SonTek/YSI Argonaut-XR Technical Manual - HydroScientific West

Argonaut Principles of Operation (March 1, 2001)
SonTek/YSI
acoustic pulse is 0.5-m long. The receive window is based up the settings of CB and CE. The
length of the receive window is equal to the measurement volume length (the difference between
CB and CE). The measurement volume weighting function of the SL is the same as for the XR
(shown in Figure 4).
5. Argonaut Data
5.1. Velocity
In general, Argonaut velocity data is used directly as output from the system without any
postprocessing. The Argonaut velocity response will not change or drift with time, and the
system never requires recalibration. The Argonaut provides several diagnostic parameters with
each sample to verify the quality and accuracy of the data. The remaining parts of this section
describe the diagnostic data and give some general guidelines for its use.
5.2. Signal Strength
Signal strength is a measure of the strength of the acoustic return signal from the water; it
decreases with distance from the transducer due to geometric spreading and sound absorption.
The maximum measurement range of the Argonaut is determined by the distance at which signal
strength approaches the electronics noise level. The noise level can be directly measured using
the system diagnostic software (ARGCHECK) with the transducers in the air (i.e., out of the water).
Signal strength data is reported in internal logarithmic units called counts (1 count = 0.43 dB). In
addition to its diagnostic use, signal strength provides an excellent qualitative measure of
suspended sediment concentration. For more information about using signal strength data to
monitor suspended sediment, contact SonTek.
For the MD, the location of the measurement volume is fixed and is well within the maximum
measurement range of the system. The signal strength reported with each sample reflects the
mean value over the measurement volume and will vary depending on water conditions. For
good operating conditions, the signal strength should be at least 10 counts above the noise level.
For the XR and SL, the location and size of the measurement volume is programmable over a
range up to 15.0 m from the system. The signal strength returned by the system is the mean signal
strength over the specified measurement volume. Signal strength decreases with range from the
transducers and will vary with conditions in the water. For good operating conditions, the mean
signal strength reported with each sample should be at least five counts above the noise level.
The vertical extent of the measurement volume may be adjusted by the Argonaut in two cases.
The XR may adjust the measurement volume based up pressure sensor data using dynamic
boundary adjustment (see §6.1 for details). Additionally, both the XR and SL may adjust the
measurement volume limits based up signal strength data as described below.
With each sample, the XR and SL monitor the signal strength profile within the specified
measurement volume. If at any point the signal strength is too low for reliable velocity
measurements, the Argonaut will end the measurement volume at that range.
In most conditions, the Argonaut is able to measure out to the specified maximum range of 15 m.
In some environments, the return signal strength from the water will be too low and the Argonaut
will have a reduced measurement range. In this situation, the system will automatically cut off
the measurement volume at the maximum effective range. The exact limits of the measurement
volume are recorded with each sample.
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