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Old 06-22-2011, 06:07 AM   #1
Vance71975
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Default Priming question

Ok i have been kegging for awhile now, and i prefer to naturally carb my beers, IE with priming sugar. I have read that you are suppose to use less priming sugar when kegging than you do when bottling, but what i have not found is a reason WHY you are suppose to use less when kegging? Maybe i have read it somewhere but i can not remember for the life of me lol.

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Old 06-22-2011, 08:14 AM   #2
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I can't give you an exact answer because I am not a scientist, nor do I completely understand the physics of gas. But I can give you my understanding of it, which I'm sure someone will chime in and tell me I don't knwo WTF I'm talking about.

Of course theriare many variables at play...however, when priming a keg, you have a fairly small headspac which plays a role in the carbonation of your beer, since once the headspace is compressed with gas, it can then beginto be absorbed into solution, (albit at different rates dependent upon temperature). The volume of beer, the volum of headspace contributes to the difference in carbonation volumes between bottles and kegs.

I guess it partially comes down to volumes of liquid vs. air/co2. Again, many variables are at play, but a single larger voulme is much more "efficient" than many smaller volume sif that makes sense....**** it I have had a few beers tonight, and I don't think I can do you justice, but maybe the coments I have made can get yout to think of things in a slightly different manner.

Cheers! sorry I am druK

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Old 06-22-2011, 08:17 AM   #3
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Quote:
Originally Posted by Schnitzengiggle View Post
I can't give you an exact answer because I am not a scientist, nor do I completely understand the physics of gas. But I can give you my understanding of it, which I'm sure someone will chime in and tell me I don't knwo WTF I'm talking about.

Of course theriare many variables at play...however, when priming a keg, you have a fairly small headspac which plays a role in the carbonation of your beer, since once the headspace is compressed with gas, it can then beginto be absorbed into solution, (albit at different rates dependent upon temperature). The volume of beer, the volum of headspace contributes to the difference in carbonation volumes between bottles and kegs.

I guess it partially comes down to volumes of liquid vs. air/co2. Again, many variables are at play, but a single larger voulme is much more "efficient" than many smaller volume sif that makes sense....**** it I have had a few beers tonight, and I don't think I can do you justice, but maybe the coments I have made can get yout to think of things in a slightly different manner.

Cheers! sorry I am druK
Actually that makes a lot of sense, if you make this much sense drunk im afraid to see what you would say sober lol!
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Old 06-22-2011, 01:25 PM   #4
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I've pondered this too.

So, basic chemistry stoichiometry says you get a certain number of moles of gas per mole of sugar. This is immutable. Volumes of carbonation reduce down to moles of gas per gallon of beer, also not changed by going from bottle to keg. By this logic, it should take the same amount of gas to carb up a keg as a bottle, no?

What changes is the headspace involved. Each bottle has, say, 1.5 oz of headspace in it. Over a 5 gallon batch of beer, this comes out to approximately 72 ounces of headspace in all the bottles. Each keg has, say, 1/4 gallon of headspace. This comes out to 32 ounces of headspace, or a little less than half.

To get the same volume of carbonation in either a bottle or a keg, you need the same headspace pressure of CO2. Since the keg has a smaller headspace, you need fewer moles of CO2 to reach that same pressure in that headspace. Theoretically, this is where the difference arises: smaller headspace in keg = less sugar required to produce CO2 to pressurize it.

What confuses me still is that having only 40 oz less headspace in the keg doesn't account for all the sugar. The amount of sugar required to produce an extra 40 oz of CO2 at say 30 PSI and 65*F isn't much. Based on this, 1 mole of sugar produces 4 moles each of ethanol and CO2. 40 oz CO2 at 30 PSI and 65*F is about 0.1 moles of CO2, which requires about 0.025 moles of sugar to make, which is about 8.56 grams of sugar.

Typical doses of sugar for bottling are 4 oz for bottles, and thus 2 oz for kegs. Thus we withhold (4-2=) 2 oz when we keg instead of bottling. 2 oz is around 60 grams of sugar. So if we only need 8.56 grams sugar extra in the bottles to fill that extra headspace, why do we cut out 60 grams of it when we go to kegs?

I don't know the answer, but the math was fun.

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Old 06-22-2011, 02:14 PM   #5
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When you put your beer in the keg do you use CO2 from your tank to displace the oxygen from the keg to minimize oxidation and to help get a good seal on the keg? This adds CO2 to the headspace above the beer meaning you need even less co2 to come from the sugar which could make up for the difference.

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Old 06-22-2011, 02:29 PM   #6
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I thought of that, but even then, assuming you pressurize to 30 PSI, you're only adding another 32 oz of CO2 at 30 PSI and 65*F, which is another ~7 grams of sugar. So now we're up to 15 grams out of 60.....why do we eliminate the other 45 grams?

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Old 06-22-2011, 06:59 PM   #7
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Quote:
Originally Posted by Nashbrewer View Post
When you put your beer in the keg do you use CO2 from your tank to displace the oxygen from the keg to minimize oxidation and to help get a good seal on the keg? This adds CO2 to the headspace above the beer meaning you need even less co2 to come from the sugar which could make up for the difference.
I hit it with 20 lbs pressure, flush the air, and hit it again to seat the lid.
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