The hydrometer is used to measure specific gravity in the context of fermenting alcohol as it relates to the density of water. This is important as the readings help us determine several things. The potential alcohol we can get from our fermentation and when the fermentation has finished. The first reading is taken before the addition of yeast at 60 degrees F. If your temperature is different use the adjustment table that comes with the hydrometer to make changes. The final reading should be taken after fermentation is complete.
Let's start by learning how we read the hydrometer then we will move on to determining our abv (alcohol by volume). Fill your hydrometer tube about 2/3” from the top with the wash/mash you wish to test. Insert the hydrometer slowly not allowing it to drop. Next give the hydrometer a spin using thumb and index finger. This will remove the bubbles that may have formed.
The reading should be taken at the bottom of the meniscus (see image) In the following example above you would read the SG as .98
To determine the potential alcohol for our purposes we suggest that you try to keep your wash or mash between 8% and 14%, however some fruits and mashes will be below the 8%. We do not recommend going above 14% for product quality purposes. Below is a table for potential abv of a wash taken with an OG (original gravity) reading prior to adding yeast. This is a rough estimate and may be helpful in determining how much fermentable sugar to water ratio you aim for, however this can be off depending on your actual fermentation as you will see in the example below.
POTENTIAL ALCOHOL CONTENT FROM ORIGINAL GRAVITY
|Specific Gravity||Potential Alcohol|
Next we will calculate the actual potential after fermentation. Below is a second image that explains the steps involved. OG is your original SG reading and FG is the final reading.
Thus using the image above if our OG reading is 1.092 according to the chart above we have a potential abv of 11.75%. However if we take a FG reading after fermentation and we have a reading of .99 which is below what the chart above used to calculate then we come up with a different answer. Using the formula take the (1.092-.99) we get a difference of 0.102. If we then multiply 0.102 x 131 we get 13.36%. An easier method is using the link below to the calculator on the parent site.
Alcohol Content Calculator
The parent site calculator multiplies by 129 instead of the 131 used in this example but both give similar answers and the difference is nothing to worry about as everything is an estimate with many variables involved.
Using these figures on a 10 gallon wash I should get somewhere near 1.3 gallons. This again is a rough estimate at best as many factors determine how much you will actually get but should give you an idea depending on how fast you run your still and what type of still you are running.
Another method to determine the concentration of a solution of alcohol and water using specific gravity can be done using the following formula. This should get you a rough estimate especially if you forgot to take an OG reading at the beginning.
First measure the specific gravity of the solution, for this example we will use 0.98 as our FG reading.
X = unknown volume of water
(1-X) = unknown volume of alcohol
Then X + (1-X) = 5 gallons
We know that the SG of water = 1.0 and the SG of ethanol = 0.785
Therefore our equation becomes
(X) (1.0) + (1-X) (0.785) = SG of solution
Solve for X
Example: Assume the measured specific gravity is 0.98
X (1.0) + (1-X) (0.785) = 0.98 (Add value for water & alcohol to equation)
X + (0.785 - 0.785X) = 0.98 (Multiply)
0.215X + 0.785 = 0.98 (Combine like terms)
0.215X = 0.195
X =0.90 (Divide each side by 0.215)
(0.90) (100%) = 90% water
(0.25) (100%) = 10% alcohol