Air backup for a nitrox dive?

[Diver-Drex]

New in town?


Bob

I wasn’t thinking clearly when I typed that...

 

[happy-diver]

The point of the EAN spare air is that it ships O2 clean so it safe to fill off of an EAN cylinder. Nothing more and certainly not worth 5 pages of bickering...

The Rouses were nicknamed the Bicker Brothers, they made everyone on the boats uncomfortable

and it killed them

 

[CuzzA]

I find no compelling reason to fill any bail out bottle with NitrOx in the first place. What possible benefit could be realized with NitrOx in a bailout? None, nada and squat in precisely that order. Air is all you need. While some of the speculation is based on a quick ascent to the surface, except for a lack of attention, many of the reasons you need extra air may require a bit of time to resolve. I'm thinking specifically of an entanglement scenario.

There is simply no excuse for simply running out of air. That's a failure of both the diver and his buddy. Buddies need to know their partners air supply and relative SAC in order to be a buddy. If simply forgetting to check your SPG is not a possibility, then the air is for other issues, such as the entanglement I mentioned.

I can give you a quasi compelling reason. My 13 cu ft. pony has somewhere between 21% and 32%. Why? Well, when you're offshore and lose a little bit of gas with each dive, I end up whipping it full with back gas. It won't get anymore o2 than 32% and wouldn't be taken past it's MOD because it would be the wrong tool for the job. Come VIP time it will be filled with air again and the procedure will be repeated the next year.

I see nothing wrong with this, but agree there's no reason to fill a pony with EANx from the start.

 

[Bob DBF]

I'd breath 18%O2 if it'd get me to the surface!

Actually the quote is:

Frankly, I'd suck a fart out of a donkey's ass if that's what it took to make it to the surface alive ...

... Bob (Grateful Diver)

 

[Rechno]

So, how do you rig a donkey? and what size?

 

[Bob DBF]

So, how do you rig a donkey? and what size?

You'll have to see if Bob stops by to explain it to you.



another
Bob

 

[EireDiver606]

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.

Yes, you think you’d take less than half a dozen breaths because you don’t want to breath much if it’s over 1.6 (should be max 1.4 but anyway) but you should realise that you can’t plan exactly how your breathing is going to react in an emergency that could potentially be unsurvivable at 40m.

Let’s face it, this is a stupid argument because if you’re at 40m and have 40% EAN you’re doing something wrong not the equipment.

To have a PO2 of 1.4 (rec) you can only go to 25m with EAN 40, you’re 15m deeper than that which isn’t ignorable. You shouldn’t be doing that dive in the first place if you’re planning to use the spare air or tank. It’s just bad dive planning. Even if you want to go to PO2 of 1.6 I think it’s still 30m max depth for EAN 40....

Complacency kills especially at rec depths where many are complacent.

 

[EireDiver606]

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].

It’s never good to extrapolate in diving and do be careful what you write, as new divers seeing this might believe you.

 

[EireDiver606]

You're making the same mistake @EireDiver606 did.

Within recreational limits, breathing Nitrox at or below 40% does not limit your depth for all practical purposes. Regardless of what sort of tank you're breathing from. Nitrox may only limit your time at certain depths. When you're referring to a Spare Air only, within recreational limits, Nitrox has no practical connection to depth whatsoever.

I know that, you’re splitting hairs. For all practical reasons, you could dive to 40m on EAN 40 because it doesn’t have a depth limit (you could say) but how long are you going to stay there? 15 seconds? It’s 15m past it’s MOD of PO2.1.4

What you’re saying is technically not wrong but it promotes bad practices (“Oh I’ll just breath EAN 40 at 40m for exactly 15 secs while I get to a shallower depth -(15m away!) and I definitely won't encounter any problems while ascending and it will all go according to plan” -that’s not real life. If you fail to prepare you prepare to fail. The biggest killer in technical diving is breathing the wrong gas or % at the wrong depth.

I said before and I’ll say it again, this is dangerous stuff where new divers could get the wrong idea. You’re whole argument is a misplaced perspective of safety and complacency. If you’re going to a depth for however long, use the right ******* gas. It’s simple.

 

[EireDiver606]

I find no compelling reason to fill any bail out bottle with NitrOx in the first place.

I would go further to say there is no compelling reason to use a bailout bottle if you’re not using twins already.

18 year old graph

You can’t count either... 2019-1991?

Sure, nobody "advises even considering breathing such a mix," but the anecdotal reports I recall suggest to me that a diver may very well NOT tox from a few tens of seconds or maybe even a minute of breathing a high ppO2. The difference is we're talking about an emergency ascent, breathing from a tiny canister of gas, which lasts a mere five or six breaths as the diver (regrettably) rockets to the surface, not NOAA's recommended "limits."

Sure but we know little about CNS toxicity and it’s hard to research, the only way to safeguard against it is to add conservatism and not be complacent. To have the mindset of using a gas not for its intended depth is bad.

It´s not really about the higher amount of oxygen but rather about the lower amount of nitrogen, which makes Nitrox beneficial.

And due to this, pure air is pretty much about the "worst" gas, for any given depth. Within recreational limits there is no depth that you can not safely dive with a certain mix of Nitrox (e.g. EAN 28).
However, I do not say, that you should not dive air. I usually do, but it is plain convenience, concerning logistcs/ fills that makes me choose air over nitrox.

^^ This ^^