SonTek/YSI Argonaut-XR Technical Manual - HydroScientific West

Argonaut Principles of Operation (March 1, 2001)
SonTek/YSI
The wave spectral estimates are presented as an array of coefficients, each giving the mean wave
amplitude (square root of the energy) within a period band. Ten bands are used, which
correspond to two-second periods ranging over 2-20+ seconds.
For each band, the Argonaut computes and reports the mean wave amplitude (A) for waves
within the period range in the band. If Ai is the amplitude for band, i, the total wave energy is
given by:
Total energy (σA 2 ) = Σ( Ai 2 ) for i=1 to 10
A generally accepted estimate of the significant wave height can be easily obtained from the
amplitudes using:
Hmo=4 * √(Total energy)
6.5. Sound Speed
The Argonaut uses sound speed to convert the Doppler shift to water velocity. This section
discusses how to correct Argonaut velocity data for errors in the sound speed used for data
collection. Since the Argonaut uses an internal temperature sensor for automatic sound speed
compensation, user corrections are rarely needed.
In shallow water, the speed of sound in water is a function of temperature and salinity. Generally,
a temperature change of 5°C or a salinity change of 12 ppt results in a change in sound speed of
one percent. The full range of typical temperature and salinity levels, from -5 to 50°C and 0 to
60 ppt, gives a sound speed range of 1375 to 1600 m/s (total change of 14%). SonTek can
provide an algorithm for sound speed calculations upon request.
Argonaut velocities scale directly with sound speed: a 1% error in sound speed results in a 1%
error in velocity measurements. The following formula is used for postprocessing corrections and
can be directly applied to the output velocity data of the Argonaut.
Where
Vtrue = Vorig (Ctrue / Corig)
Vtrue = Corrected velocity measurements
Vorig = Uncorrected (original) velocity measurements
Ctrue = True speed of sound
Corig = Speed of sound used in original calculations
Errors in sound speed also affect the physical location of the Argonaut measurement volume,
although these errors are generally very small. To calculate the correct location of the Argonaut
measurement volume, use the formula:
Where
Ztrue = Zorig (Ctrue / Corig)
Ztrue = Corrected measurement volume location
Zorig = Uncorrected (original) measurement volume location
Ctrue = True speed of sound
Corig = Speed of sound used in original calculations
11