Is it possible to put 3 gallons into a 5-gallon cornie and have it force-carbonate properly with all that empty headspace? I imagine I'd go through a fair amount of CO2 trying to purge 2/5 of the headspace of oxygen. For those whose immediate reaction is to ask why I would want to do that - to bottle the other 2 gallons.
Yes, you can do this, no problem. And yes, it will take more CO2 to purge the headspace than if the keg was full. But less than it took you to purge the keg before you filled it with beer. You do that too already, right?
As the others said, yes it's possible. Your beer does not know or care if there is one inch of headspace or ten inches. As long as you maintain the correct pressure, the CO2 will establish equilibrium in your brew. Yes you will use more CO2, but just like your beer, when you need more you get more. You might also consider bottling from your keg instead of splitting the batch. This can provide for more consistent carbonation, but you will need something like a Blichmann Beer Gun. If you are into the hobby this deep, what's another 75 bucks?
It will use a bit less CO2 than a full keg. Think of two quantities of CO2 - one quantity that gets dissolved in the beer, and the other pushes the beer our of the keg. You'll add 2.5-ish volumes of CO2 to three gallons instead of five - that's the first ('less') part. However, by the time the keg kicks, you will have added a 5 gallons volume of CO2 gas at 12-ish psi either way.
I always try to cheat when I know I'm getting down the last couple of pints. I turn the CO2 off and let the pressure drop a bit as I finish the keg. I think I'm saving CO2 this way.
No, I don't, and it has been a nagging worry. Can you also suggest guidelines for purging a 5 gallon corny without wasting CO2? I realize that sounds a little penny-wise, pound-foolish (better to waste CO2 than beer). Still, how do you know when you are finished purging a 5 gallon keg?Now that I have a 10 lb CO2 tank instead of a 5 lb one, maybe I can be a little more liberal in my purging.
You can actually do the math and figure out how much O2 you'll still have left in the keg, but I haven't done that. (I think hopfenunmaltz has IIRC.) I set my regulator to 20 PSI and purge 5 or 6 times. By purge I mean bleed off a burst of pressure and let the keg come back up to pressure again. I have a 10 lb tank and two 5 lb tanks. Sometimes I wish I had more. But I would strongly recommend to anyone that they have at least one backup.
So after purging the keg, do you just disconnect gas, bleed the pressure, pop the lid, and rack? That's what I've been doing (with a piece of sanitized foil over the keg opening), banking on the CO2 blanket to keep things relatively oxygen-free while I rack. Is this the best approach short of transferring under pressure?
Thanks all, I gave this a try. 3 gallons in a 5 gallon keg, purged the headspace, gas line on the out post at 30 psi for 24 hours, then gas line on the in post for four days at 12 psi and it carbonated nicely. Vikeman, thanks for the tip on purging the empty keg first, I don't usually do that, but will from now on.
One thing to think about besides purging is that if this is an extremely hoppy beer you will want to add some extra dry hops. The larger dead volume in the keg will suck off some of the aroma compounds, so its best to add some extra hops to compensate
