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 Home Brew Forums > Oxygenation under pressure
02-22-2012, 03:38 PM   #1
MalFet
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 Oxygenation under pressure

I ferment my beers in a pressurized corny keg, as described in WortMonger's thread. This kind of set up seems to offer a particularly efficient way of oxygenating. After I fill the corny and close it up, I can hook up my O2 welding can to one of the posts of my keg and let the thing pressurize to a couple PSI. The trouble is, I really have no sense of quantity here and likewise I don't know what kind of dissolved oxygen numbers I'm getting.

I can do the basic math, but I get stuck:
I fill to about 19L, which leaves me approximately 1.5L of headspace. To hit 12ppm by weight, I need roughly .25g or 0.016 mol of O2.

The trouble is, I don't know where to go from there. I tried looking at the solubility and pressure equations, but they're well above my pay-grade. Is there a reasonably reliable way to calculate the pressure to which I should be filling the headspace with O2, given fixed volumes and temperatures? I'm not looking for razor precision here, but right now I have really no clue what kinds of oxygenation I would be getting.

Thanks in advance for any suggestions!

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02-22-2012, 04:57 PM   #2
ajdelange
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At room temperature 0.2 atm is going to give you about 8 mg/L. Thus pure oxygen at 1 atmosphere (open the bleed valve, put some O2 pressure on and let the O2 sweep out the air and then shut off the O2 and close the bleed valve) will result in about 40 mg/L. It might make more sense to pressurize to 2 atm (14.7 psig) with air as that gives a partial pressure of O2 of 0.4 atm and an equilibrium oxygen level of 16 mg/L.

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02-22-2012, 05:48 PM   #3
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Thanks, as always, ajdelange.

Would it be accurate then to say that beginning with a headspace filled with air and then pressurizing with pure O2 to a total of 2.5 psi would give me a gas composition of roughly 1 atm air and 0.15 atm O2, which would give me a combined O2 partial pressure of around 0.35 atm? Would this in turn give me a dissolved O2 rate near 14mg/L?

And, with standard ale pitching temps and corny dimensions, what kind of time are we talking to reach equilibrium? I imagine this depends on a number of factors, but are we talking minutes, hours, days?

I really appreciate the help!

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02-23-2012, 03:51 AM   #4
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Roger the math. Bear in mind that the solubility of O2 in wort is dependent on temperature and wort strength so the numbers are nominal.

As to time - that depends on how much things get agitated. Just sitting would probably take weeks. With vigorous shaking it wouldn't take as long. That's why most people run the oxygen in with a stone giving, for example, 2 minutes with enough O2 pressure that the fine bubbles just break the surface.

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02-23-2012, 12:49 PM   #5
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Great, I think I've got a grip on the relationships.

I ordered some DO test chemicals to see how things play out in practice. I'll post back here when I've got some results.

Thanks again!

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02-23-2012, 03:12 PM   #6
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What I'd suggest you do is obtain an oxygenating stone and use it to experiment with oxygenating 5 gal of water. The reason for this is that Winkler's method (the usual chemical test) relies on the oxygen in the sample to convert Mn(II) to Mn(IV) and subsequent reduction of that back to II by means of iodide ion which, in the process gets turned into free iodine proportional to the amount of oxygen in the original sample. One then titrates that with sodium thiosulfate or phenlyl arsine oxide until the solution turns clear. I think you see the obvious problem in trying to do this with wort. Iodine solutions and wort are very nearly the same color. Often starch is added to the mix so that the color change is from blue to clear rather than from redish orange to clear but the color of wort (high absorption at the blue end of the spectrum) is going to hurt you with starch too. The other problem is that any oxidizing or reducing agents in the wort (and we hope it is full of reductones) will throw the measurement off.

If, OTOH, you find experimentally, that a 2 minute blast through the stone oxygenates 5 gal of water to 14 mg/L DO you can be reasonably sure that it will do approximately the same to wort. You can improve control over the oxygen dose by the use of a flow meter in the O2 line.

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02-23-2012, 04:07 PM   #7
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OP, just curious:

Spunding is best used after fermentation is done right? And is this a great way to lager in a keg in the keezer before carbing?

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02-23-2012, 04:22 PM   #8
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Quote:
 Originally Posted by ajdelange What I'd suggest you do is obtain an oxygenating stone and use it to experiment with oxygenating 5 gal of water. The reason for this is that Winkler's method (the usual chemical test) relies on the oxygen in the sample to convert Mn(II) to Mn(IV) and subsequent reduction of that back to II by means of iodide ion which, in the process gets turned into free iodine proportional to the amount of oxygen in the original sample. One then titrates that with sodium thiosulfate or phenlyl arsine oxide until the solution turns clear. I think you see the obvious problem in trying to do this with wort. Iodine solutions and wort are very nearly the same color. Often starch is added to the mix so that the color change is from blue to clear rather than from redish orange to clear but the color of wort (high absorption at the blue end of the spectrum) is going to hurt you with starch too. The other problem is that any oxidizing or reducing agents in the wort (and we hope it is full of reductones) will throw the measurement off.
Good call.

Quote:
 Originally Posted by badbrew OP, just curious: Spunding is best used after fermentation is done right? And is this a great way to lager in a keg in the keezer before carbing?
People do it differently. I've had the best luck starting at around 5 psi from the beginning and then raising that to my desired carbonation level towards the very end. I'm not sure how this would help with layering, though, since fermentation (and thus CO2 production) should be all done by then.
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02-23-2012, 07:06 PM   #9
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Malfet... where did you get your 12PPM target figure?

Not saying it is incorrect at all, just curious of the source. We had a old club member who went off and got his PHD in Microbiology specializing in yeast and he did a presentation just a week ago and according to his presentation, he thought the proper amount was about 8PPM and that anything over that would actually be determinental to the beer. He said he's done a few rough "experiments" and comes out with about a minute on a O2 stone as being a reasonable rule of thumb. Of course... what kind of stone.. what PSI... etc etc... again... rule of thumb.. but he was trying to get to 8 PPM.

Again... not saying 12 is wrong... and not saying this guy is right... just curious as to the source.

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02-23-2012, 07:58 PM   #10
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Quote:
 Originally Posted by Cape Brewing Malfet... where did you get your 12PPM target figure? Not saying it is incorrect at all, just curious of the source. We had a old club member who went off and got his PHD in Microbiology specializing in yeast and he did a presentation just a week ago and according to his presentation, he thought the proper amount was about 8PPM and that anything over that would actually be determinental to the beer. He said he's done a few rough "experiments" and comes out with about a minute on a O2 stone as being a reasonable rule of thumb. Of course... what kind of stone.. what PSI... etc etc... again... rule of thumb.. but he was trying to get to 8 PPM. Again... not saying 12 is wrong... and not saying this guy is right... just curious as to the source.
I picked the number casually just for the sake of figuring out the math behind solubility. I'm not extremely attached to it. But, the Wyeast website recommends 10-15 ppm and the Chris White yeast book says something similar if I remember correctly. Based on the little I know about yeast metabolism, I suspect that once you get to 8 ppm or so it's not critical to hit any particular number dead on. But, if someone with the microbiology background wants to chime in, I'd be interested in hearing.
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