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Old 01-30-2009, 02:53 PM   #11
david_42
 
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Oct 2005
Oak Grove, Oregon, USA
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If you have a precision scale, it's fairly easy. Add exactly 100 grams of brew to a small jar. Heat the jar in a waterbath at 180F. Monitor the temperature of the beer, when it goes over 175F all of the alcohol is gone. Weigh again. The difference is your percent alcohol by weight.
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Old 01-30-2009, 02:54 PM   #12
remilard
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Nov 2008
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A couple of points:

None of these formulae are going to be exact, they are approximations.

Gas chromatography is indeed in the best way to measure this and so we generally prefer approximations that can be performed with a number of hydrometer readings.

I believe the Cape Brewing approximation is better than the 131 approximating however I think Cape Brewing is doing it wrong. In the numerator you ought to be subtracting the real final extract and not the apparent final extract. Also this ought to be performed in degrees plato not SG terms.

The two approximations ought to agree quite closely if the Cape Brewing approximation (due to Balling) is being used correctly.

Here is an HBD post from George Fix on the Cape Brewing method.

HOMEBREW Digest #880 Wed 13 May 1992

Note that Fix claims that the Balling approximation is subject to error as large as 3%-5% (percent error, not in absolute terms) and the 131 approximation is a bit rougher.

 
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Old 01-30-2009, 03:01 PM   #13
remilard
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Nov 2008
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Quote:
Originally Posted by david_42 View Post
If you have a precision scale, it's fairly easy. Add exactly 100 grams of brew to a small jar. Heat the jar in a waterbath at 180F. Monitor the temperature of the beer, when it goes over 175F all of the alcohol is gone. Weigh again. The difference is your percent alcohol by weight.
Not all of the alcohol will be gone, just like not all water is gone once you have heated it to 100 C. Confounding the issue is that water dissolves alcohol extraordinarily well. Also some of the water will have evaporated.

Empirical evidence suggests that alcohol in beer can not be reduced much more than by half in a household kitchen. Mary Beth Raines posted about this on HBD. Nobody but her appears to have ever performed this experiment and actually measured the result and then written about it publicly.

 
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Old 01-31-2009, 04:29 PM   #14
Quincer
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Jan 2009
Lincoln, Nebraska
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Quote:
Originally Posted by YooperBrew View Post
If you post the recipe you used, we could probably give you a really good guestimate.
I didn't realize the recipe had the ABV. It's 4.0% to 4.5%. A little vague but close enough. Thanks for your help.

American Light
3.3 lbs. Plain Light Malt Exract
1 lb. Plain Light Dry Malt Extract
1 lb. Rice Syrup Solids
1 oz. Sterling Hops (Bittering)
1/4 oz. Sterling Hops (Finishing)

 
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