How Deep Is Air Safe As A SCUBA Breathing Gas?

[The Iceni]

I appear to be in a minority since I consider we should never use air to dive.

Why? Because it contains about 80% of that devil's gas, nitrogen, that's why.

I am sure Bruce Weinke will forgive me if I am wrong but I think this is also his belief.

Nitrogen => inert gas bubbles and narcosis

Helium => inert gas bubbles, which are easier to eliminate, but there is no risk of narcosis (at recreational depths).

Oxygen => whether you believe it is narcotic or not (I do not) it certainly does not cause inert gas bubbles.

This is how I see it; -

Maximise ppO2 to within what is considered safe and minimise ppN2; - the lower the better.

If a diver needs an oxygen mix of 21% (the same as air) to attain a maximum ppO2 of 1.4 at depth he must be diving to 57 metres, (185 fsw) where the ppN2 is an extremely narcotic 5.3 bar.

A Nitrox mix can therefore be used to any depth shallower than this (i.e any mix from 22% to 99% according to target depth) but of course the ppN2 of Nitrox 22 (with a ppO2 of 1.4 bar) at 54 metres is still highly narcotic at 5.0 bar, so it must be diluted with helium.

It would seem sensible for divers to use a nitrox mix for shallow dives and trimix for deeper dives in order to minimise nitrogen load.

I gather logistics and cost often dictate accepting a ppN2 of 3 bar in open circuit diving but this is not a problem with closed circuit rebreathers.

 

[DeepTechScuba]

Dr. Paul,

Using nitrox mixes at every depth seems to be the cutting edge of modern diving. That's just my read on the current climate of though on air vs nitrox diving. The only question becomes, what mix?

There is a bit of a paradox however in that our empiracal data suggest that short of habitat-saturation diving, that ordinary air poses no problems at 20 ft or shallower for scuba.

This poll is intended to sort out where everyone here stands on the issue, and what their understandings are.

So far I am surprised at the low percentage of "never" responses since I know this to be a popular doctrine among at least one of the training organizations.

I am also surprised at the responses deeper than 185 ft, since I know of no training organization that teaches to dive deeper than 185 ft.

Thanks for your post.

 

[Walter]

What is the advantage of using nitrox on a 15 ft dive?

 

[Scubatooth]

well put pug about narcosis, its true, to true. Narcosis was explained to me this way, as "martinis law" for every atmosphere (33ft 10m) is the equivalent of drinking one martini on a empty stomach, and so on and so forth to your depth.

For me personally i know i have been narced and i was able to recognize it when going through my AOW training because the simple math problem on the surface that i got in 3 seconds took me over 10 at 75 feet, and this was in a dark lake (semi cold lower 60's) and then when i went to cozumel when we were at 80 feet i was feeling a little silly. it seems like when im narked i get a stupid grin on my face similar to this smiley :bonk: or ut:

then when i was in Grand Cayman and diving on 32% at 110 i felt alot better was slightly effected but was able to keep everything running smoothly by checking on my buddy, gauges(every so often) and doing my roll to check on the rest of the group (that DM stuff is really stuck in my head) and also just singing a song in my head.

It seems like if keeping my mind busy im not as effected as if nothing is going through my head.

YMMV

Dan

 

[cobaltbabe]

Could someone explain this to me. I am sure I have heard of it before but I forget. I have dove to @80-90 feet on the Wolfe islander. I did feel a little more calm then usual but I didn't think I was narced but now I am rethinking it.

 

[The Iceni]

Hi DTS & Walter

Have a quick look at my last sentence and you will get my drift. using a CC rebreather the mix is infinitely variable!

The poll was about safety was it not?

Perhaps the UK diving scene is different but would very seldom consider diving at a depth of 20 ft (6 mtres) or less.

Snorkel, yes!

 

[DeepTechScuba]

Dr. Paul,

20 ft is actually a perfect depth for training new divers on scuba, especially for such currently controversial and often risky drills such as Emergency Swimming Ascents. The mask removal and replacement drill is also very appropriately done at a 20 ft depth.

With instructors then operating at this depth and executing numerous ascents and descents at this depth with individual students, it does in fact become a nitrox issue, for some instructors, even this shallow.

A lot of scuba does actually take place at 20 ft, believe it or not. Several training organizations define a minimal training dive as 20 ft for 20 minutes. And conservative instructors therefore often have no inclination to go unnecessarily deeper during the opening dives of the training.

 

[Walter]

Paul,

I'd skin dive most times at 15 ft as well, but when looking for fossils in <2 ft of viz, you really need to stay down. Nitrogen loading is minimal at 15 ft and so is narcosis. I can't imagine an advantage to nitrox at that depth.

 

[Genesis]

might apply to alcoholics, but it certainly doesn't to me.

I can tell 'ya this with certainty - at 4 martinis on an empty stomach I'm blasted. That much narcosis and I'd be lucky to know what a regulator WAS, say much less worry about having it in my mouth.

 

[Scubatooth]

Well it was a anology that was described to me by my dad, then again(here goes another anlogy) how you hold your liquor its dependent on the person(as is narcosis) my dad had a couple of frat brothers who were could drink a case of beer and be fine and then they had one real big (ie tree trunk) who 1/2 a beer and he was blasted out of his mind

man i knew that was going to come back at me but i was thinking it was going to pug or walter, but genesis you got me.

again your mileage may very