YSI SonTek ADVField, Technical Documentation Manual

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SonTek/YSI
Acoustic Doppler Velocimeter Principles of Operation (September 1, 2001) 12
possible to use a higher velocity range and the ADV will be forced to use a reduced velocity
range. The most significant limitations occur when trying to measure high flows (> 1 m/s) very
close to the boundary. These measurements can typically be made, but require careful adjustment
and monitoring.
One important result of the potential boundary interference described above is that changes in the
distance from the sampling volume to the boundary during data collection can potentially cause
problems. This is because the ADV only checks for interference at the start of data collection. If
the probe is moved with the respect to the boundary (e.g., making a profile), data collection
should be restarted at each depth. This allows the ADV to check for interference and adapts its
operation to avoid it. Care should also be taken when working near moving beds or near the sur-
face, where the height of the boundary can change with time.
7.4. Reduced Sampling Volume
As described in Section 4, the horizontal extent of the sampling volume is defined by the trans-
mitter beam pattern and the vertical extent by the convolution of the pulse length with the receive
window size. Both pulse length and receive window size are controlled by the ADV software.
Thus, with software modifications, we can reduce the vertical extent of the sampling volume.
This section describes the changes that can be made, and the tradeoffs associated with them.
When altering the vertical extent of the sampling volume, we can reduce both the length of the
acoustic pulse and the time window over which the return is sampled. Reducing both of these as
much as possible makes the height of the sampling volume 1.2 mm for 16-MHz/10-MHz ADVs,
and 2.4 mm for 5-MHz ADVOceans.
There are two effects associated with reducing the size of the sampling volume. First, by reduc-
ing the receive window, we reduce the number of points averaged for velocity calculations and
increase the temporal variability (noise) of the measurements. Reducing the receive window to its
minimum value increases the noise in individual measurements by a factor of about two.
The second effect occurs because of shrinking the size of the transmit pulse; using a shorter pulse
reduces the strength of the return signal from the water. Reducing the length of the acoustic pulse
to its minimum level reduces the instrument Signal-to-Noise Ratio (SNR) by about 6 dB. This is
not a concern in applications where signal strength is high (SNR > 15 dB), but it can be a factor
in relatively clear water as it can introduce or increase seeding requirements. If the SNR ratio re-
mains sufficiently high, reducing the length of the transmit pulse has no effect on the noise in ve-
locity measurements.
7.5. Low Flow Measurements
One significant advantage of Doppler current meters, particularly when using pulse-coherent
processing, is that there is no minimum measurable velocity. Doppler systems have no potential
for a zero offset or zero drift. Since they compare the return signal from the water with the trans-
mitted signal, a change in the transmit signal (which might be thought to cause a drift) will have
no significant effect since the instrument is looking at the change in frequency.
The ADV specifications state that the zero offset velocity is
±
0.25 cm/s. This is done largely be-
cause it is very difficult to establish what the true flow is at very low velocities; there is no poten-
tial for any zero offset. In laboratory experiments, the ADV has been used to measure calibrated
flows as low as to 0.04 cm/s.