Headspace pressure in fermenter

[bjacokes]

I've been gently rousing my beer in secondary while dry hopping, which causes a bit of CO2 to bubble out of solution. This got me thinking about the following hypothetical scenario:

Say I have my beer in secondary - assume it's completely done fermenting and has stopped bubbling (maybe it's been sitting there for several months), and it's at a constant 70F. That should mean that the pressure of CO2 in the headspace is one atmosphere. Now I rouse the beer, some CO2 bubbles out of solution and increases the pressure so that some of the CO2 escapes from the airlock. Now if I wait around then some of the CO2 in the headspace will dissolve back into the beer, but in the end, the pressure in the headspace must be less than one atmosphere (because all conditions are the same as before the rousing, but there is less CO2).

Shouldn't this pressure difference mean that the airlock starts pulling air back into the fermenter? And how come I don't see this happen after rousing?

 

[TipsyDragon]

CO2 in the head space is not going to dissolve back into the beer once you stop rousing it. now i have seen the air lock pull liquid back in when the temperature drops and then push it back out when the temp rises. typically at that point i switch to a carboy cap with both plugs in.

 

[bjacokes]

My understanding is that CO2 in the headspace always equalizes with the CO2 in solution. This is like what happens when you shake a beer bottle - CO2 leaves the beer and enters the headspace, but if you give it time then that CO2 will dissolve back into the beer. If a similar process took place when you roused a fermenter, that CO2 dissolving back into the beer should cause a pressure differential that sucks air back into the fermenter, even at a constant temperature. Unless my logic is flawed...

 

[TipsyDragon]

yes your logic is flawed. the reason the CO2 in the beer bottle disolves back is because of the increase in pressure in a closed system. the CO2 has nowhere to go. but in a carboy that extra gass goes out the airlock and equalizes the pressure. now when the temp falls the CO2 contracts and no air can get in to fill the space because of the water in the lock. the only way the pressure can equalize is to suck water in from the airlock till either the pressure equalizes or air is pulled in from outside and equalizes that way. rousing will expell some gass and equalize the preasure but there is only so much gass to outgass. at some point you will get nothing for your efforts and still have the vaccume problem.

 

[bjacokes]

My claims are that:
(1) there is less overall CO2 in the carboy after rousing pushes bubbles out of the airlock (assuming fermentation is finished)
(2) if there is less overall CO2 in the carboy, this must mean that, at equilibrium, there is less CO2 in the beer *and* less CO2 in the headspace
(3) if there is less CO2 in the headspace, that means the pressure in the headspace is less, which means that air should be sucked back in through the airlock

Anyone else want to chime in on this?

 

[Scimmia]

You're forgetting why the CO2 is dissolved in the beer in the first place, though. It was produced in the beer and stayed in the beer. With enough time, it would come out on it's own as it equalizes with the partial pressure of CO2 in the headspace. If it's coming out of the beer, it was not in equilibrium in the first place.

 

[radamadar]

Interesting thread... I think when you agitate, you're rousing a system out of an relatively unstable status. In order to kick off some nucleation or whatever, you provided an activation energy.It's like a book standing on its side... you have to give it a kick to get it up and over an edge, then it can quickly fall to a more stable condition. This kicks out some CO2, and the system levels out at some new level, closer to the natural equilibrium one would expect at standard temperature and pressure I would think.

It's also important to remember that though the system may be in equilibrium, that doesn't mean CO2 isn't being absorbed into the beer; all it means is that the rate at which CO2 effuses from the beer and the rate at which it is being absorbed are approximately equal. That's probably important given the partial pressure comment above... other gases are doing the same thing (including oxygen), though it sounds like most practical experience suggests there's enough coming out of solution to scrub a reasonable headspace under most common racking timeframes.

 

[Edcculus]

I think what you are saying makes sense if you dont have an airlock. I'm no scientist, but I think it would take a good bit of negative pressure inside the bucket/carboy for air to suck back in. Remember that it would have to push, or suck, depending on how you look at it, a good bit of water through the tube first. That won't happen unless you have a significant pressure difference. Like when you pick up a bucket full of beer. The bucket warps, and sucks water in. I don't think that pressure difference will happen from a change in temp, CO2 dissolving, or not etc.

 

[radamadar]

I think what you are saying makes sense if you dont have an airlock. I'm no scientist, but I think it would take a good bit of negative pressure inside the bucket/carboy for air to suck back in. Remember that it would have to push, or suck, depending on how you look at it, a good bit of water through the tube first. That won't happen unless you have a significant pressure difference. Like when you pick up a bucket full of beer. The bucket warps, and sucks water in. I don't think that pressure difference will happen from a change in temp, CO2 dissolving, or not etc.

Hmmm... airlock. The cap on a bubbler can't weigh more than 1 or 2 ounces right? 1 atmosphere is about 14.7 pounds per square inch. A standard bucket has about a six inch radius? (2/16) / (3.14*6^2) is pretty darn small. I don't think the airlock has much of an impact. Maybe for a blow off type setup there could be though...

Definitely here you on warpage and sucking though. I'm more careful since that first 5 gallon batch!

 

[WillPall]

I would think about it like this: The beer is exerting more pressure on the CO2 that is trapped inside the liquid than the air outside is exerting on the headspace. The CO2 is actively trying to come out of the beer, but it just takes a long time because of the pressure of the thicker substance. When you agitate it, you help that CO2 come out of the beer and once it's in the headspace there's not enough pressure to force it back into the beer.

And yes, temperature variations can cause suckback, but both problems are alleviated by filling your airlock with vodka instead of water.

And yes, I'm drunk posting, so I may be completely wrong.