pss78 - derivation of practical salinity

The equation relating salinity to conductivity, temperature and pressure is the Practical Salinity Scale of 1978, often referred to as PSS78 (see Practical salinity of seawater). If the salinity calculation leads to an aberrant value, which generally happens 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 our example:

• n0 is the label of the temperature channel, temperature_00 in this example.

• n1 is the label of the absolute pressure channel, pressure_00 in this example.

• n2 is the label of the conductivity channel, conductivity_00 in this example.

• n3 is the label 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 reports a small negative value. This does not apply if one of the parameters is already flagged as an error.

Examples

>> channel salinity_00 equation
<< channel salinity_00 equation=deri_salinity

Confirm the equation type.

>> calibration salinity_00
<< calibration salinity_00 datetime=20251001000000 equation=deri_salinity offset=0.0000000e+000 slope=1.0000000e+000 n0=temperature_00 n1=pressure_00 n2=conductivity_00 n3=value

Request 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 salinity_00
<< calibration salinity_00 datetime=20251001000000 equation=deri_salinity offset=0.0000000e+000 slope=1.0000000e+000 n0=temperature_00 n1=value n2=conductivity_00 n3=value