The numeric difference between psu and ppt is small; both indicate ocean salinity. Prior to 1978, oceanographers referred to the physical quantity ppt (kg salt per kg water in parts per thousand). In 1978, the Practical Salinity Scale (PSS-78) was adopted, which yields a practical salinity from equations, smooth expansions of conductivity ratio, which were carefully fit to the real salinity of diluted North Atlantic seawater. The numeric unit from PSS-78 is psu (practical salinity unit). The primary motivation for psu was consistency; it focused on a trace to a primary conductivity standard (K15) and recognition that ocean ion ratios were not identical. Salinometer work was plagued by an inconsistent standard and the ppt equations included ion ratios from different oceans. So, the trade was a consistent standard and equation that works for a single ion mix instead of exact salinity in other ocean basins. G. Siedler and H. Peters highlighted where PSS-78 and EOS-80 formulas deviate from real salinity and density (e.g., Baltic Sea is difficult, but the deep Pacific has EOS-80 deviations of up to 0.02 kg/m3, implying salinity errors of order 0.02 psu).
In June 2009, a new Thermodynamic Equation of State of Seawater, referred to as TEOS-10, was adopted by the Scientific Committee on Oceanic Research and the International Association of Physical Sciences of the Ocean Working Group 127. The new equation incorporates a more accurate representation of salinity known as Absolute Salinity. Application Note 90 discusses this new equation, and Sea-Bird’s implementation in SBE Data Processing.