deri_sos - derivation of speed of sound
Full, in-situ derivation of speed of sound requires salinity, hydrostatic pressure and temperature. The equation used in the logger relating speed of sound to the three underlying parameters is the Chen and Millero equation reviewed by Wong and Zhu, often referred to as UNESCO equation. For further information about this equation, please refer to the paper: G.S.K. Wong and S. Zhu, Speed of sound in seawater as a function of salinity, temperature and pressure (1995) J. Acoust. Soc. Am. 97(3) pp 1732-1736.
The equation is a series of polynomials combined in various ways: the coefficients are all empirically determined constants, and all values are embedded in the logger.
Speed of sound is a “pure” derived parameter which has its own channel assigned to it, but there is no underlying measurement hardware for speed of sound itself; it simply uses the outputs of the salinity, temperature and hydrostatic pressure channels. There are no coefficients in either of the c or x groups; all that is needed is to specify the indices in the n group.
In our example:
n0 is the label of the temperature channel, temperature_00 in this example
n1 is the label of the sea pressure channel, seapressure_00 in this example
n2 is the label of the salinity channel, salinity_00 in this example.
Examples
>> calibration speedofsound_00
<< calibration speedofsound_00 datetime=20251001000000 equation=deri_sos offset=0.0000000e+000 slope=1.0000000e+000 n0=temperature_00 n1=seapressure_00 n2=salinity_00Request confirmation of all channel indices.