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K111er
03-12-2007, 02:19 PM
Over the weekend, I was explaining to my brother how a hydrometer works, and how we can use OG and FG measurements to calculate an approximate alcohol content. As part of the explanation of how this works, I wrote out some of the equations, and, after seeing it written out like that, I had an insight that I thought I'd share.

ABV ~= delta_SG/0.00736 where delta_SG = OG - FG

I thought about it some, and really understood how this equation defines the relationship between ABV and SG. So, I did some substitutions, using D47 as an example. D47 has an advertised PABV of 14%, so:

14 ~= delta_SG/0.00736

or delta_SG ~= 14 x 0.00736 = 103.04

This means that D47 has the capacity to chew through about 100 points of sugar density to produce 14% alcohol.

So, if I know what my target FG is (based on taste preference), I can easily add the appropriate amount of honey to get the OG required for the yeast under consideration to reach the target. This has likely already been discussed somewhere, but perhaps my equations might be useful to some other newbees.

10% PABV needs 74 points of extra SG
12% PABV needs 88 points of extra SG
14% PABV needs 100 points of extra SG
16% PABV needs 118 points of extra SG
18% PABV needs 133 points of extra SG
20% PABV needs 147 points of extra SG

This is, of course, if you wish to load honey once, without backsweetening at the end.