Narcosis with Altitude dives?

[goonsquad]

Just wondering if you can get nitrogen narcosis at a shallower (actual) depth if it is at altitude. I know you have to do conversions for DCS but is that all its for, does a diver who is going to say a 70ft depth at 7000ft have to worry about nitrogen narcosis?

 

[knotical]

Good question.

My understanding is that narcosis is related to absolute pressure, while DCS is triggered by changes in relative pressures. So as far as I know, you do not need to make any special adjustment for narcosis when diving at altitude.
Unfortunately, I currently have failed to find a definitive reference for this.

 

[ajduplessis]

No, the partial pressure of nitrogen at 70 ft is not enough to induce narcosis . Generally speaking narcosis starts at around 100ft, but differs depending on diver and environment (low vis, cold, dehydration etc) Altitude dives needs to be managed from a DCS perspective as bubble formation occurs easier at lower pressures . You will have shorter bottom time at altitude that at the coast.

 

[fdog]

The environment - cold water from being in a mountain lake, dehydration from dry air at altitude - may generate the feeling that it's easier to get narced.

From a purely partial pressure point of view, diving at altitude actually would delay symptoms a bit, due to reduced surface pressure. For example, surface pressure at Lake Tahoe is 0.8 ATA, so this would push onset of narcosis ( 0.2 x 34 = 7 feet) 7 feet deeper.

I'd submit that everyone is narced below depths of 50 feet, with narcosis increasing with depth, of course. Those that say they aren't narced usually haven't dove with helium. Almost everyone surfaces from their first helium dive just astonished at how much they were missing, even at relatively shallow depths.


All the best, James