The sudden release of gas will push other gases out of the way,
that is true. But in a closed system like a keg where there is
no mixing, you will not see stratification of the gases, they
mix evenly. And let's be clear, a CO2 extinguisher does not need
to push all of the O2 out of the space - the atmosphere is ~21%
O2. The amount of O2 required for sustained combustion varies by
material, but in general people throw around the number 14% O2
to sustain burning. So the CO2 extinguisher only needs to get
rid of 1/3 of the O2. For breathing, we collapse when the O2
level drops below ~8%, and suffocate in minutes if it is below
~5%. More importantly, CO2 levels above 5% cause problems, with
immediate loss of consciousness somewhere above that (10%? 15%?
I'm not sure). So again, not all of the O2 and other gases need
to be evacuated from the space to be dangerous. that's why a
balloon filled with helium rises. You might have posted pre-
coffee, but you know this has nothing to do with what we're
talking about. Gase