Dictionary of Ichthyology  

practical salinity unit = the ratio K of the electrical conductivity of a sea water sample of 15C and the pressure of one standard atmosphere, to that of a potassium chloride (KCl) solution, in which the mass fraction of KCl is 0.0324356, at the same temperature and pressure. The K value exactly equal to one corresponds, by definition, to a practical salinity equal to 35. In this definition, salinity is a ratio and parts per thousand () is therefore no longer used, but an old value of 35 corresponds to a value of 35 in the practical salinity. Practical salinity is a ratio and strictly no units should be used but often PSU or psu is added to the value.

Annual mean sea surface salinity 2001 from Wikimedia Commons: http://commons.wikimedia.org/wiki/File:Wiki_plot_04.png

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