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Old 09-14-2010, 11:16 PM   #1
kanzimonson
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Default Skeptical about the CO2 blanket

I'm having doubts about a common brewing concept and I'm hoping somebody with some more chemistry/physics experience can chime in.

I'm doubting that the CO2 created during fermentation truly creates a thick, impenetrable blanket over the top of our wort. Yes, CO2 is denser than air, and overall I think that the airspace in our fermenters stratifies into layers with mostly CO2 towards the bottom. I just don't think it's a 100% layer of CO2, on top of which is a layer of air.

Take the earth's atmosphere as a whole. If all the different kinds of gases in air separated into layers, then we'd be living on the surface of the earth under a CO2 blanket as well (or some other dense gas).

In wort, people seem to think of it like oil and water - almost instantaneous separation, but I think a better analogy would be something more like combining honey and molasses - they have similar densities, but not exactly the same, they flow but slowly, and they mix (slowly). If you added honey to the bottom of a container of molasses, the honey (it's denser) would force MOST of the molasses out the top, but there would be a little mixing.

This principle is basically the reason why we "purge" our kegs with CO2 three times. Each time you dilute the air content, and by the third time the dilution percentage is so high that we might as well say there's no air in there.

Anybody have any real insight into what this actually means for our fermenters and other applications?

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Old 09-15-2010, 12:23 AM   #2
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At the begining of fermentation the air in the headspace doesn't matter, we aerate the wort to introduce oxygen so the yeast can multiply easily. I have read somewhere that the volume of CO2 produced is approx 40-50x that of the must for wine (I'm sure it is at least 25x for wort base on the diferences in alc content) so for a fermenter with a 1:2 split between headspace:wort that would mean the headspace gets "purge" 50 times during fermentation. If we are happy with 3 for kegging it should be ok.
I'll try find that link later

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Old 09-15-2010, 12:27 AM   #3
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The air in our atmosphere is constantly being blown around and mixed. Sure, if you're opening up your fermenter and packing it around, then the CO2 layer will get disturbed and mix with outside air. But if you have a lid on your pail and an airlock, then that CO2 isn't getting blown off. the CO2 is just going to sit there until some outside force makes it move.

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Old 09-15-2010, 12:27 AM   #4
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I'm not chemist. All I know is that I ferment in corny kegs and when I stick my nose down into the opening to check the scent, I'll sometimes stick it below the rim of the opening and get knocked back onto my feet by the Co2. It's THICK in there. That's enough of a blanket to provide me with some assurance. Any o2 in there is not a concern.

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Old 09-15-2010, 12:30 AM   #5
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Originally Posted by JetSmooth View Post
I'm not chemist. All I know is that I ferment in corny kegs and when I stick my nose down into the opening to check the scent, I'll sometimes stick it below the rim of the opening and get knocked back onto my feet by the Co2. It's THICK in there. That's enough of a blanket to provide me with some assurance. Any o2 in there is not a concern.
+1 That's the perfect demonstration of the concept!
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Old 09-15-2010, 12:36 AM   #6
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Originally Posted by JetSmooth View Post
I'm not chemist. All I know is that I ferment in corny kegs and when I stick my nose down into the opening to check the scent, I'll sometimes stick it below the rim of the opening and get knocked back onto my feet by the Co2. It's THICK in there. That's enough of a blanket to provide me with some assurance. Any o2 in there is not a concern.
Haha, no kidding.

The day I got my first set of kegs I pressure tested one then popped off the top and looked in it out of curiosity. I saw the mist that had formed and was fascinated. My face was right up to the opening and it was still in the upright position. Without thinking I blew into the keg which not only displaced the CO2 in the keg but also triggered the reflex to inhale... damn near passed out. Learned my lesson quick about CO2 from that.
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Old 09-15-2010, 12:45 AM   #7
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People DIE in commercial breweries by getting their faces next to fermentation vessels. They pass out, fall in, and drown.

How's THAT for a serious Co2 blanket??

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Old 09-15-2010, 12:51 AM   #8
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Originally Posted by JetSmooth View Post
People DIE in commercial breweries by getting their faces next to fermentation vessels. They pass out, fall in, and drown.

How's THAT for a serious Co2 blanket??
that is how i would want to die. at the age of 85
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Old 09-15-2010, 12:59 AM   #9
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Quote:
Originally Posted by JetSmooth View Post
I'm not chemist. All I know is that I ferment in corny kegs and when I stick my nose down into the opening to check the scent, I'll sometimes stick it below the rim of the opening and get knocked back onto my feet by the Co2. It's THICK in there. That's enough of a blanket to provide me with some assurance. Any o2 in there is not a concern.
Ever burned your nose trying to drink the last bit out of a fermenter?
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Old 09-15-2010, 01:06 AM   #10
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I'm not bringing this up because I'm terrified of oxidation, and I'm obviously aware of the CO2 haze... hell, I'm a baker and you should experience it when you take the lid off a 140qt bowl full of bread dough - it burns so much you cough for 30 seconds or so. But do you think you'd get pretty much the same CO2 burn from 99% CO2 versus 100%? So where's the cutoff?

I'm just curious how much gasflow and mixing goes on in there. I mean, the liquid inside a fermenter maintains temp better than the gas, and the gas towards the edge of the fermenter is probably going to be a different temp than the inner gas. Therefore I'm guessing there's going to be some flow and mixing.

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