Good morning folks - I hope this is the correct place to post this question. I would like to have the flexibility to scale between 5 and 20 gallon batches. If I buy one fermenter that can handle 20 gallons, what would the implication be for smaller batches? Would the extra space potentially cause a problem? I'm thinking no because the CO2 from the yeast activity will eventually fill it but would like to hear some thoughts on it ;-) Thanks!
excess space in fermenter - OK or not?
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I ferment 2 1/2 gallon batches in a 6 1/2 gallon fermenter with no problems. I don't think your bigger fermenter for small batches will be a problem either.
I think there is a potential problem. There will be alot of oxygen compared to wort/beer.
It will reduce during the fermentation cycle but with 5 gallons in a 23? gallon fermenter, there will be quite alot of oxygen left after fermentation is complete.
It will reduce during the fermentation cycle but with 5 gallons in a 23? gallon fermenter, there will be quite alot of oxygen left after fermentation is complete.
It's not entirely a bad bet. From a physics perspective, roughly equal weights of ethanol and CO2 are produced. CO2 is roughly a thousand times less dense than water (this is our reference density=1 gram/mL). (Normal air is 1.2 grams per liter, so I'm rounding CO2 to 1.5 g/L.) At 5% ABV, you know how much ethanol you'll produce. Multiply by the density of ethanol to get weight. And since weight of ethanol almost exactly equals weight of CO2 produced, divide by the approximate density of CO2 to get its volume.
I plugged all that into google as follows:
5% * 5 gallons * 0.79 grams/ml / (1.5 g/L) to gallons
You will produce 132 gallons of CO2. That is clearly enough absolute volume, but the question is whether it will mix too much. There were about three months when I was in university when I would have been able to do that math problem. For all the time before and since, it's been way too hard.
I wouldn't risk it. Is there any reason you can't get a 5-gallon fermenter? They cost practically nothing if you don't mind plastic and are willing to wait until you find something suitable.
* An error in my wording and a mistake in the CO2 density was corrected. Previously I had come up with 195 gallons, which just goes to show that "back of the envelope" math is fairly tolerant of small mistakes!
I plugged all that into google as follows:
5% * 5 gallons * 0.79 grams/ml / (1.5 g/L) to gallons
You will produce 132 gallons of CO2. That is clearly enough absolute volume, but the question is whether it will mix too much. There were about three months when I was in university when I would have been able to do that math problem. For all the time before and since, it's been way too hard.
I wouldn't risk it. Is there any reason you can't get a 5-gallon fermenter? They cost practically nothing if you don't mind plastic and are willing to wait until you find something suitable.
* An error in my wording and a mistake in the CO2 density was corrected. Previously I had come up with 195 gallons, which just goes to show that "back of the envelope" math is fairly tolerant of small mistakes!
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I get the whole math is a problem thing - I look at old calculus tests, recognize the writing as mine but have no clue what it all means any more! I have 5 GAL bucket for fermenting - I just want to be able to heat/chill as needed to maintain accurate temp in the fermenter. We oxygenate before going into the fermenter..... I COULD fill the space with CO2.....
If you can adequately purge the headspace after pitching yeast than it's really a non-issue. You'll waste some CO2 but it should really amount to just a few cents. Unless you're paying exorbitant prices for CO2 that is...
If you're really concerned with that, you could try to partially purge the space with CO2, and it probably would work fairly well.
One can purge a keg with the CO2 that's off-gassed during fermentation. I can't recall the exact source of this, but 50 points of gravity is worth about 25 volumes of CO2--and if you purge something 25x, you'll have almost pure CO2. So if you have an OG of 1.062 and it drops to 1.012....there's your 25 volumes.
But that presumes the space is 5 gallons that you're purging, and your extra headspace is much larger than that. So i wouldn't expect it to be completely purged when doing a 5-gallon batch...which you already know/suspect.
Purging a 15-gallon headspace with a 5-gallon fermentation...won't completely work, if my math is correct. So the solution is to A) purge that headspace w/ CO2 after oxygenating the wort, or B) get a smaller fermenter, as @piojo suggests.
FWIW: there probably is some absorption of the oxygen in the headspace back into the wort, where the yeast makes use of it. This isn't fundamentally different than expecting oxygen to be drawn into a starter flask on a stir plate. How much? I don't have any idea other than "some."
FWIW: there probably is some absorption of the oxygen in the headspace back into the wort, where the yeast makes use of it. This isn't fundamentally different than expecting oxygen to be drawn into a starter flask on a stir plate. How much? I don't have any idea other than "some."
Henry's law is your friend.
O2_concentration = O2_partial_pressure / 770
Where partial pressure is in atmospheres and concentration is in mol/l. For oxygen multiply concentration in mol/l times 32 to get concentration in g/l.
This is however all theoretical, as soon as the fermenting wort reaches the high kräusen stadium the kräusen itself will provide a barrier to oxygen ingress. The problems start if you still have too much O2 in the headspace once kräusen fall.
