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Old 12-31-2009, 01:14 AM   #1
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Default What's the best way to purge oxygen from a keg?

There is a generally accepted belief that beers that are drunk a few weeks after kegging or bottling won’t show oxidation-related defects. I generally don’t have beers hanging around for months, but now and then I do age an IPA or a holiday ale for 6 to 12 months. I haven’t noticed any oxidation-related flavors, in any of my beers, so I’m content that my method of purging my corny kegs of oxygen is good enough, but I want data, not beliefs. Also, I don’t have the most sensitive palate, so maybe I’m missing the flavors.

I’m not interested in resuming this debate, so let’s assume for the sake of this thread that oxidation defects in home brewed beer are real, and that one source of oxygen is encountered during kegging.

So I need some help that only you brewing mavens can provide.. There is an on again-off again discussion on this and other boards about the best or right way to purge a corny keg to minimize oxygen content in the purged keg. Some folks pressurize and release the pressure several times (Method 1), some fill with CO2 from the liquid post to push the air out through the pressure release valve (Method 2), and some fill with sanitizer and push it out with CO2 (Method 3). No one seems to have any real data or can offer a diffusion equation that can be used calculate/estimate the amount of O2 that ends up in the CO2 purge gas. I want to find a diffusion rate constant for O2 diffusing into CO2, so I can calculate how much O2 is getting into my purge gas.

My belief is that Method 1, pressurize and release, is the least effective purge method, since it would take five 30 psi purges to get to 0.1 % O2, which I suspect is enough oxygen to cause trouble, even when most of the gas in the keg is pushed out by the incoming beer.

So, here’s my math to back up that statement. 30 psi (2 atmospheres) CO2 dilutes a corny full of air (one atmosphere of air) a dilution of 2:1. So starting with a keg full of air at atmospheric pressure and pressurizing and releasing with 30 psi CO2 reduces the O2 content in the keg like this, assuming good mixing in the keg: First purge cycle: 22% O2 to 7% O2, second purge gives 2.67% O2, third purge leaves 0.9 %, a fourth leaves 0.3 %, a fifth 0.1 %. If the purge gas pressure is lower than 30 psi, then even more oxygen is left in the keg after purging. Also I suspect few of us do more than 3 purge cycles, we leave even more O2 in the keg. Also this method uses a lot of CO2, roughly 88 grams, or about 3 ounces per purge, so three purges at 30 psi is a half pound of CO2.

I use Method 2, purging empty, sanitized cornies through the liquid in line with the cover in place and the pressure release valve open. I want to figure out how much oxygen is diffusing into the CO2 as I purge this way. I purge slowly enough that I think I can ignore gas mixing. It takes about ten minutes for me to get a strong CO2 odor at the pressure release valve, so I know I’m purging at about 2 liters per minute. I know that gases diffuse rapidly into other gases, but I can’t find any web based reference that gives a definite rate constant, and I can’t find my old physical chemistry text, so if you have any references, let’s have them. I’d like to know if my method is useless, or practical.

It seems intuitive to me that the most effective purge is Method 3, fill the keg with sanitizer, and push it out with CO2, since most of the air is already displaced with sanitizer. But, any air left in the keg will still let some O2 diffuse into the purge gas, or mix the air with the CO2 if the purge gas goes into the keg too fast. But how much?

Anyone here who can help? I believe that my method is adequate, because I don’t detect oxidation defects, and no one else has mentioned them, but I still want to be able to compare these methods on a quantitative basis.

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?

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Old 12-31-2009, 02:43 AM   #2
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Since CO2 is heavier than O2 won't the CO2 just sink to the bottom of the space and not mix with the O2?

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Old 12-31-2009, 04:40 AM   #3
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Nope. All gases, heavy or light, will mix by diffusion. Heavy ones mix more slowly than light ones, but they all mix over time. They mix fast if the flow into a container is turbulent. Even if you could perfectly layer air on top of say, Xenon (atomic weight 131), a pretty heavy gas, they would end up completely mixed and stay mixed in time.

I want to figure out how fast O2 from the air mixes with CO2, so I can figure out how much ends up in my purged keg, and then in my beer.

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Old 12-31-2009, 05:01 AM   #4
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assuming you have a small amount of head space in your keg when you first fill it, there's a relatively small amount of O2 in there to start with. If you purge multiple times after first connecting the gas, you will force out most of that small amount of oxygen before it has a chance to mix with the CO2. After that all you introduce is CO2, so the percentage of O2 by volume is going to decrease steadily over the consumption time. You will never get all of the oxygen out of the keg. I have never had an issue. So, the question is: what amount is acceptable to you? Personally I say that if there's no detectable change in the beer, then I'm below that threshold.

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Old 12-31-2009, 02:10 PM   #5
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Are you using corny kegs?

I've found that I can fill them completely with liquid by going in the 'out' valve and bleeding sanitizer out the pressure relief valve. Then just force the liquid out with gas and you can be assured that the keg is void of air.

I also feel confident in 'blanketing' vessels by using a sparge stone on the end of the CO2 tubing. There are certain conditions in my cellar that cause the CO2 to become visible as it leaves the stone (picture Elvira), and it's then easy to see that the stone causes the gas to come out gently filling the vessel from the bottom up (assuming the stone is on the bottom, of course) with no mixing of air. This is also an effective method of completely evacuating a vessel of air, but I rarely go there with the size of my tanks. I have no data for you on this but I've seen it with me own eyes, and practice it on a regular basis.

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.
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Old 12-31-2009, 02:14 PM   #6
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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.

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Old 12-31-2009, 02:24 PM   #7
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Quote:
Originally Posted by AiredAle View Post
and some fill with sanitizer and push it out with CO2 (Method 3). No one seems to have any real data or can offer a diffusion equation that can be used calculate/estimate the amount of O2 that ends up in the CO2 purge gas. I want to find a diffusion rate constant for O2 diffusing into CO2, so I can calculate how much O2 is getting into my purge gas.
Regardless of the equation, I suspect that the 3rd method will drive o2 down to as low a level as homebrewers are likely to get.
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Old 12-31-2009, 02:25 PM   #8
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Hope you are fond of Roggenbier.

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Old 12-31-2009, 02:59 PM   #9
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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.

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Old 12-31-2009, 03:14 PM   #10
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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.
lol, good motto
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