PHYSICAL METHODS OF SALINITY MEASUREMENT
Pure water is a poor conductor of electricity. However, the presence of ions in water enables it to carry an electric current. In the 1930s, it was established that the electrical conductivity of seawater is proportional to its salinity. Conductivity is inversely proportional to resistivity, and for many decades conductivity salinometers were based on simple electrical bridge circuits, using ‘standard seawater’ of known salinity (close to 35) for calibration.
Conductivity is also affected by temperature, however, which can lead to appreciable errors. Ideally, physical oceanographers require salinity measurements to be accurate to + 0.001, requiring conductivity to be measured to 1 part in 40 000. A change of this magnitude can be induced by a temperature change of only 0.001oC so careful control of temperature is essential.
In the past, precision thermostatting was used to maintain both sample and standard seawater at constant temperature, but the equipment was bulky and measurements took a long time because samples had to be heated or cooled to working temperature before measurement could begin. Such problems have now been largely circumvented, and modem salinometers are compact and rapid in operation, and can measure salinity to + 0.003 or better.
Conductivity sensors have been incorporated into in situ temperature salinity instruments for use in shallow waters, and into conductivity temperature- depth (CTD) probes for use in the deep oceans.
THE FORMAL DEFINITION OF SALINITY
Since the mid-1960s, the definition of salinity has been based (by international agreement) on empirically determined and rather complicated looking formulations involving a conductivity standard.
The salinity of a sample of seawater is now measured in terms of the conductivity ratio, R, which is defined by:
the concentration of the standard KC1 solution being 32.435 6 g kg-1. Salinity is related to the conductivity ratio at 15o C and 1 atmosphere pressure (R15) by the following equation:
Seawater Second Edition : Its Composition, Properties and Behaviour