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Old 03-27-2011, 09:46 PM   #1
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Default Gravity / ABV maths questions. Do my homework for me.

Recently brewed a 5G recipe using a 2.5G boil volume. On finishing, I realised I didn't have the fermenter available that I thought I had. all I had available was a 5G plastic bottle.

So, I decided to throw in my 2.5G of wort, and add water until reaching 4G, leaving 1G worth of head space.

My OG was 1.067.

I'm going to estimate my FG will be 1.015 (stay with me here). ABV roughly 6.85%

question: Once fermentation is complete, and I add an extra gallon of water to bring the total volume up to 5G, what will my FG and ABV be? Can you show me the maths required to figure this out?

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Old 03-27-2011, 09:56 PM   #2
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Yup,that bottle is def too small. A 6.5G bottle is what you need. Adding water after ferment is done is not a good idea. It seems like with a full 5G in the fermenter,the OG would be 1.045-ish,1.050 maybe.
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Old 03-27-2011, 10:23 PM   #3
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Your OG was 1.067 after cutting with water?

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Old 03-27-2011, 10:26 PM   #4
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I get 1.054OG, 1.012FG, and 5.5% ABV

(67*4)/5 and (15*4)/5

But, I would never advocate topping off post-fermentation....Just roll with your four gallons and get a bigger fermenter for next time...

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Old 03-27-2011, 10:31 PM   #5
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Huh, what's the deal with not topping up after fermentation?

This is a belgian golden ale, so there's no problems letting it go to the higher ABV, but I am curious.

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Old 03-27-2011, 10:34 PM   #6
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Topping up post fermentation is tricky. You can't have oxygen in your water, and because he diluted it before it may get very thin. I do post fermentation dilution on occasion and you have to know your stuff or it is horrible. That's why I wanted to know his true OG.

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Old 03-27-2011, 10:37 PM   #7
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I had about 2.5G left in the pot after boiling was done. I then added enough water to make 4G. The OG of those 4G was 1.067

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Old 03-27-2011, 10:48 PM   #8
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Ahh, well c1v1 = c2v2 when diluting. V= volume, and C= concentration.

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