See also mustimeter :
Apparatus for measuring the density of grape juice or must during fermentation.
The measurement of the density of unfermented grape juice makes it possible to know with a good approximation the sugar rate.
The probable alcoholic degree is deduced from this.
The cellar master then follows the evolution of the alcoholic fermentation by frequently (once or twice a day) measuring the density of each vat. By plotting a curve of the density as a function of time, he can see a possible slowing down and take action before the fermentation stops completely.
The densimeter (or mustimeter) is therefore a simple but essential instrument for the winemaker.