Narcosis & Altitude

[dlarbale]

All,

Can someone clear up the confusion over narcosis and altitude. One of the questions in a PADI staff instructor exam implies that nitrogen narcosis will occur at a shallower depth when diving at altitude.

I'm obviously missing something here as the PPN2 is less at altitude and it would therefore be logical that it would be less at depth. So what am I missing, is it something to do with the human metabolism?

Thanks - Dave.

[roakey]

PADI thinks you have to apply an altitude correction to the PPO2 calculation for Nitrox, so I imagine that they're equally uninformed in this area as well.

Roak

[dlarbale]

Can anyone provide an example of the mentioned calculation, is this physics, biology or guestimation?

Also, what do the other agencies teach and what would a nitrox dive computer do in this situation?

Thanks - Dave.

[omar]

The general barometric equation to determine pressure at altitude is:

Palt = 33 * exp (-0.038 * (alt/1000))

So for a 2000 foot change (sea level to 2000) the pressure change would be:

Palt = Psea * exp (-0.038 * 2) = Psea * 0.9268

As a general rule of thumb a 1 inch change of mercury (Hg) in barometric pressure is about a 1000 foot change in elevation.

The actual equation is:

P = Po * e^-[(M*g*h)/(kB*T)]

Where M is the molecular mass of air
g is the acceleration due to gravity
h is the altitude
kB is the Boltzman constant
T is the absolute temperature

this will reduce to:

P = Po * e^ -0.0383*(h/1000) (for units feet)

omar

different computers do different things for altitude correction.

[roakey]

Originally posted by dlarbale
Can anyone provide an example of the mentioned calculation, is this physics, biology or guestimation?

Also, what do the other agencies teach and what would a nitrox dive computer do in this situation?

Thanks - Dave.
I’m not quite sure what you’re asking here, so let me shotgun an answer…

It’s physics and biology. DCS is a completely different animal than oxygen toxicity or narcosis. The former has to do with nitrogen tension in the tissues and you have to be careful not to exceed a critical amount. Because at altitude you surface to a lesser pressure than you would at sea level, the Cross corrections are applied to your depths. For example, at 10k feet a dive to 70 feet may, after correction be equivalent to a 100 foot dive (I don’t have my tables and my scientific calculator’s batteries are dead, so this is a guess).

Here’s a flippant statement for diving at altitude that really works if you think it through:

If you’re using a capillary depth gauge, no altitude correction need be applied for decompression.

Oxygen toxicity and nitrogen narcosis are completely different. They’re dependent on ONLY the absolute partial pressure of the gas. Because the 1ATM we always talk about pressing on the surface of the sea is actually less than 1ATM at altitude, if you want to get extremely nit-picky you can actually go a smidgen deeper both in terms of oxygen toxicity and nitrogen narcosis at altitude.

Nitrogen narcosis is expressed in “equivalent narcotic depth” (END) expressed in feet and you try and keep it below 100 fsw (talking WKPP standards here). So the partial pressure of N2 at 100 feet is 100 / 33 + 1 = ~4 *. 79 = 3.18ATA. But now assume you’re at an altitude such that the atmospheric pressure is actually .75ATA. So at 99 feet you’ll be at 99 / 33 + .75 = 3.75 * .79 = 2.96 ATA. To get the same narcotic effect at such an altitude (0.75 ATA) you’d have to go to 108 feet (108 / 33 + .75) * .79 = 3.18.

Same goes for O2. I won’t run the numbers but given omar’s calculation, you’ll find that when we dive a 10k lake up near Leadville, Colorado that we can actually go on pure O2 at the 30’ stop and not exceed a 1.6 ATA of O2!

All the so called "technical" agencies teach that you don't bother to do any kind of correction for O2 exposure at altitude. Only PADI, and only if you call them and ask, since their Nitrox book completely ignores the issue.

Roak

[dlarbale]

Roak,

As stated in the original post:

"One of the questions in a PADI staff instructor exam implies that nitrogen narcosis will occur at a shallower depth when diving at altitude."

I believe that the reverse is true (ie Narcs at a deeper depth - marginal).

Does anyone have any info to back up what the PADI exam question implies?

Regards - Dave.

PS// I'll try and find out which exam it was and get the precise wording over the weekend.