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Old 12-31-2009, 03:52 PM   #11
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Originally Posted by Bobby_M View Post
I flood the keg with CO2 for one minute with the lid off, rack the beer in to the bottom gently, seal it, flood it, vent it. If there's still O2 in there after that, F it.
I use the same method .
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Old 12-31-2009, 04:13 PM   #12
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Viinc:

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CO2 (or argon) will have a temporary blanketing effect that lasts for at least an hour or so before significant diffusion occurs; a useful fact for transfers.
Vinic, you may be right about the one hour, but what do you base the statement on? How do you know it's an hour? I can state that the time to significant diffusion is 1 minute, and neither of us can argue this out on a factual basis. That's why I want quantitative information or a method to estimate diffusion rates.

WildWest:

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No offense, this is way over thinking the matter, and is not "brew science". Most everyone on here "burps" their keg to purge, why, because it works.
I know it's overthinking, but I still am interested in understanding the underlying physics that determine the amount of mixing. And if that's not 'brew science" what the devil is it? It certainly meets my definition of brew science.

Sure, burping works for most folks, but how do you know you aren't leaving oxygen behind if you can't quantify it. Taste varies among individuals and is subjective. I looking for a quantitative way to estimate oxygen levels, short of buying an expensive O2meter.
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Old 12-31-2009, 04:29 PM   #13
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Viinc:



Vinic, you may be right about the one hour, but what do you base the statement on? How do you know it's an hour? I can state that the time to significant diffusion is 1 minute, and neither of us can argue this out on a factual basis. That's why I want quantitative information or a method to estimate diffusion rates.

WildWest:



I know it's overthinking, but I still am interested in understanding the underlying physics that determine the amount of mixing. And if that's not 'brew science" what the devil is it? It certainly meets my definition of brew science.

Sure, burping works for most folks, but how do you know you aren't leaving oxygen behind if you can't quantify it. Taste varies among individuals and is subjective. I looking for a quantitative way to estimate oxygen levels, short of buying an expensive O2meter.
It is going to be impossible to calculate the o2 pickup in a practical situation that can only be controlled to a certain extent. You would have to measure this empirically with, you guessed it, an expensive meter.

Why not just sulfite the beer and call it a day?
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Old 01-03-2010, 05:20 PM   #14
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determining an explicit equation would be difficult, but here are a few considerations:

geometry is almost certainly a concern. diffusion can only happen where the gases are in contact.

you'd also probably want to account for the co2 that escapes the beer while it's being transferred.

i don't think you can assume a complete mixing of the two gases inside the keg before it is burped in method one. there will be some mixing, certainly, but i'd be surprised if the gas inside was completely homogeneous.

graham's law states that the relative diffusion rates of two gases is inversely proportional to the square root of their relative molecular masses.

also co2 is typically very cold when it comes out and i'd guess that any complete description would have to take thermal effects into account.

calculation of o2 content for the various methods is an interesting exercise, and you might get lucky and find a (relatively) simple equation that describes diffusion as a function of time, room temperature, co2 pressure, and corny keg cross section. but with so many variables and competing effects i don't think i'd be confident in any sort of model without good experimental data using cornies, a standard co2 tank, real beer, etc. if i were to embark on that sort of undertaking, i'd probably pick a phenomenon that has a greater effect on the flavor of my beer.

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Old 01-05-2010, 09:33 PM   #15
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You left out cask conditioning as a method of removing oxygen. You rack your beer into the corny with some corn sugar solution added (same as you would add to your bottling bucket if bottling). Seal the corny, purge a few time with CO2. Leave at a few psi and let the yeast consume the O2 and generate the CO2. It takes a little longer but you have efficient removal of O2 by the yeast and you have active yeast for sometime in the keg which is known to extend shelf life of bottled (and kegged) beers. No equations needed.

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Old 01-05-2010, 09:39 PM   #16
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Oh, and if you have a different purge method, what is it, and why do you think it effectively gets the O2 out of the keg?
I just turn my keg upside down. O2 falls right out.
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Old 01-06-2010, 05:45 PM   #17
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BierMuncher,

Why didn't I think of that? I assume you have some of Maxwell's demons assisting you with sorting the O2 from the N2 in the air? Can you send me a breeding pair?

Dr Malt,

Good idea, kegging before complete fermentation would work as well.

John from DC,

I know (well I believe...) oxidation isn't a major issue with my beer; I am more interested in this issue because it is a frequent question/discussion point on this and other boards, and was hoping we could come up with some quantitative basis for deciding what's the best way to purge a keg. Your points are valid; it is a difficult phenomenon to pin down.

I was hoping to get some better idea of how fast O2 from air diffuses through the CO2. Something along the lines of it takes one minute to diffuse 100 ppm of O2 one inch into the CO2 at the air-CO2 interface, just so I have some basis to decide on how I want to purge.

At this point, I think I will switch from purging air filled cornies to sanitizer filled cornies. No math needed, just displace the maximum amount of air with water and then displace the water with CO2.

So, thanks to all for the ideas.

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Old 01-06-2010, 05:56 PM   #18
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Quote:
Originally Posted by Bobby_M View Post
I flood the keg with CO2 for one minute with the lid off, rack the beer in to the bottom gently, seal it, flood it, vent it. If there's still O2 in there after that, F it.
I do the exact same thing. I have an 8 tap kegerator and don't drink very much beer, so my beer is usually around for months and I have never had an issue with oxidative staling.
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Old 01-06-2010, 05:57 PM   #19
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but you have efficient removal of O2 by the yeast
Not really, in a bottle the yeast will consume about a third of the o2 in the headspace. Headspace/liquid space in a keg is smaller, but I wouldn't consider this dependable near zero oxygen packaging a priori.
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Old 01-06-2010, 07:09 PM   #20
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Remilard:

In a bottle, air can diffuse under the cap resulting in O2 in the head space. The major brewers use oxygen scavenging caps to absorb this oxygen to extend shelf life. So whether the yeast are consuming all the O2 or it is equalibrium with diffusion is a question. I know that active brewers yeast in a closed container removes O2 to less than 1 ppb.

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