-=> Quoting Jay Hanig to Ray Harkness <=-
RH>> cabin fires being fueled by emergency oxygen.  Barry also said that
he thought compressed air could be used instead of oxygen.
 
CM>> In the event of a loss of pressurization at altitude,
compressed air could NOT be used instead of oxygen.
 
RH> Hi Charles,  How long does it take for a modern airliner to descend from
altitude to say 15,000 feet after a decompression ?  I agree compressed
air is not suitable for a long period but surely it only takes a few
minutes to make an emergency descent.
JH> I think the answer here has to do with the partial pressure of oxygen,
nominally 3 psi at sea level.  As the total pressure drops (and the
aircraft rises), the partial pressure also drops (directly in
proportion). 
If you will accept that atmospheric pressure is roughly 15 psi at sea
level, and that oxygen makes up approximately 20% of air, then the
partial pressure of O2 is 3 psi (20% of 15 psi).  Atmospheric pressure
at 18,000 feet is half that of sea level, so the partial pressure of O2
at 18,000 feet would be about 1.5 psi (20% of 7.5 psi).  Switching to
pure O2 will raise the partial pressure 5 times that of air alone (now
100% of 15 psi at sea level, or 100% of 7.5 psi at 18,000 feet).
JH> Whereas 1.5 psi of O2 may not be adequate for clear thinking, 7.5 psi
is more than enough.  You need to use O2, not compressed air in a
decompression incident.
One of the hot items in SCUBA diving is enriched oxygen mixtures, 
generally called NITROX.  Do you suppose carrying a 50/50 O2/N2 
mix would be signifigantly safer than 100% O2?
... ARRRRRGGGHHH!!!! ... Tension breaker, had to be done.
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