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Old 03-11-2012, 04:10 PM   #1
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Default 3 gallons brew in 5 gallon keg

I am a newbie having just received my keg setup.

I have a 3 gallon brew in my fermenter. I would like to keg it in a 5 gallon corny. This of course leaves a very large headspace.

Is this a wise thing to do?
Will I be using a lot more CO2, especially to initially purge the O2?
WHich carbonation scheme is best?

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Old 03-11-2012, 04:27 PM   #2
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I do it all the time. you'll use less c02 than if you put a 5 gallon batch in there. I assume you're force carbing and if you are then you have less beer to carb. The empty space gets filled in every keg whenever the beer leaves regaurdless of how much beer was initially in there.

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Old 03-11-2012, 04:46 PM   #3
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Quote:
Originally Posted by jester5120 View Post
you'll use less c02 than if you put a 5 gallon batch in there.

I too have done it, but use much much more CO2 at first. In order for it to not go bad there ideally would be no air in there. With 3 gal in a 5 gal corney there starts out to be 2 gal of air. This must be replaced, as well as you can, with CO2. I bring it up to 15psig (**) and then purge. This would leave ½ air and ½ CO2. Do it again and you yeild ¼ air, do it again and yeild 1/8th air, again, 1/16th, etc. I usually do it 4 times for a 5 gallon batch to leave very little air in there. For three gallons I have gone 4 or 6 times, depending on how stingey I am feeling. Once the beer if flowing, hooked up to the CO2, it is all CO2 going in. I always start it off with as little air as possible.



** (standard room air pressure that we breathe is ~14.8psi because of the weight of the 600 miles of air there is abouve us. When you read 15psi on your gage, that is really 15psi more than the std room pressure and technically is abbreviated psig. If you add 15psig CO2 to the keg the real pressure becomes 30psi, 15 from the room air and 15 from the CO2)
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Old 03-12-2012, 01:55 AM   #4
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Quote:
Originally Posted by Eisendrath View Post

I too have done it, but use much much more CO2 at first. In order for it to not go bad there ideally would be no air in there. With 3 gal in a 5 gal corney there starts out to be 2 gal of air. This must be replaced, as well as you can, with CO2. I bring it up to 15psig (**) and then purge. This would leave ½ air and ½ CO2. Do it again and you yeild ¼ air, do it again and yeild 1/8th air, again, 1/16th, etc. I usually do it 4 times for a 5 gallon batch to leave very little air in there. For three gallons I have gone 4 or 6 times, depending on how stingey I am feeling. Once the beer if flowing, hooked up to the CO2, it is all CO2 going in. I always start it off with as little air as possible.
you don't need to do all that. you're wasting gas, time, and time away from your beer. all you need to do to make sure no air is in the keg is transfer beer into the keg regaurdless of volume, after that the only air left in the keg is what's sitting on top of the beer. to get rid of that turn the gas on with the lid of the keg still off. since c02 is heavier than air the co2 will take the place of the air that was sitting on your beer and the air will rise out the top. then close up the keg and give your relief valve a tug to let out a little gas to get every bit of air out and you're done. no fractions involved and a lot less gas used
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Old 03-12-2012, 08:23 PM   #5
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Gases mix. Yes CO2 is heavier than "air", but gases mix.

The Earth's atmosphere is 78% Nitrogen, 20% Oxygen,
1% Argon, .04% Carbon dioxide (CO2), etc.
If gases didn't mix we'd be suffocating in argon and CO2.
Specific Gravity of Nitrogen is .97
Specific Gravity of Oxygen is 1.10
Specific Gravity of Argon is 1.38
Specific Gravity of CO2 is 1.52

Gases mix. When you blast CO2 into your keg it mixes with the air in there. Thinking that the CO2 floats to the bottom and air comes out the releif valve, doesn't compute. Sorry.

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Old 03-13-2012, 01:46 AM   #6
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Quote:
Originally Posted by Eisendrath View Post

I too have done it, but use much much more CO2 at first. In order for it to not go bad there ideally would be no air in there. With 3 gal in a 5 gal corney there starts out to be 2 gal of air. This must be replaced, as well as you can, with CO2. I bring it up to 15psig (**) and then purge. This would leave ½ air and ½ CO2. Do it again and you yeild ¼ air, do it again and yeild 1/8th air, again, 1/16th, etc. I usually do it 4 times for a 5 gallon batch to leave very little air in there. For three gallons I have gone 4 or 6 times, depending on how stingey I am feeling. Once the beer if flowing, hooked up to the CO2, it is all CO2 going in. I always start it off with as little air as possible.
Exactly how long is the relief valve open each time you purge? because unless you have it perfectly timed at equal intervals that would make your fractions increasly incorrect. In theory i'm sure this is true but you have no way of knowing whether this untimed method or pulling the relief valve and holding it open for a few seconds works any better.

