Alcoholometer

Alcoholometer is a special type of hydrometer which is used for determining the alcoholic strength of liquids. Alcoholometers have scales marked with volume percents of "potential alcohol", based on a pre-calculated specific gravity. A higher "potential alcohol" reading on this scale is caused by a greater specific gravity, assumed to be caused by the introduction of dissolved sugars. A reading is taken before and after fermentation and approximate alcohol content is determined by subtracting the post fermentation reading from the pre-fermentation reading.

ample calculation

Suppose a wine "must" (unfermented grape juice) is tested and reads "potential alcohol" of 15% on the alcoholometer's scale. The wine is tested again after fermentation and now reads 3%. The calculated alcohol strength, therefore, is:

15% - 3% = 12% alcohol by volume (with 3% unfermented sugars remaining)

The second reading also determines the sweetness of the beverage as it is an indication of the remaining (unfermented) sugars in the liquid - the larger this number, the sweeter the drink.