Example 13: deri_sos, speed of sound
Full, in-situ derivation of speed of sound requires salinity, hydrostatic pressure and temperature, so a simple RBRconcerto³ C.T.D will be used as an example.
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 involves a rather fearsome looking series of polynomials combined in various ways: mercifully 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. This makes its specification rather sparse: 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 index of the temperature channel, 3 in this example,
n1 is the index of the hydrostatic pressure channel, 4 in this example,
n2 is the index of the salinity channel, 6 in this example.
Examples
>> calibration 7 type
<< calibration 7 type = sos_00Confirm the channel type.
>> calibration 7
<< calibration 7 type = sos_00, datetime = 20171201000000, n0 = 3, n1 = 4, n2 = 6Request confirmation of all channel indices.