What's the best way to purge oxygen from a keg?

[remilard]

The oxygen in the headspace isn't doing anything. Nothing happens
until it dissolves in the liquid and a reaction can take place. So you really
need to know how much of the O2 in solution gets taken up. I would guess
all of it until the yeast dies or becomes dormant.
Jim

Right, and according to numerous sources the yeast die or become dormant with around half of the oxygen remaining.

 

[Stuntman]

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.

Or Drink It!

 

[Dr Malt]

Thanks AiredAle. I was about to get out some corny kegs and do some testing with a really expensive O2 meter I was hoping to borrow.

On the practical side, like others here, I have not experienced any oxidation type off flavors in my kegged beer using either the cask conditioning with the 3 purges of CO2 on filling, or the 3 purges on filling followed by forced carbonation. It is not that I consume the product fast either.

Dr Malt

 

[AiredAle]

What is happening when your thiophenol bottle is opened is that mixing is taking place by both diffusion and convective mixing of the air and the thiophenol. The thiol diffuses out of the bottle and is blown around by the air currents in the room. Plus, the odor threshold for thiophenol is about 1 ppb, so very little will escape the bottle before you smell it and wish you had opened it in a fume hood.

You’re right the numbers only apply in a static situation. My point in my post was that this is a baseline, and that one assumption was no mixing by any method but diffusion.
I also wanted to get some real data regarding this issue, because it is so often debated on this and other forums but no one has any basis in fact to argue from. One only reads, “diffusion is fast”, and “I never have any problems with oxidation with my way of keg purging, so it must be good”.

In fact the numbers I posted overstate the rate of diffusion because the rate is also dependent on the pressure of the gases diffusing into each other. The pressure of oxygen in air is not one atmosphere but 21 % of one atmosphere, since the concentration in air of oxygen is 21%.

 

[AiredAle]

Dr. Malt,

Please do the measurements, that's exactly what we really need here, some experimental data to answer the question. I'd do the measuring but I don't have access to an O2 meter. Pity you're in the NW, I'd show up and help you do it.

 

[sudbuster]

Thanks for all your answers to this question. Happily, for me at least, I have found the answer about how fast O2 diffuses through CO2. I had a conversation with a colleague of mine who is a PhD material scientist, and she handed me a book written by one of her professors at the University of Minnesota. The book is called “Diffusion, Mass transfer in fluid systems”. Lest you doubt me, the author is E. L.Cussler, and it’s published by Cambridge Press. < clip>........

Thanks, AiredAle, for sharing that information and for this thread. Ifound it relevent to the hobby and stimulating. I have enjoyed it. In fact, I have looked for one particular physical chemistry book from college days that I thought contained the info, but haven't been able to locate it. thanks again..

 

[C2H5OH]

If you really, really, REALLY, want to get the O2 out, and you have some cash and skills.

Buy one of these:

http://www.pemed.com/physof/madapump01_95.jpg

Its called an aspiration pump. (Used in respiratory)

Hook it up to the out post, turn the keg upside down, and turn it on. After a few minutes, most all air will be pulled from the keg. Once that is accomplished, fill with CO2 on the in post.

Personally, the 'burp' method is sufficient for me.

 

[AiredAle]

C2H5OH,

Thanks, I'll stick to my slow fill from the "out" post method. I suspect you would either collapse the corny with your vacuum pump, or the lid O-ring will leak like a sieve since it is designed to seal under pressure not under vacuum. Visually a cool experiment to try though, imploding a corny keg. You could also put a pint of water in the keg, seal it up but open the PRV, put it on its side over your turkey fryer burner, boil the hell out of it for ten minutes to purge the air with steam, turn the flame off, close the PRV and see if it impodes on cooling....