Argonaut-ADV and FlowTracker Principles of Operation
Note: Although only the Argonaut-ADV is referenced here, these principles of operation
apply to both the Argonaut-ADV and to the FlowTracker Handheld ADV systems.
The SonTek/YSI Argonaut-ADV (Figure 1) is a
single-point, Doppler current meter designed specifically for low-power measurements in
shallow water. The Argonaut-ADV Doppler processing techniques provide several important
advantages: 3D (or 2D) velocity measurements in a remote sampling volume; invariant
factory calibration (no periodic re-calibration required); simple operation; and excellent
low-flow, shallow-water performance. This document presents the basic operating principles
of the Argonaut-ADV. To learn more about specific Argonaut-ADV configurations and
applications, contact SonTek/YSI.
The Argonaut-ADV measures the velocity of water using a physical principle called the
Doppler effect. If a source of sound is moving relative to the receiver, the frequency of
the sound at the receiver is shifted from the transmit frequency.
Fdoppler = -2Fsource (V / C)
In this equation, V is the relative velocity between source and
receiver, C is the speed of sound, Fdoppler
is the change in frequency at the receiver, and Fsource
is the transmitted frequency.
Figure 2. Bistatic Doppler Current Meter
Figure 2 illustrates the operation of a bistatic Doppler
current meter such as the Argonaut-ADV (bistatic systems use separate acoustic transducers
for transmitter and receiver). Both transmitter and receiver are constructed to generate
very narrow beam patterns. The transmitter generates sound with the majority of the energy
concentrated in a narrow cone, and the receiver is sensitive to sound coming from a narrow
angular range. The transducers are mounted such that their beams intersect at a volume of
water located some distance away. The beam intersection determines the location of the
sampling volume (the volume of water in which measurements are made).
The transmitter generates a short pulse of sound at a known frequency, which propagates
through the water along the axis of its beam. As the pulse passes through the sampling
volume, the acoustic energy is reflected in all directions by particulate matter
(e.g., sediment, small organisms, bubbles). Some portion of the reflected energy travels
back along the receiver axis, where it is sampled by the Argonaut-ADV and processed by the
electronics to measure the change in frequency. The Doppler shift measured by one receiver
is proportional to the velocity of the particles along the bistatic axis of the receiver
and transmitter. The bistatic axis is located halfway between the center axes of the
transmit and receive beams.
Each transmitter/receiver pair measures the projection of the water velocity onto its
bistatic axis. The Argonaut-ADV uses one transmitter and two or three acoustic receivers
(for 2D or 3D probes; Figure 3).
Figure 3. ADV Probe Arrangements
The
receivers are aligned to intersect with the transmit beam pattern at a common sampling
volume (Figure 4). The velocity measured by each
receiver is referred to as the bistatic velocity, and is the projection of the 3D velocity
vector onto the bistatic axis of the acoustic receiver. Bistatic velocities are converted
by the Argonaut-ADV to XYZ (Cartesian) velocities using the probe geometry. XYZ velocities
give the 3D velocity field relative to the orientation of the probe. As it is not always
possible to control instrument orientation, the Argonaut-ADV can be equipped with an
internal compass and tilt sensor. The compass/tilt sensor allows the Argonaut-ADV to
report velocity data in an Earth (East-North-Up or ENU) coordinate system, independent of
probe orientation.
The location of the sampling volume is determined by the physical construction of the
probe, and is 10 cm from the tip of the probe. The default size of the Argonaut-ADV
sampling volume is 0.25 cc, though this can be reduced.
Because of the remote 3D velocity measurements, the Argonaut-ADV is extremely well
suited to flow studies in boundary layers. The Argonaut-ADV automatically measures and
records the distance to the boundary at the start of each data collection cycle (the
boundary measurement can be made when the sampling volume is between 2 and 25 cm from
the boundary).
Under good operating conditions, the leading edge of the sampling volume can be placed
within about 0.5 mm of a boundary. The vertical extent of the sampling volume is precisely
defined; thus, this leading edge can be placed very close to a boundary without
interference.
One significant advantage of the Argonaut-ADV is that there is no minimum measurable
velocity, with no potential for a zero offset or zero drift. The lowest ADV velocity range
will yield good results for flows down to about 0.0001 m/s. If working in an
environment with extremely low flows, the Argonaut-ADV software can be modified to use
lower velocity ranges to further improve performance.
More details about the Argonaut-ADV and FlowTracker can be found at:
