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Old 03-12-2010, 07:54 PM   #1
jfkriege
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Default Calculating ABV without OG

Is there a way to calculate the abv of a brew without having the OG?

I have both a hygrometer and refractometer. Is there perhaps a way using the FG from the two instruments and calculating from that?

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Old 03-12-2010, 07:58 PM   #2
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If it's an extract recipe, yes, the OG will be easy to calculate. If it's not, then if you know your efficiency you can get a good estimate of the OG, but it may not be exact.

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Old 03-12-2010, 08:09 PM   #3
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if you have a good scale it is pretty easy to do. Measure the final gravity of the beer with a hydrometer. Take a small sample and weigh it. Then dry the sample with gentle heat, removing all the moisture. Then weigh it again - calculate the weight% solids. This will give you your final sugar% (or Final Brix). Send me the two numbers and I will tell you what % alcohol you have.

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Old 03-14-2010, 01:25 AM   #4
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ok mansfield, let me test your skills. my FG is 1.017. I used your dry-out method to find a %sugar of 5.93. So tell me, what is my %ABV?

I already know the answer cuz I measured my OG, I am just wanting to see if you can actually do this.

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Old 03-14-2010, 01:32 AM   #5
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The amount of solids will probably depend somewhat on the ABV and OG (or vice versa) because they both get in there during the mashing process but it seems unlikely that one can provide a reliable estimate of the other.

Putting your recipe into Beersmith or some other software to estimate the OG is probably as close as you can get without sending a bottle off to an analytical lab. Like Damdaman said, if it was from extract then you should know almost exactly how much fermentable sugar you started with.

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Old 03-14-2010, 05:23 AM   #6
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Now that I think about it I'm pretty sure you can get the ABV from the FG and the %age by mass of solids if you know the specific gravity of the solids. I don't know whether that is a known quantity or not. I'm used to doing those kinds of calculations with soils and and air and water, not with beer solids, water, and alcohol.

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Old 03-14-2010, 05:30 AM   #7
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This link has a method for calculating ABV without the OG -

"A quite simple way that will give accuracy up to 0.1% is to boil off all the alcohol and substitute by water. This means boiling down to less than a third of the original volume in most cases, it's not that hard to smell if there are alcohols in the vapour. Fill with water so you have your original volume and take the difference in gravity, then look up alcohol content in the table:"

http://www.realbeer.com/spencer/attenuation.html#alcohol

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Old 03-14-2010, 06:15 AM   #8
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The two things that I am wanting to know the ABV for are both wines. If it helps the situation, they have both fermented to dry (0.996).

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Old 03-14-2010, 04:25 PM   #9
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Yorbag,
Sounds like you made a pretty sweet beer - that is a good bit of sugar. So here is my answer: You have a FG of 1.017 and you measure the %sugar to be 5.9 wt% - this means you started with an OG of 1.058 and you have an ABV of 5.4%. My numbers will only be as good as your measurements. What kind of beer is it? How close are my numbers to your measured OG?

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Old 03-14-2010, 04:41 PM   #10
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jfkriege - if you have a good scale go ahead and dry out a sample. For a typical wine with an OG of 1.085 you would have a finished product of 2.6wt% sugar (BRIX) and 11.4% ABV for a final gravity of .998

withak - you are on the right track, need to have a good understanding of sugar effect on SG.

gxm - that is an interesting concept, I need to try it out. I am thinking it is easier to go ahead and dry the sample out and do the sugar analysis -if you have a scale. Also, I don't think all of the alcohol is gone until you almost completely dry it out. However, if you don't have a scale this method would work pretty well.

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