Hydrometers 

A hydrometer is an instrument used for determining the specific gravity of liquids. A Hydrometer can be made of glass and consist of a cylindrical stem and a bulb weighted with mercury or shot to make it float upright. The liquid is poured into a tall jar, and the hydrometer is gently lowered into the liquid until it floats freely.

Using water as a reference material can make calculations using SI units convenient, since density of water is (approximately) 1000 kg/m³ or 1 g/cm³. Determining the specific gravity (same as relative density of an object relative to water) entails little effort, as the density of the object only needs to be divided by 1 or 1000, depending on the unit,

The function of the hydrometer is based on Archimedes principle that a solid suspended in a liquid will be buoyed up by a force equal to the weight of the liquid displaced. Thus, the lower the density of the substance, the lower the hydrometer will sink. All of our refractometers act as hydrometers as well letting you determine the specific gravity of the substance you are testing.  This is made easier by models that have denisity scales, However density can be converted from brix, RI, % concentration., etc into a density measurement on any refractometer