Air backup for a nitrox dive?

[Lorenzoid]

You'd be comfortable breathing 40% (what nitrox cert allows to be used) at 130' (still in rec depths)?

I realize you're addressing caruso's specific assertion, but nobody should be relying on a Spare Air to get them to the surface from 130 ft. It gives you, what, five or six breaths? If you're diving that deep--on the edge between rec and tec depths--you probably should carry a lot more reserve than a Spare Air.

As for oxygen toxicity, my guess is that in the few seconds you have to get to the surface while breathing from a Spare Air, you're not going to tox. I would say that if one is foolish enough to try to get to the surface from 130 ft. on five or six breaths from a Spare Air--a dangerously fast ascent--toxing is not going to be their biggest risk.

edit: caruso replied while I was typing

 

[caruso]

I realize you're addressing caruso's specific assertion, but nobody should be relying on a Spare Air to get them to the surface from 130 ft. It gives you, what, five or six breaths? If you're diving that deep--on the edge between rec and tec depths--you probably should carry a lot more reserve than a Spare Air.

^^This. I maintain that Spare Air has it's place in recreational diving but at depths closer to 50-60 ft max.

 

[jlcnuke]

Let's think this through. I've got 40% Nitrox in my spare air at 130' and I'm OOA with no other options. First thing I'm doing is going up. I doubt I'd even take 1 breath at that depth I'd be already 5-10 feet shallower and moving upwards at a pace far greater than the recommended 60 fpm.

Odds are I won't take more than around a half dozen breaths at a depth that exceeds PO2 1.6, and remember PO2 is a function of exposure over time. Those numbers are going to calculate at close to zero.

Most importantly- if you're at 130', OOA and reliant on a Spare Air for your only survival you've got bigger issues than worrying about exceeding your MOD.

Mod Edit:
If you're at 130' alone with just a spare air, the fact is that it isn't "safe" to do so with 40% as your backup gas at that depth. Regardless of what you think you'd do and I don't think we should be making statements implying it's fine. That was my point.

 

[caruso]

the fact is that it isn't "safe" to do so with 40% as your backup gas at that depth. Regardless of what you think you'd do and I don't think we should be making statements implying it's fine. That was my point.

To clarify- In a situation where diver is breathing Nitrox 40% from a Spare Air at 130' they won't be doing it long enough to exceed safe P02 limits.

I just took a look at the P02 tables. The last line on the table states that a diver can stay at 113 ft for 45 minutes while breathing EAN36. I think that can be extrapolated to mean that it's fine for a diver to be at 130' for perhaps 15 seconds breathing EAN40 without a CNS hit.

[To put in context as per a subsequent post by @Caveeagle. I am in no way advocating a diver blow past recommended max P02 limits because it's ok to take a few breaths at any depth regardless of the MOD of the gas mix. I'm simply saying in the scenario outlined in this thread, the odds of a diver getting a CNS hit are so remotely low as to be considered nonexistent].

 

[jlcnuke]

^^This. I maintain that Spare Air has it's place in recreational diving but at depths closer to 50-60 ft max.

A spare air (larger version) is probably fine for shallow dives. So for someone who never dives deep with the need for a backup air source they're acceptable (if probably still not ideal). When you can just get something better that works in a lot more situations, I can't see the appeal of them though. But my opinion doesn't mean much outside of my own decision making in general so...

 

[jlcnuke]

To clarify- In a situation where diver is breathing Nitrox 40% from a Spare Air at 130' they won't be doing it long enough to exceed safe P02 limits.

If everything goes well other than the situation already being all jacked up that they got to that point... If you've looked at the CNS curves you'll note that at 2.0 you get around 2 minutes before you're crossing the CNS line on the graph. Not knowing what caused them to run out of air at depth etc, I don't know if they got tangled up in something and lost their primary reg and may need 3-5 minutes to sort everything out while breathing on their backup, or if they're going to casually switch over and make an ascent immediately. I try not to plan for the rainbow and unicorn scenarios myself however.

 

[caruso]

If everything goes well other than the situation already being all jacked up that they got to that point... If you've looked at the CNS curves you'll note that at 2.0 you get around 2 minutes before you're crossing the CNS line on the graph. Not knowing what caused them to run out of air at depth etc, I don't know if they got tangled up in something and lost their primary reg and may need 3-5 minutes to sort everything out while breathing on their backup, or if they're going to casually switch over and make an ascent immediately. I try not to plan for the rainbow and unicorn scenarios myself however.

They don't have 3-5 minutes if they're OOA at 130' and all they've got is a Spare Air.

You're turning this into an entirely different conversation that has nothing to do with Spare Air, which is clearly the subject of this thread and what all of my posts have clearly addressed.

 

[Lorenzoid]

You guys crack me up. I would add that a Spare Air might be a reasonable choice for shallow dives AND in environments where getting "tangled up" while losing their primary reg is not a possible scenario. If a Spare Air makes a diver feel good on shallow, tropical reef vacation diving, I suppose that's a reason to take it along.

 

[broncobowsher]

I didn't know that a helicopter pilot would need Nitrox to escape from a downed whirly bird.

Perfect world, same gas. Same perfect world, pointless.

The gas in the spare just needs to be breathable, with at least enough oxygen. For as little as it would be used, that is you are a missile for the surface, anything between 10% and 100% oxygen will work. The 10% is hypoxic, at the surface, but at 20' or more is enough. Even at 130' 100% won't be breathed long enough to be an issue, You won't tox in the rocket ride to the surface that you will need before you run out.

It really doesn't matter. That simple. Overthinking it. Fill it with air, fill it with Nitrox, results will be exactly the same.

 

[jeffsky29]

If you were properly trained for nitrox, or trained at all you probably shouldn’t be asking this question. And that’s not being rude, it’s just reality. Put simply, with increased O2 you cannot go down as far.

To the OP, your question was a good question. In fact it showed you paid attention during your Nitrox training, creating awareness and concern about mixing O2 percentages, with your computer not being aware of the mix. As others have said, just get to the surface breathing. After I received my Nitrox cert, I had the same question for my instructor, with respect to my 30cf pony bottle. Don’t let one rude response stop you from chiming in and asking questions.