Oceanographers measure the salinity of ocean water by using a hydrometer, a hydrometer set, a chemical test kit or a salinometer. A hydrometer is a quick test tool used in aquariums, and it works by reading the water density. Higher dissolved salts result in higher density.
Hydrometers are quite easy to use, but they do not adjust the density when the temperature changes. A hydrometer set consists of a cylinder, a hydrometer, a thermometer, and a temperature, salinity and density, or TSD, graph. It measures the water density from a water sample. Higher salt content means the water is denser, causing the hydrometer to float higher in the cylinder. Oceanographers determine the salinity by reading the water density and the temperature in a standard TSD graph, as temperature, salinity and density are related variables. A hydrometer set is easy to operate on solid land but hard to use on a rocking boat.
A chemical test kit applies chemical titration with a sample of water to identify one of the salts and estimate the remaining salinity. This method is not highly accurate due to the interaction of other salts. Lastly, oceanographers use a salinometer to determine salinity by measuring electrical conductivity, but the instrument is not very accurate. All of these tools measure salinity in parts per thousand.