High+.
It is about 6dB difference in SNR between High+ and High -, 10 dB between High- and Low + and
about 6dB between Low + and Low -. Adjusting the power level should thus be related to the signal
strength, so if the scattering level is low then the power level should be set to High.
Take a look at the Probe Check section for a couple of examples on how to determine whether the
power level should be increased or decreased from the default setting.
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