Quote:
Originally Posted by Eisendrath View Post
Gases mix. Yes CO2 is heavier than "air", but gases mix.

The Earth's atmosphere is 78% Nitrogen, 20% Oxygen,
1% Argon, .04% Carbon dioxide (CO2), etc.
If gases didn't mix we'd be suffocating in argon and CO2.
Specific Gravity of Nitrogen is .97
Specific Gravity of Oxygen is 1.10
Specific Gravity of Argon is 1.38
Specific Gravity of CO2 is 1.52

Gases mix. When you blast CO2 into your keg it mixes with the air in there. Thinking that the CO2 floats to the bottom and air comes out the releif valve, doesn't compute. Sorry.

This forum is here to help people not to belittle them or try to prove yourself better. While i appreciate people like you that have a passionate drive to learn the fine scientific details of brewing, your rude in your face attitude doesn't help anyone.
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Old 03-13-2012, 02:21 AM   #7
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What if you filled the keg with sanitizing solution, put the lid on, use low pressure CO2 to push the sanitizer out turn the gas off, keg now has nothing but CO2 in it. The hold the relief valve open or crack the lid and rack the beer in through the outlet fitting so that it fills from the bottom and the blanket of CO2 is pushed up and out the relief or lid.

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Old 03-13-2012, 02:49 AM   #8
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Quote:
Originally Posted by jester5120 View Post
Exactly how long is the relief valve open each time you purge? because unless you have it perfectly timed at equal intervals that would make your fractions increasly incorrect. In theory i'm sure this is true but you have no way of knowing whether this untimed method or pulling the relief valve and holding it open for a few seconds works any better.
Sincere apologies if I seemed in your face. This is a great and helpful forum and I would not want to make anyone feel that way.

Answering your question: I fill to 30psi, then turn the CO2 off and purge to 0. Repeat three times. No timing required. This gets out 15/16ths of the air. Takes under a minute with a full keg. Uses much less CO2 than during cleaning and sanitising. In my opinion getting the air out is not a waste of gas or time. It may also speed up carbonation, but that might be just wishful thinking.
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Old 03-14-2012, 03:14 PM   #9
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Quote:
Originally Posted by Eisendrath View Post

I too have done it, but use much much more CO2 at first. In order for it to not go bad there ideally would be no air in there. With 3 gal in a 5 gal corney there starts out to be 2 gal of air. This must be replaced, as well as you can, with CO2. I bring it up to 15psig (**) and then purge. This would leave ½ air and ½ CO2. Do it again and you yeild ¼ air, do it again and yeild 1/8th air, again, 1/16th, etc. I usually do it 4 times for a 5 gallon batch to leave very little air in there. For three gallons I have gone 4 or 6 times, depending on how stingey I am feeling. Once the beer if flowing, hooked up to the CO2, it is all CO2 going in. I always start it off with as little air as possible.

your math in the first part is correct, but it doesnt matter if there is 1 inch of head space or half a keg; you use the same number of purges to get the same reduction in O2%. you will just use more gas because you have a larger amount of volume. you dont have to do more purges, the more headspace there is.

Quote:
Exactly how long is the relief valve open each time you purge? because unless you have it perfectly timed at equal intervals that would make your fractions increasly incorrect. In theory i'm sure this is true but you have no way of knowing whether this untimed method or pulling the relief valve and holding it open for a few seconds works any better.
i dont understand what you mean here... if you fill it to 15psi (until the gas stops entering the keg), then purge back down to atmospheric pressure, which takes 2 seconds, that is pretty accurate to consider 1 whole purge, or an O2 reduction of 50%. no timing necessary.

sure if you dont wait for the keg to reach a full 15 psi each time before purging, the fractions would be increasingly off... but then you arent following correct procedure, which is your own fault.

Quote:
Thinking that the CO2 floats to the bottom and air comes out the releif valve, doesn't compute. Sorry.
in theory, CO2 would go to the bottom... but in reality you are correct here- if you pressurize your keg to 15psi with 100% CO2, you will effectively dillute the oxygen concentration by (around) half. its not a completely thourough mixture, but its far closer to a homogenous mix, than to complete stratification of CO2 on the bottom.
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Old 03-14-2012, 04:25 PM   #10
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So if you were that concerned about absolutely no oxygen in the keg, what would be wrong with filling it water/sanitizer till there was no air at all in the keg, using low pressure CO2 to push that out - then releasing the relief valve as you siphon your beer in through the dip tube to let just enough of the CO2 out to make room for the beer?
Wouldn't that leave you will a 100% (or as close to it as your CO2 bottle is) blanket over your beer?

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