Another classification is based on the starting sugar content, as listed in Table 1. Classification of wines according to the sugar content. Use of a Hydrometer A hydrometer is a floatation device used to measure the density of a liquid. Because the liquid exerts a buoyancy force equal to the weight of the volume displaced by the hydrometer, the meter will float higher in a denser fluid than in a lighter fluid. In general, a denser solution has more dissolved solids.
Many scales are available on a hydrometer.
Of course, this represents the maximum value that is rarely reached. Usually, residual sugar is present and is sometimes desirable for taste. Note that for historical reasons, the alcohol content is commonly measured in units of volume percent or proof, while the sugar content is expressed in weight percent. Both units are widely used in wine industries. The proper way of using a hydrometer is to spin it gently in the grape juice, must, or wine for which the specific gravity is to be measured.