Calculating Specific Gravity
The specific gravity is the ratio of the density of a liquid to the density of water. Water has a specific gravity of 1.000. In case of wine, as you add sugar (water soluble) the specific gravity increases, for example 1.015. The specific gravity indicates the amount of fermentable sugar or potential alcohol in the must (or wine).
A hydrometer is used to measure the specific gravity and has three scales, Specific Gravity, Balling or Brix and Alcohol Content. The Balling or Brix scale expresses the amount of sugar as a weight percent and the Alcohol scale indicates the potential alcohol content. To determine the alcohol content, you must take two readings, one before fermentation and another after fermentation is complete. Subtract the two numbers (Alcohol Scale) and the difference is the alcohol content of the wine.
To use a hydrometer, collect a sample of must (or wine) and pour into hydrometer tube (See Figure 2). Spin the hydrometer to remove bubbles and read at eye level the scale of your choice. Record the temperature of the must (or wine) and reading prior to fermentation.
Figure 1 Must
Figure 2 Pouring must into hydrometer tube
Figure 3 Read specific gravity at eye level
The specific gravity in Figure 3 is 1.096 (reading just below the air bubbles. Next, correct the specific gravity for temperature if the temperature of the must is above or below 60°F according to the correction chart. For example,
Temperature of the must is 70°F you must add 0.001. So the corrected specific gravity is 1.096 + 0.001 = 1.097
Adjust the sugar content according to the type of wine you wish to make. For a dry wine the ideal range is 1.085 to 1.100. In the example above no adjustment is necessary.
To adjust specific gravity use the following guidelines:
14.2 ounces of sugar in 5 gallons of must (or wine) will raise the specific gravity 0.005 units.
The formula to increase the specific gravity is:
(A-B)/0.005 X C/5 X 14.2 = Ounces sugar to add per 5 gallons
A= Target specific gravity
B= Actual specific gravity
C= Volume of expected wine in gallons
Dissolve the sugar in warm water 1-2 quarts (or gallons) depending upon the amount of sugar needed. Be sure to completely dissolve the sugar in water before adding to the must. Add the sugar solution in increments, stir and check after each addition. Be careful not to increase the specific gravity over your target.
The formula to decrease the specific gravity is:
(A-B)(5)/B X 10.9 = gallons of water to add per 5 gallons
Add water to the must in increments and check after each addition. If you are adding large quantities of water, don't forget to check acidity after adding the water.