Salinity

Salinity is a value for the salt content for strongly saline solutions and is dimensionless (no unit). It is given in ppt = promill = g/kg = PSU (practical salinity unit) or percent. Salinity is a temperature-independent parameter. Determining salinity by evaporation is extremely laborious and is not used in everyday aquaristic practice.

A widely used way to indicate salinity is the density of the solution. However, the density is temperature dependent. Without the indication of the temperature and a suitable conversion table, a density measurement, e.g. with a spindle, is not useful. The spindle, or more precisely the areometer, should have a high design so that a precise reading can be made. The spindle should only ever come into contact with water for measurement. If it is stored in seawater for a long time, fouling will form and falsify the results. Preferably, the areometer should be read in a suitable measuring cylinder.
Float meters use the same effect.

The second practical solution is to measure the temperature-dependent electrical conductivity. This increases in proportion to the salinity of the water. The measuring device should be suitable for seawater and should be checked at least annually with a fresh standard (reference). This method also requires a table to calculate the salinity. Some meters can also calculate the salinity.

The salinity can also be determined by the refractive index of the seawater. A refractometer is used to make this measurement easily and quickly. Care should be taken that the instrument has the appropriate scale and can be calibrated. The refractive index is also a temperature-dependent variable. Therefore, a temperature-compensated instrument is preferable.