Narcosis & Altitude

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.

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

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.

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.

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.

Click to expand...

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

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.

Sorry to be a bit 15 years late. My take: Partial pressure of nitrogen in air at 10 metres depth in salt water at sea level is 1.56ATA, exactly double that at the surface. Assume you're up a mountain and atmospheric pressure has reduced to 0.8ATA. In a *salt water* lake, at 10m depth, PN2=1.422. This is 2.25x the PN2 at the surface of this lake. Looks like ratio of PN2 at depth to PN2 at surface is more at altitude than at sea level. Assumes acclimating first, say waiting six hours before diving.

Bramham:

Sorry to be a bit 15 years late. My take: Partial pressure of nitrogen in air at 10 metres depth in salt water at sea level is 1.56ATA, exactly double that at the surface. Assume you're up a mountain and atmospheric pressure has reduced to 0.8ATA. In a *salt water* lake, at 10m depth, PN2=1.422. This is 2.25x the PN2 at the surface of this lake. Looks like ratio of PN2 at depth to PN2 at surface is more at altitude than at sea level. Assumes acclimating first, say waiting six hours before diving.

Click to expand...

Your last sentence should be deleted. The partial pressures do not depend on any acclimatization; your sentence is relevant to residual nitrogen levels in your tissues when arriving at altitude from sea level, so is about DCS.

Thank you Roaky, I apologize for falling asleep halfway through your post.

Dave, I am assuming that you have a fundamental understanding of mathematics and you understand that altitude skews the bell shaped curves for nitrogen/inert gas loading, gas toxicity and narcosis to the left.

This means that if you are measuring proportional partial pressures against less that one sea level atmosphere the impact will be greater.

The example I could give is (theoretical) if you went for a dive in Mexico City. (Altitude 2400 meters) 0.825 of an atmosphere ambient pressure.
1/x
0.175 increase of impact.

Now, if you want to talk about narcosis, i suggest we look at 30, 40 50, 60, 70, 80 and 100m on air and compare to trimix.

There are four separate issues that are getting conflated in the PADI information:

The difference physical depth that occurs between fresh versus salt water,
The residual nitrogen that occurs when you travel from a low altitude to a higher altitude,
The differences in atmospheric pressure at sea level and at altitude, and it's effects on deco calculations, and
The absolute pressure of the gas mix being used - it's Maximum Operating Depth (MOD) and Equivalent Air Depth (END).

1. The difference between fresh and salt water is mostly a moot issue. Some high altitude dive capable computers adjust the depth display to reflect the greater actual depth that occurs at a given pressure in fresh water due to its lower density than salt water, but the computer itself will still be crunching the numbers for the deco calculations based on the water pressure, and the atmospheric pressure, not the physical distance below the surface. If you're diving tables with a regular depth gauge, that gauge is reading pressure in feet sea water. It's your max depth that you adjust on the table to correct for the higher altitude and the few feet difference in physical distance between seas water and fresh water depth is a non issue, as it's already accounted for in the altitude table correction.

2. Residual nitrogen can be an issue depending on where you came from prior to starting the dive.

For example, back in the day I slipped my boat in a 150 ft deep lake at 4575 ft MSL altitude. If I drove up from Rapid City (3200 ft MSL) and made a dive, I'd be a group "C" diver on arrival due to the "residual" nitrogen from 1,575 ft my ascent up the hill from a lower altitude to a higher altitude. I'd have to take that into account in the deco plan for the first dive of the weekend. On the other hand if I drove to the lake from home in Lead SD at an altitude of 5,400 ft MSL, I didn't have to worry about it, as I was on gassing at the lake already due to the lower altitude I had descended to.

Based on 1 & 2, a caution with using a dive computer at altitude is needed as the computer has to account for the lower atmospheric pressure and thus "correct" the depth used in its calculations accordingly. And ideally, it will ask you to either input the elevation of where you came from, or ask you to turn it on before you leave home so it can address any RNT issues. If not, you need to dive it conservatively and be aware that you've got some RNT that it's not taking into account.

3. The altitude at the lake also meant that one atmosphere was a little less than 33 feet of fresh water. Given the lower atmospheric pressure at the surface at 4,575' MSL, at a depth of 110 fsw depth at 4575' MSL, the pressure isn't equal to 4.33 ATM, it's actual equal to 5.0 ATM, and the corrected depth used for decompression calculation purposes is consequently deeper. In this case the 110 fsw depth is corrected for altitude to 132 fsw for deco calculation purposes and that's the depth I'd use for decompression planning.

The stops depths also varied a bit as a 10' stop would be corrected to 8' and a 20' stop would be corrected to 16' to adjust for the lower atmospheric pressure at the surface to which I would ultimately surface.

While the decompression schedule had to be based on the altitude corrected depth of 132 ft, I could still use the actual 110 ft depth of the dive for gas planning purposes as that was the actual pressure I was at, and the volume of gas in each breath is based on actual pressure and depth, not altitude corrected depth.

4. Similarly, the MOD and END of the mix were still the same as if I were diving at sea level. If I were diving 32% at the pressure depth of 110 ft, the PPO2 was still 1.38, not the 1.6 I'd have a pressure depth of 132 ft, because it's the absolute pressure of the oxygen and nitrogen that have the effects on my body. Which means I'd have the same amount of narcosis on a 110' dive in a lake at 4,575' MSL as I'd have on a 110' ft dive at sea level.

In summary, the only things that are different in a high altitude dive area a) a few feet difference in actual physical depth at the same pressure, and b) deco schedules and stop depths based on the lower atmospheric pressure at the surface. The MOD and END of the mixes used are exactly the same at the same depths measured on a gauge calibrated in feet sea water.

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That's apparently too much for the folks that develop PADI instructional materials to grasp.