Example 7: pss78 - derivation of practical salinity (1978)
Full, in-situ derivation of salinity requires that conductivity, temperature and pressure are all measured, so a simple RBRconcerto³ C.T.D will be used as an example.
The equation relating salinity to the three underlying parameters is the Practical Salinity Scale of 1978, often referred to as PSS78: for further information refer to the section Practical Salinity of Seawater. 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. If the salinity calculation leads to an aberrant value, which happens generally when the conductivity sensor is in the air and reads a slightly negative value, the logger will saturate the salinity value to zero instead of generating an error.
Salinity is a 'pure' derived parameter which has its own channel assigned to it, but there is no underlying measurement hardware for salinity itself; it simply uses the outputs of the conductivity, temperature and 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.
Because hydrostatic pressure is used in the salinity equation, it also accommodates the presence of a channel to measure atmospheric pressure. In practice most loggers, like the RBRconcerto³ C.T.D in this example, will not have an "atmospheric pressure" channel, so the 'n' group index will be set to the text "value", and the logger will use the substitute value specified by the "settings atmosphere" command.
In our example:
n0 is the index of the temperature channel, 3 in this example,
n1 is the index of the pressure channel, 2 in this example,
n2 is the index of the conductivity channel, 1 in this example,
n3 is the index of the atmospheric pressure channel; not present in this example, so set to "value".
If the PSS78 calculation generates an error, the datalogger will report a salinity of 0. This might occur when, in air, the conductivity report a small negative value. This does not apply if one of the parameters is already flagged error.
Examples
>> calibration 4 type
<< calibration 4 type = sal_00Confirm the channel type.
>> calibration 4
<< calibration 4 type = sal_00, datetime = 20171201000000, n0 = 3, n1 = 2, n2 = 1, n3 = valueRequest confirmation of all calibration coefficients.
It is not uncommon to monitor salinity using a logger with only conductivity and temperature (C.T) channels, deployed
at a constant depth. In this case we might have:
>> calibration 4
<< calibration 4 type = sal_00, datetime = 20171201000000, n0 = 2, n1 = value, n2 = 1, n3 = valueRequest confirmation of all calibration coefficients.