An hydrometer is an instrument used to determine the relative density of a fluid. For example, to measure the relative density of a sugar/water mixture before and after fermentation. The scale is usually in S.G. (Specific Gravity) units where 1.000 is the S.G. of pure water. Some hydrometers also have a scale to indicate the potential percentage alcohol that can be achieved if all of the sugar in the mash is converted into alcohol.A reading of less than 1.000 is usually an indication that the sugar in the mash has been converted to alcohol. It is NOT an indication of the percentage alcohol in the wash. It could be used to indicate the concentration of alcohol but ONLY in a mixture of pure water and pure alcohol.