HOMEBREW Digest #5907 Fri 02 March 2012

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  Re: Keg Purging..Gas Mixing .etc (Pete Calinski)
  Gas stratification ("Spencer W. Thomas")

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---------------------------------------------------------------------- Date: Fri, 2 Mar 2012 09:58:23 -0500 From: Pete Calinski <pete.calinski at gmail.com> Subject: Re: Keg Purging..Gas Mixing .etc I'm just an electrical engineer so I don't know much about this gas stuff but back in the early 80's I worked on a project that required a very precise mixture of different gasses. My job was the computer that controlled the flow of the gasses. My equipment was accused of not delivering correctly because the GAMS (Gas Analysiser Mass Spectrometer) was reporting the concentrations were wrong. I spent I don't know how many hours trying to find my problems. Then someone noticed that, as the gas was depleted, the concentrations changed. DUH... the gasses were not mixing in the tanks. They were stratified. The "experts" on that decided the tanks had to be "mixed". They began by putting the tanks on a roller and slowly rotating them for "a while". Well "a while" turned into a day, then a week. Finally after a lot of experimentation they ended up with some mechanism that rotated, jostled, tipped, and who knows what else before they were satisfied that the gasses were properly mixed. And the tanks went through this for, like a week. Since I started the story, I'll go a little further. The project was a NASA project and the tanks were going on the shuttle for medical experiments. The big question became, what would happen inside the tanks in micro G conditions. Furthermore, from the time the tanks were charged and mixed until launch and use it could be many months. What would happen during that time. To tell you the truth, I don't know how that turned out. Between the time we delivered the system and it's first flight the Challenger blew up and shuttle flights were suspended for about three years. All I heard was that when our system was finally flown it worked successfully on two missions. My $0.02 Pete Return to table of contents
Date: Fri, 02 Mar 2012 10:28:46 -0500 From: "Spencer W. Thomas" <hbd at spencerwthomas.com> Subject: Gas stratification Pete describes a work experience where gases were "stratifying" in a pressure tank. I've got to wonder if one or more of the gases were liquid at the pressure and temperature of the tank. At the pressures obtaining inside a corny keg (and especially at room temperature and atmospheric pressure) normal gas diffusion will cause the CO2 to mix into the air pretty quickly. The "black & tan" example someone gave recently is a good one. Liquids spontaneously mix much more slowly than gases, because the molecules in a liquid are more tightly bound and move more slowly (ignoring situations like oil and water, where other forces come into play in keeping them separated), but if you pour a black and tan and let it sit long enough, the stout and the ale will eventually become fully mixed. And unless you pour it very carefully, they'll be mixed quite quickly. That said, I have often wondered... If I fill a cornie with CO2 (by pushing water out of a full one, so there was very little other gas in there to start with), then remove the lid, how long until the gas inside comes to an equilibrium mix with the air outside? Can I siphon beer into the cornie through the open lid and still maintain a "CO2 blanket" over it? At what rate would I have to continue to feed CO2 into the tank through the dip tube to maintain a "blanket" of CO2 in the tank? If I take my tank of CO2 (but no liquid) and take the lid off and turn it upside down, does the mixing happen more quickly? (Presumably, but how much more quickly?) I've read many stories of farmers going down into a manure pit and dying because of the CO2 in the pit. At what rate does CO2 have to be outgassing from the pit contents to effectively exclude enough oxygen from the pit so that this happens? Think about the a scenario on the flip side: you're sleeping in a 1-person backpacking tent that is all closed up because it's really cold outside. You breathe in O2 and breathe out CO2. If air diffusion wasn't sufficiently fast, even through the walls of the tent, you'd suffocate in your own CO2 by morning, or at least feel short of breath and sluggish. Never had it happen to me. =Spencer in Ann Arbor Return to table of contents
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