Mathematics Behind Salinity Calculation
Salinity is computed in two main ways: Practical Salinity (PSU) and Absolute Salinity (PPT).
1. Practical Salinity (PSU) is calculated using the conductivity ratio:
\[
\text{SP} = f\left(
\underbrace{\frac{C}{C_{\text{KCl}}}}_{\text{Conductivity ratio } R},
\quad T, \quad P
\right)
\]
where:
- C = Sample conductivity (mS/cm)
- CKCl = Standard KCl conductivity at 35 PSU and 15°C
- T = Temperature (°C)
- P = Pressure (dbar)
Practical Salinity is unitless and defined by an empirical polynomial fit to conductivity measurements.
2. Absolute Salinity (PPT) approximates the actual mass of dissolved salts per kilogram of seawater:
\[
\text{SA} = \text{SP} + \delta(\text{SP}, \text{lat}, \text{lon})
\]
where \(\delta\) is a location-dependent correction factor, typically around 0.2 g/kg for open ocean waters.
In this calculator, latitude and longitude are assumed zero, so:
\[
\text{SA} \approx \text{SP} + 0.2 \quad \text{(approximate absolute salinity in g/kg)}
\]
Units summary:
- SP (Practical Salinity): Unitless (PSU)
- SA (Absolute Salinity): Grams of salt per kilogram of seawater (g/kg ≈ PPT)