Air backup for a nitrox dive?

[jlcnuke]

It certainly will not "extrapolate to zero". There is no way to know where that line is going to end up- there's no data. Even given your rather basic 18 year old graph that really says nothing about what happens after the P02 level of 1.6 is reached, I maintain that taking a breath or two from EAN40 at 130' (or more likely on the way up during a rapid ascent) is not going to be a concern.

You're welcome to that opinion with no data to support it. I certainly won't ever consider testing it myself and I strongly recommend no one else does either...

I'd be willing to bet that no one posting that it would be fine will risk their lives to prove themselves correct on that unsubstantiated guess either. And I don't recommend they do (even if I disagree on an issue, I hope no one would be dumb enough to risk dying for such a dumb thing).

 

[DeepSeaExplorer]

Nitrox Spare Air... No way... I’m holding out for the Trimix version.

 

[DeepSeaExplorer]

It takes time for the lungs to get the O2 into your system. No one would immediately TOX by exceeding the MOD while shooting to the surface. I have inadvertently dropped to 30fsw on O2 while doing drift deco with no reference and task loaded. Lots of divers have.

The lungs would probably start out O2 deficient anyway since the diver is probably holding his breath while deploying the spare air.

 

[DeepSeaExplorer]

Spare Air takes a lot of bashing, and I personally have no use for one. I also don’t need a 40cf bailout bottle on a recreational dive either. If I’m that concerned, I’ll just strap on the double AL80s with dual regs, that’s what they’re for.

However, a SA on your BCD beats an AL40 left in the car every time - even if it’s a Nitrox one.

 

[Caveeagle]

First, I will agree with doc that air is your best bet for almost any bail out gas. I can think of some exceptions, but they Wouldn’t apply to most.

And if a few extra breaths can give you some safety at recreational depths, that’s fine. Just don’t confuse this with true redundancy, or allow it to make you less careful with gas planning.

Now for this..

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.

Seriously dude.. EAN40 has a P02 of 1.61 at 100’. So 40% at 130’ is TOTALY unsafe by any reasonable gas planning logic. Will you get away with a few breaths? ..and is it less risky than drowning? Ok.. but statements like yours can be taken out of context and could lead to a death!

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.

I get that, but you can’t insert faulty “extrapolation” gas planning and expect it to go unchallenged.

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.

This is just untrue. Diving air (21%), most agencies recognize a max depth of 130’ for recreational, OW diving.

Let’s say you are diving EAN38.
You get a PO2 of 1.42 at 90’. (Exceeding what most rec limits for Nitrox set at 1.4)
You get a P02 of 1.6 at 106’ (Most agencies consider 1.6 as an absolute po2 limit)
*reference IANTD EAD/MOD Tables. C-3200

In this example, diving 38% Nitrox, you absolutely should respect a shallower max depth vs diving on air, 21%.

 

[Diver-Drex]

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...

 

[yle]

You’re assuming that the OP filled it from their Nitrox tank. Given the OP/question, I have to assume that they had it filled from a regular air cylinder.

Oh, actually... I was just pointing out that the only reason to differentiate a "nitrox" Spare Air from a regular one would be the seemingly necessary oxygen cleaning for the system. But O2 clean is only necessary if the cylinder is being filled by partial pressure, and that's not how SA cylinders are filled. Whether they're filled from a big cylinder that just as air or has a nitrox blend, the SA doesn't need to be O2 clean.

Which means (my point) that a regular SA is the same thing as a "nitrox" SA.

 

[yle]

As to the off topic comments about Spare Air being useless, I suggest you simply ignore them. It's a good product, it does have it's place in recreational diving, it's infinitely better in an OOA situation than no gas at all. Just know the limits and consider using a larger source of backup gas if you're spending some time at depth.

Yeah, I've never understood the "it's only a few breaths, not enough to get you to the surface from 100'!!"

The normal Spare Air is 3 cu ft, which is enough gas to ascend from 100' depth at a reasonably slow ascent rate, if you have and RMV of 0.5 cu ft/min. If someone needs a 19 cu ft pony bottle to ascend for a couple minutes, then I will agree that Spare Air is definitely not for them.

 

[yle]

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...

They actually ship O2 clean?!? Hmmm... weird.

Would a regular spare air be unsafe to fill off a 32% cylinder? I've always been under the impression that O2 clean is necessary only for partial pressure fills.

BUT... yes, the 5 pages of discussion is pretty silly. It's called Spare AIR for a reason. If you have one, fill it with air.

 

[Rechno]

Keep in mind that there's a tendency to think nitrox is better than air because nitrox has more oxygen. But your body doesn't use that extra oxygen... whether breathing regular air or nitrox, your body only "consumes" a fraction of the oxygen you breathe (the rest is exhaled.)

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.