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Salinity can be defined as the weight of all dissolved salts in water in the ratio of parts per thousand. Hydrometer, refractometer, and salinometer are all used to measure salinity.
The hydrometer measures the relative density of liquids. It can be indirectly used to measure salinity. As the salinity of seawater rises, its density increases as well.
The refractometer measures the refractive index of liquids. It can also be indirectly used to measure salinity. As the salinity of seawater rises, the amount of salt added rises, so the refractive index rises.
The salinometer measures the amount of dissolved salt content.