Re: Xenon and vent placement

["Dale E. Boyce" <dale@radpro.uchicago.edu>]

Density/specific gravity are "bulk" effects.  As long as xenon
is locally in a high concentration that region is more dense than
surrounding areas and that region, including the oxygen and 
nitrogen in that region, will sink relative to the surrounding
areas.  Once dispersed, the xenon will not reconcentrate even
in a completely sealed and stagnant room.  This is treated
in thermodynamics.  Atoms and molecules of a given atomic or
molecular weight will distribute throughout the atmosphere.
There will be a gradient in the concentration of the different
gases which is governed by a thermodynamic equation, which since
I don't have my thermo text here, I won't reproduce at this
time.  However, the gradient effect is much different from
the bulk action effect of density.  The thermodynamic gradient
is a pretty small effect, but has been measured.  Individual
atoms and molecules do not exhibit density as such, since the space
they actually occupy in a gas is small compared to the space
available.

All that said, the observation that you can carefully fill a
container from the bottom up with a gas more dense than air
is quite accurate.  If turbulence is avoided, diffusion is the
primary means of mixing.  Diffusion happens pretty slowly.
However, the open container of dense gas will eventually mix
evenly with the room air even under ideally sealed and stagnant
conditions.

Dale Boyce
dale@radpro.uchicago.edu