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knotical1 |
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Post subject: Nitrox and altitude cancel
 Posted: Oct 06, 2006 - 08:50 PM
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Guppy
Joined: Dec 31, 1969
Posts: 9
Status: Offline
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Altitude divers know that with increasing altitude, theoretical depth increases.
Nitrox divers know that with increasing O2 percentages, theoretical (“equivalent air”) depth decreases.
At what altitude/Nitrox% combinations do the theoretical depth effects cancel?
I’ve never seen a book or article that answered this question, so I posted the following on another board, and thought I’d share it here, too.
Find the atmospheric pressure at altitude, and add sufficient oxygen to the breathing gas to reduce the nitrogen fraction proportionally.
The formula for the correct oxygen percentage is:
O2% = 1 - 0.79Pa
Where O2% is the Oxygen content of EANx expressed as a decimal.
And Pa is the atmospheric pressure at altitude, in atm.
Example: at 4000 feet elevation, the atmospheric pressure is 0.8637, which can be offset approximately by EANx32.
Caveat: This is theoretical. There are divers who use this technique to dive at altitude with sea-level air tables or computers, but I don’t know if any agency teaches it. Before anyone even considers diving this way, they should be at least Nitrox certified and keenly aware of the serious risks associated with elevated oxygen partial pressures.
Simplified derivation:
Altitude theoretical depth = (D + 33)/Pa – 33
Where Pa is the atmospheric pressure (in atm.) at altitude.
And D is the actual depth
Nitrox theoretical depth = N(D + 33) – 33
Where N is the ratio of nitrogen in the Nitrox to nitrogen in air, which could also be: (1-O2%)/0.79
Assume some depth D1 and apply first the altitude formula, and then the Nitrox formula. For the effects to cancel, we should end up at D1 again, or:
N((D1 + 33)/Pa – 33 + 33) – 33 = D1
(We just substituted “(D1 + 33)/Pa – 33” for D in the Nitrox formula, and set it equal to D1)
This reduces to:
N((D1 + 33)/Pa) = (D1 + 33), or: N/Pa = 1
As mentioned above, N is (1-O2%)/0.79, we can substitute and rearrange to get:
O2% = 1 - 0.79Pa
n.b. You can find atmospheric pressure (Pa) at various altitudes either from tables or from:
Pa = (1 – H*6.87535E-6)E5.2561
Where H = Altitude in feet
And E means: “raise to the power of” |
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Lsdeep |
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Post subject:
Posted: Oct 06, 2006 - 10:50 PM
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Site Admin
Joined: Feb 18, 2007
Posts: 125
Status: Offline
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i think i have to leave that alone for rite now (in my slightly mindaltered condition) and give it a re-read over the weekend  .
it is an interesting topic - since yes - altitude should have an effect on your savely usable nitrox mixes. i have to admit also - i never spend any thought on it - since i come from the flats, spend my live on the flats and never really attempted to dive at altitude (well the handful of times i did it was on air anyway). the expert on it should be sharki!!!! as avid swiss diver (and therefore at constant altitude) he should be able to just rattle it down like his bedtime prayers :rofl:.
i really have to think that thru with a clear head  . |
_________________
Idealism is what precedes experience - cynicism is what follows.
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Lsdeep |
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Post subject:
Posted: Oct 06, 2006 - 10:52 PM
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Site Admin
Joined: Feb 18, 2007
Posts: 125
Status: Offline
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hmm - off topic - really have to spend some time to improve the smilies, they s*ck as they are  .
please ignore this non relevant comment! |
_________________
Idealism is what precedes experience - cynicism is what follows.
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