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The hydrometer is a calibrated cylindrical glass "rod" that is used to measure the sugar content of wine (or the must). The rod is floated in a test-tube containing the wine or must. As the density or specific gravity of the liquid varies with the sugar content, the buoyancy, or how the hydrometer will float in the wine or must will vary according to the amount of sugar. The more sugar in the wine or must, the higher the rod will float. The cylindrical glass rod contains markings along the glass rod for measuring the sugar content. Often the glass rods will contain makings to measure in one of several units-- Specific Gravity, Brix or Balling, or potential alcohol by volume.
For reference, water has a specific gravity of 1.000. Typically juice is brought up to a specific gravity of around 1.085 or 1.090, corresponding to about 21 brix. If all of this sugar is converted to alcohol, then the wine will end up about 11% alcohol.
As wine ferments, the yeast eats the sugar and produces carbon-dioxide gas and alcohol. By measuring the sugar content at the before fermentation begins, and then once fermentation ends, the difference in sugar content can be determined; and therefore the percent alcohol content of the wine can be calculated.
Brix (or Balling) Scale
The brix scale (or sometimes referred to as the balling scale) ranges from -10 to 40 degrees. As indicated above, in indicates the amount of sugar in the must. To calculate the potential alcohol of a must solution, multiply the brix reading by 0.575. This will approximately predict the alcohol content of the wine once fermentation is completed. This assumes that the must fully ferments.
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