The Practical Salinity Scale 1978 (PSS-78) has been considered by the Joint Panel on Oceanographic Tables and Standards and recommended by all oceanographic organizations as the scale in which to report future salinity data.

Practical salinity, symbol S, of a sample of seawater, is defined in terms of the ratio K₁₅ of the electrical conductivity of the seawater sample at the temperature of 15°C and the pressure of one standard atmosphere, to that of a potassium chloride (KCl) solution, in which the mass fraction of KCl is 32.4356E-3, at the same temperature and pressure. The K₁₅ value exactly equal to 1 corresponds, by definition, to a practical salinity exactly equal to 35, The practical salinity is defined in terms of ratio K₁₅ by the following equation:

formulated and adopted by the UNESCO/ICES/SCOR/IAPSO Joint Panel on Oceanographic tables and Standards, Sidney, BC, Canada, 1-5 September 1980.

PSS-78 is based on an equation relating salinity to the ratio of the electrical conductivity of seawater at 15°C to that of a standard potassium chloride solution (KCl). It has already been pointed out that a "conductivity ratio" defined salinity scale is better than a "chlorinity" scale for density determination; and added to this is the study of Farland showing that in the hands of average observers, titration is a less precise than is conductivity measurement. In order to eliminate the ambiguity exhibited by (1) and (2) under conditions of ionic ratio variation, the Practical Salinity Scale 1978 breaks the existing chlorinity-salinity tie in favor of a definite salinity-conductivity ratio relationship; all waters of the same conductivity ratio then have the same salinity.

A standard seawater of 35 has by definition a conductivity ratio of unity at 15°C with a KCl solution containing a mass of 35.4356 g KCl in a mass of 1 kg of solution. In practice Merk "Suprapur" KCl has been found to be of adequate purity being consistent within a bath and between batches. It is worth nothing that the major impurity is NaCl, and at the level of interest the molal conductivities of the two salts are sufficiently near to minimize the effect of the impurity. From measurements on weight diluted (with pure water) or weight evaporated North Atlantic surface water, the following relationship was established:

	a0 = 0.008                            b0 = 0.0005                
	a1 =-0.1692                           b1 = -0.0056             
	a2 = 25.3851                          b2 = -0.0066
	a3 = 14.0941                          b3 = -0.0375
	a4 = -7.0261                          b4 = 0.0636
	a5 = 2.7081                           b5 = -0.0144

	k = 0.0162                            -2°C < T < 35°C
	

In all cases temperatures are measured according to the International Practical Temperature Scale (1968).

The values of coefficients in PSS-78 are based on experiments carried out on existing standard seawaters that were diluted and evaporated by weight. This ensures the conservatism of a salinity so defined and its local reproducibility. The standard deviation was 0.0013 in salinity.

This equation is used for calculation of the practical salinity measured only by laboratory salinometers. A supplementary equation was also established for converting conductivity ratios measured at pressure greater then one atmosphere (i.e. for CTD-measurements):

i.e.

where:

where:

The standard deviation was 1.3 ppm in salinity. Given R, T, and p, a may be computed and the factor rt, the temperature coefficient of standard seawater, may be obtained from the polynomial equation:

where:

This equation enables practical salinity (S) to be calculated from conductivity ratio (Rt) measured at temperature T C and atmospheric pressure and is valid in the temperature range -2°C to +35°C and salinity range 2 to 42.

Notwithstanding the fact that definition of Practical Salinity (UNESCO,1981a,b) omits units or a designator, some members of the panel and of the oceanographic community felt that the issue of a possible designator (or units) should be considered further. For these reason the JPOTS Chairmen polled the panel as well as interested expert on this matter in order to establish a consensus. The main result of this survey is that cogent reason exist that no designator or units should be assigned to practical salinity. Henceforth practical salinity should be expressed by dimensionless number only and should be written as, e.g. S = 35.034.