09 Sep 97 21:04, Ray Harkness wrote to Charles Mielke:
 RH>> cabin fires being fueled by emergency oxygen.  Barry also said that
 RH>> he thought compressed air could be used instead of oxygen.
 CM>> In the event of a loss of pressurization at altitude,
 CM>> compressed air could NOT be used instead of oxygen.
 RH> Hi Charles,  How long does it take for a modern airliner to descend from
 RH> altitude to say 15,000 feet after a decompression ?  I agree compressed
 RH> air is not suitable for a long period but surely it only takes a few
 RH> minutes to make an emergency descent.
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).
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.
Jay
--- GoldED/386 2.50+
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