I would like to brew a low ABV beer and split into two 5-ish gallon batches but I'm not sure how to create a recipe. I have a 10.5 gallon BIAB kettle, so I'm pretty sure I can't just brew it all and split it in half. I think I need to brew with less water in the kettle and top it off in the fermenters but how would I calculate all that?
How to calculate ABV for a split batch?
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By itself a 10.5 gallon kettle is not going to give you two 5-ish gallon batches. If you find a way however you just measure the gravity in each fermenter before pitching the yeast. Measure the final gravity when fermentation is complete and make your ABV calculations. It will be no different at all from making a single batch.
If you brew a 1.090 wort and top off with a equal volume of water, you'll have a 1.045 wort in the fermenter.
((90*1)+(00*1))/(1+1)=45
2 parts 1.090 wort with 1 part water, 1.060
((90*2)+(00*1))/(2+1)=60
Etc.
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Yep, sorry about the confusion. I can start with 5 or 6 gallons of wort at 1.090 (or something high) and split that into two fermenters. Then add enough water so that I finish with 5-ish gallons of beer in each fermenter with lower gravitys. I'm just not sure how to calculate that. Ideally I want to end up with two beers that are 4-5% ABV. I hope that makes more sense.
Thanks! is there a name for this formula?
Not that I know of. It's just an exercise in averages.
I should have paid more attention to my high school math classes lol .
To target an ABV you need to have a good estimate of your yeast's attenuation. Then you can do some algebra on the yeast attenuation equation and the ABV equation and solve for a target OG.
Formula for yeast attenuation is:
Let's say you want 5.5 gal of 1.040 SG wort in each of two fermenters, and you can brew a 6.5 gal batch. The total fermenter volume will be 11.0 gal, and the target OG for the brewed batch will be:
Brew on
Formula for yeast attenuation is:
Attenuation = (OG - FG) / (OG - 1) [multiply by 100% to get % attenuation]
Formula for ABV is:ABV = (OG - FG) * 131.25
If we solve the ABV formula for FG, and then substitute for FG in the Attenuation formula, we can then solve for OG:ABV / 131.25 = OG - FG
FG = OG - ABV / 131.25
Attenuation = (OG - (OG - ABV / 131.25)) / (OG - 1)
Attenuation = (ABV / 131.25) / (OG - 1)
OG - 1 = (ABV / 131.25) / Attenuation
OG = 1 + ABV / (131.25 * Attenuation) [where Attenuation is fractional, not percentage, 75% = 0.75]
We can use the last equation above to determine our target OG from our desired ABV and the yeast attenuation. Let's do an example. If we want 4.0 ABV, and our intended yeast has an attenuation of 75%, the calculation looks like this:Target OG = 1 + 4.0 / (131.25 * 0.75) = 1.0406
Target FG = 1.0406 - 4.0 / 131.25 = 1.0101
The above target OG is the SG in the fermenter(s). If you want to brew a higher OG batch and dilute it into two lower OG batches with larger total volume than the brewed batch volume, you have to figure out the target OG for the brewed batch. In that case the math looks like this:(Brewed OG Target - 1) = (Fermenter OG Target - 1) * Fermenter Volume / Brewed Volume
Brewed OG Target = 1 + (Fermenter OG Target - 1) * Fermenter Volume / Brewed Volume
Let's say you want 5.5 gal of 1.040 SG wort in each of two fermenters, and you can brew a 6.5 gal batch. The total fermenter volume will be 11.0 gal, and the target OG for the brewed batch will be:
Brewed Target OG = 1+ 0,040 * 11.0 / 6.5 = 1.0677
Brew on
Turns out brewing the above example, with a 0.5 gal BK trub hold back allowance, wouldn't work in the OP's 10.5 gal kettle, as the mash volume would be 11 gal. That's for no-sparge BIAB with a grain absorption rate of 0.08 gal/lb, 1.25 gal boil-off and 100% conversion efficiency (would also require 17.2 lb of grain.)
So, let's see what a 5 gal + 0.5 gal BK trub allowance plus a reduced 1.0 gal boil-off would look like.
Brew on
So, let's see what a 5 gal + 0.5 gal BK trub allowance plus a reduced 1.0 gal boil-off would look like.
Brewed Target OG = 1+ 0.040 * 11.0 / 5.0 = 1.088
This ends up with 19.3 lb of grain (due to reduced lauter efficiency of less strike water), and a 9.6 gal mash volume (tight but doable.)Brew on
And here comes Doug with the real math!
Just looking for an update as I would like to do something similar. How did it turn out?
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