SonTek/YSI Argonaut-XR Technical Manual - HydroScientific West

5.4. Percent Good
Argonaut Principles of Operation (March 1, 2001)
SonTek/YSI
The MD records one additional quality parameter with each sample – percent good. Percent good
is based on a statistical analysis of signal strength data from each beam. The MD looks for large
variations in the signal strength from each beam, and throws out points that fall outside a predetermined
threshold. This helps prevent data contamination from fish that may occasionally
cross the acoustic beam, particularly on moorings that may attract schools of fish.
For the MD, percent good is the ratio of samples used for velocity calculations to the total
number of samples taken. Percent good is not used by the XR or SL.
6. Special Considerations
6.1. Dynamic Boundary Adjustment – Argonaut XR
One of the most powerful capabilities of the XR is its ability to automatically adapt to changing
conditions using a technique called dynamic boundary adjustment. When mounted on the
bottom, the XR can adjust the limits of the measurement volume based up the water level
reported by an internal pressure sensor. Thus, with changing tide or river stage, the XR will adapt
its operation to match the environmental conditions.
There are two forms of dynamic boundary adjustment. When measuring vertically-integrated
flow, the start of the measurement volume is typically as close to the XR as possible. The end is
set as close to the water surface as possible. Velocity data from the XR represents an integration
over the entire water column and can easily be used for total flow calculations.
The second form of dynamic boundary adjustment is called layered flow. The user specifies a
layer of water (relative to the surface) that is of interest; typically, the XR will measure velocity
in the top N meters of the water column. The XR will adjust the location of the measurement
volume relative to itself, maintaining the desired layer at a constant range from the water surface.
When using dynamic boundary adjustment, the XR takes into account both the mean surface
elevation and changes in the surface level during the averaging period. Thus, if the XR is
operating in a wave environment, the top end of the measurement volume will be placed below
the level where velocity data could be contaminated during the trough of a wave. To aid analysis,
each sample includes the mean and standard deviation of pressure as well as the limits of the
measurement volume.
6.2. 2D Horizontal Current Measurements – Argonaut SL
The SL is normally mounted from an underwater structure looking to the side. The SL measures
the 2D water velocity in the plane formed by its two acoustic beams, parallel to the water surface.
The SL is typically installed at mid-water depth and can measure over a range of up to 15 m. This
allows the SL to be safely and easily installed on an underwater structure, but still measure the
water velocity away from any flow interference generated by the structure.
The primary limitation for SL operation is the maximum measurement range relative to the total
water depth. This is expressed as the aspect ratio between the horizontal distance from the SL
(Range) and the vertical distance to the surface or bottom (Height). Aspect ratio is defined as
Range/Height.
While Argonaut transducers generate very narrow beams, these beams will spread after some
distance and may see interference from the surface or bottom. The SL can operate without
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