Nitrox and Altitude Diving

[omar]

... planning an Altitude dive on Nitrox ........ 120' ffw dive on Nitrox at an elevation of 2000'; Mix = 28%; PO2 = 1.4; Max. Contingency PO2 = 1.6

I am going to preface this with the comment: There is such a thing as Math Narcosis!

Atmospheric pressure at 2000ft is 13.62 psi [ p= 14.69*e^(-0.038*{2000/1000}) ] This is the proper altitude equation, see some of my other posts on this.

For fresh water: 1ATM (defined as 14.69psi) = 33.899 ft water
Theoretical depth = 129.475 ft ( 120 / .92682)
EAD = 109.0 ft (4.466 atm * 72% * 33.899)
MOD = 138.07 ft [ {(1.4 / .28) - .926 }* 33.899 ]
Max = 162.29 ft [ {(1.6 / .28) - .926 }* 33.899 ]

Your body will not know the difference between the “real” and what you came up with. As I said earlier you can get carried away with this. I know because I have done it myself. I also do not consider anything an altitude dive if it is less than 3000 ft. But that is just me. :icon_idea

 

[shark.byte.usa]

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.

Exactly and thanks everyone for the answers, guess I was looking at it completely wrong, guess that's why I don't remember Nitrox being mentioned in Altitude discussions or vice versa. I regularly dive this lake and make no considerations for altitude, as everybody has eludeded to 2000' is no big woop (and a bad example on my part).
When I started thinking of it in terms of Nitrox and MOD is where I started overthinking it apparently I also didn't mean to infer that Nitrox would reduce Narcosis, that was just a little tidbit of info, sorry for confusion.

Thanks for the responses everyone, good info!

-Garrett

 

[roakey]

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

I ran the numbers once for a lake that we dive at 10,000 feet near Leadvile, Colorado and it turned out that we could go on O2 at 30' and be less than 1.6 ATA (1ATA being 14.7psi, not 1ATA at that altitude...).

I keep telling the local shops that for folks that dive locally, they should be hyping Nitrox as an altitude gas...

Roak

 

[Charlie99]

I keep telling the local shops that for folks that dive locally, they should be hyping Nitrox as an altitude gas...

Particularly since the shallower EAD of nitrox can be used to cancel out the deeper sea level equivalent depth used for altitude correction. For example, if one ignores the initial adaptation issue, at 5000'-6,000' diving EAN32 on sea level air tables pretty much works out with the same NDLs that you would get by going through the detailed calculations.

The rough approximation of Cross correction of 4% deeper equivalent depth per 1000' altitude gets balanced out by the approximation of equivalent air depth of 80% of actual depth for EAN32. Even if you don't trust the two approximations to cancel out, it gives you a quick and dirty answer as a sanity check on your more detailed calculations.