Nitrox and Altitude Diving

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shark.byte.usa

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If planning a Altitude dive on Nitrox do you use the theoritical depth to figure out "best mix"? I'm thinking you would, and if so it there a formula to convert theoritical depths back to actual depths for the purpose of EAD, MOD and Max Contingency depth? Any altitude tables I've used are not that precise, there has to be a formula.

For Example:
Planning a 120' ffw dive on Nitrox at an elevation of 2000'

Theoritical depth = 129' (from a table)
PO2 = 1.4
Max. Contigency PO2 = 1.6
Mix = 28%
EAD (Theoritical) = 115'
MOD (Theoritical) = 135'
Max Contigency Depth (Theoritical) = 160'

What would EAD, MOD and Max. Contigency be in actual depth?

Maybe I'm looking at this the wrong way, but the question occured to me last night, based on a dive I did last year (not as deep and on air, got pretty narc'd) that I plan on doing again this year but on Nitrox.

Thanks,
Garrett
 
Your MOD is based on actual depth, not theoretical.

You'll be basically as narced on nitrox as on air. Although I'm sure someone going to try to say otherwise shortly, really there's very little practical difference. You may feel less narced due to your increased experience. You'll still be impaired until you use a He gas though.
 
If you want to get percise, at 2000' the surface pressure is 13.66 psia, making 1 ata 31.6 feet, which is all you really need to know to do the math.

But heck, it's less than 10%, so for a 120' dive I'd just WAG the thing.
 
The partial pressure of a gas within a mix is dependant on the fraction of that gas in the mix and the actual pressure. Right? P1 + P2...Pn = Pt. A lower atmosheric pressure results in a lower Pt (and therefor a lower PP) for the same depth.
 
MikeFerrara:
The partial pressure of a gas within a mix is dependant on the fraction of that gas in the mix and the actual pressure. Right? P1 + P2...Pn = Pt. A lower atmosheric pressure results in a lower Pt (and therefor a lower PP) for the same depth.

Sounds right to me. Atmospheric pressure would be lower, lowering pp (not much at 2000 ft).
 
Theoretical depth is used to explain the offgassing of inert gasses, not the actual number of molecules - the dose - disolved in the blood and tissues. The confusion here is that we use ATA to measure oxygen exposure, when the physiological impact would be more accurately expressed in actual pressure. For oxtox concerns the dose is what counts, and is measured in ATA where 1 ATA = 14.7 psi. You use actual depth. Indeed, if you wanted to shave it really thin you could add the extra depth to bring the "One" atmosphere we credit to the atmosphere up to 14.7 psi and add another foot or two to MOD, but what's the point?
Likewise, the narcotic properties of nitrogen are dependent on dose rather than pressure relative to the air above the water... Just use actual depth for both MOD and END, and theoretical depth for EAD and decompression.
Rick
 
shark.byte.usa:
[snip]...I did last year (not as deep and on air, got pretty narc'd) that I plan on doing again this year but on Nitrox.

You'll still probably get narced though. Nitrox won't help you there.
 
A great 'though' using question,,,and some interesting replies.
 
MikeFerrara:
The partial pressure of a gas within a mix is dependant on the fraction of that gas in the mix and the actual pressure. Right? P1 + P2...Pn = Pt. A lower atmosheric pressure results in a lower Pt (and therefor a lower PP) for the same depth.

2000ft = 93% of 1 ATA. 1.07 * 33ft = 2.3 ft. So add 2.3ft to the MOD, big whoop :D
 
rjack321:
2000ft = 93% of 1 ATA. 1.07 * 33ft = 2.3 ft. So add 2.3ft to the MOD, big whoop :D

Not a big woop at all but my intention was to show the OP why why we don't need to use theoretical depth for calculating MOD or "best mix" at altitude.
 

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