FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES
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Contents:
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
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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
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End of HOMEBREW Digest #5907, 03/02/12
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