What do hydrometers test?
The specific gravity (density) of a liquid.
They are called hydrometers because their primary use is in the measurement of the specific gravity of water solutions. This is an inferential measurement of the amount of water and solute in a solution.
They are based on the density (specific gravity) of water as a reference. When other compounds are mixed or dissolved in water, they change the bulk density. Thus a solution becomes more or less dense than pure water when something is added to it.
The change is NOT the same for all compounds, so a single reference to “water purity” cannot be made. However - particularly in the case of binary mixtures – a calibrated hydrometer is a very rapid and inexpensive measuring device.
They are extensively used in the beverage industries for both sugar and alcohol content.