Sound speed vs sound speed used

I have a client using an Aquadopp Profiler and noticed there are two columns for sound speed - "sound speed" and "sound speed used."  We suspected "sound speed used" is a pressure corrected velocity, but want to check to make sure.  They see at least a difference of 30 m/s between the two at the bottom, but they are essentially the same at the surface.

Thanks! 

Hi Freda,

You are right. The "sound speed used" is calculated from pressure, temperature and salinity and used to correct the measured velocity.

Reference: Chen and Millero, 1977, JASA,62,1129-1135

Calculator: http://www.es.flinders.edu.au/~mattom/Utilities/soundspeed.html

Best regards,
Oistein

This issue is related to the fact that the speed of sound is not calculated from from pressure, temperature and salinity internally by the instrument. The speed of sound to use is provided by the software, either as a user specified value ("Fixed" option) or as a table of values for different temperatures ("Measured" option). The table is calculated by the software using the user specified salinity. When the "Measured" option is checked, the instrument looks up the speed of sound to use in the table using the measured temperature.

The sound speed table does not consider the pressure which becomes relevant for the sound speed calculation at large depths. To correct for this the data conversion function calculates speed of sound using the pressure data and corrects the velocities by applying a scaling factor (Scaling = SoundSpeedUsed / SoundSpeed).

In the header file the "SoundSpeed" is the speed of sound used by the instrument when measuring, while the "SoundSpeed Used" is the pressure corrected speed of sound used to correct the velocities in the conversion.

Best regards,
Oistein