Salinity Correction From Conductivity Calculator

Practical Salinity is defined not by weighing dissolved salts but by measuring how well seawater conducts electricity relative to a standard potassium chloride solution. The PSS-78 scale fixes the reference point at a KCl solution containing 32.4356 grams per kilogram, whose conductivity at 15 degrees C and one standard atmosphere equals that of seawater with a salinity of exactly 35. That reference conductivity is 42.914 mS/cm. A measured conductivity ratio R is corrected for the instrument cell constant, then split into temperature and pressure factors to yield Rt, the ratio the same sample would give at 15 degrees C and zero gauge pressure. Salinity follows from a fifth-order polynomial in the square root of Rt with the six coefficients a0 through a5, plus a small additive term that vanishes at 15 degrees C. The corrections matter more than they might appear. Seawater conductivity changes roughly 2 percent per degree C, so an uncorrected 1 degree temperature error shifts salinity by about 0.4, whereas modern CTD instruments target an accuracy of 0.002 or better. Pressure raises conductivity by around 1.5 percent per 1000 dbar. PSS-78 is defined only over salinity 2 to 42, temperature minus 2 to 35 degrees C, and pressure to 10,000 dbar, and it is dimensionless by construction, so values are written as 35 rather than 35 ppt. Its principal weakness is that it assumes the fixed ion ratios of open ocean water, so it misreports salinity in estuaries, the Baltic, and regions with high silicate or unusual nutrient loading, where the composition anomaly can reach 0.03 g/kg.

Before conductivity, salinity was determined by chemical titration. Following William Dittmar's Challenger analyses, Martin Knudsen's 1901 Hydrographical Tables defined salinity through chlorinity measured by silver nitrate titration, with the relation salinity equals 1.805 times chlorinity plus 0.030. The method was accurate to about 0.02 but slow, requiring skilled bench chemistry on every sample, which limited the spatial and vertical resolution of hydrographic surveys for half a century. Electrical measurement changed that. Inductive and electrode salinometers developed in the 1950s and 1960s, notably by Bruce Hamon and Neil Brown in Australia, reached a precision near 0.003 and could be run at sea in minutes. Neil Brown went on to develop the continuously profiling CTD instrument, which made conductivity, temperature, and depth records of the whole water column routine. The chlorinity-based definition became a bottleneck, so the Joint Panel on Oceanographic Tables and Standards, working under UNESCO, SCOR, ICES, and IAPSO, adopted the Practical Salinity Scale 1978, which defines salinity purely by conductivity ratio and severs it from chemical composition entirely. IAPSO Standard Seawater, produced since 1975 from a facility now in the United Kingdom, provides the physical calibration standard. TEOS-10, endorsed in 2010, adds Absolute Salinity in g/kg on top of PSS-78 rather than replacing the measurement itself.