Practical salinity *S*_{P} is defined on the Practical Salinity Scale of 1978 (PSS-78) in terms of the conductivity ratio *K*_{15} which is the electrical conductivity of the sample at temperature *t*_{68} = 15 °C and pressure equal to one standard atmosphere, divided by the conductivity of a standard potassium chloride (KCl) solution at the same temperature and pressure. The mass fraction of KCl in the standard solution is 0.0324356 (32.4356 g of KCl in 1 kg of solution). When *K*_{15} = 1, the Practical Salinity P S is by definition 35. The conductivity of that reference solution is C(35,1568,0) = 42.914 mS/cm = 4.2914 S/m (Siemens per meter). Note that Practical Salinity is a unit-less quantity. Though sometimes convenient, it is technically incorrect to quote Practical Salinity in "psu". When *K*_{15} is not unity, *S*_{P} and *K*_{15} are related by the PSS-78 equation

At a temperature of *t*_{68} = 15 °C, *R*_{t} is simply *K*_{15} and Practical Salinity *S*_{P} can be determined from the above equation. For temperatures other than *t*_{68} = 15 °C, Practical Salinity *S*_{P} is given by the following function of *R*_{t} (*k* = 0.0162)

(*t*_{68 }/°C - 15)
/
[1 + *k*(*t*_{68 }/°C - 15)]

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Generalic, Eni. "Practical salinity." *Croatian-English Chemistry Dictionary & Glossary*. 20 Oct. 2018. KTF-Split. {Date of access}. <https://glossary.periodni.com>.

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