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Several postings have discussed various thoughts on Nitrox and High 
altitude diving. I think we may be digressing a bit, and have perhaps 
forgotten a little of our basic physics as it applies to diving.  One 
post stated that 33ft is 2 ATA (atmospheres absolute) at sea level and 
questioned if 33ft at altitude is still 2 ATAs. Correct on the first 
point but not as good on the second.  At sea level we have 1 ATA of 
pressure (barometric pressure) due to the "weight of the atmosphere." 
When we descend 33ft we accummulate another atmosphere of pressure for 
a total of 2 ATAs. We all know this.  However at altitude the 
barometric pressure is somewhat reduced. For example the barometric 
pressure at 7,000ft above sea level is about .77 ATAs, almost a 25% 
reduction. So at 7,000ft the total absolute pressure at 33ft would be 
.77 + 1.0 = 1.77 ATAs. If you were breathing normoxic air from a tank 
at 33 ft @ 7,000ft you would be breathing a partial pressure of oxygen 
of about .37 ATA O2 (.21 x 1.77), well above any hypoxia concerns. If 
you were breathing the air from the tank at the surface @ 7,000ft, you 
would have a O2 partial pressure of about .16 ATA (.21 x .77) which is 
where you may begin to experience some minor signs of hypoxia. 
Breathing NITROX with an elevated O2 content improves the scenario. A 
32% Nitrox at the surface at 7,000ft would have an O2 partial pressure 
of about .24 ATAs (.32 x .77), well above hypoxia concerns. Remember 
the metabolism of O2 is based on partial pressure (fraction of O2 times 
the total ambient pressure, ignoring the vapor pressure of the moisture 
in the gas).

I believe one of the original questions about this subject was about 
how do you use tables with Nitrox at altitude?  A similar question was 
asked years ago about using air at altitude. One of the methods which 
resulted was the Cross-Corrections method which calculated an adjusted 
depth which was based on the actual intended depth and the ratio of the 
barometric pressure at sea level and at altitude. You took this new 
depth, which was deeper than the actual depth, and enterd into the U.S. 
Navy tables to get your allowable bottom time and decompression 
schedule. My thought is (please do not interprete as a "how to" 
suggestion) can we now take this adjusted depth and enter it into the 
Equivalent Air Depth formula for Nitrox to get yet another fictitious 
depth (shallower than the adjusted depth) to enter into the U.S. Navy 
tables to get decompression schedule when using Nitrox? When you do 
this you are assuming the validity of the Cross-Correction and EAD 
concepts which appear to work reasonably, individually, but may not do 
so good in combination. Any comments?  A computer program to generate 
tables certainly is simpler if available.  
  
Doug Chapman

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