A hydrometer is a very simple tool. It’s function is simply to measure the relative density of a liquid with respect to water. Hydrometers can be calibrated to test the density of many different liquids. The hydrometer one uses for wine or beer making is called a saccharometer – its name denoting the fact that one measures the sugar content.
The way these instruments are calibrated has two facets. Firstly, the quantity of mass in the lower part of the instrument will cause it to sink in water but not submerge fully. Secondly, a scale is put inside the instrument to make the relative density. So the mass contained in the bottom of the hydrometers will be different depending on their purpose.
It is for this reason that hydrometers can’t be used interchangably – you can’t use a saccharometer in your wine making set to test the alcohol content of high proof spirits; for that you need an alcoholometer.
Some other types of hydrometers include Lactometers which measure the purity of milk, Thermohydrometer for petroleum products and the medical Urinometer for testing the content of solids in one’s urine.