Air backup for a nitrox dive?

[caruso]

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!

Ok fair enough. I edited the post you are referring to and added a snippet that I am referring only to the emergency situation outlined in this thread where a diver might take a breath or two that would be beyond the range of the maximum recommended P02 limits.

you absolutely should respect a shallower max depth vs diving on air, 21%.

Of course divers should respect and follow recommended safe and maximum limits of P02 when diving Nitrox, or else risk serious complications from CNS toxicity. I was simply making the point that P02 exposure is a function of depth AND time.

 

[Diver-Drex]

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.

If they are stating it is for EAN use, then it should be. I’m not concerned with transferring pre-blended gas but laws, regulations and manufacturer recommendations vary as to what O2 percentage and pressure is acceptable. From a liability standpoint it’s a safe move for Spare Air to specify that only their EAN ready units meet the requirements for service with any mix other than 21%.

But like many here, I chose to use other means of redundancy, side mount doubles or an AL40, and only use air for pony bottles.

 

[W W Meixner]

Say I have Spare Air (the air model, not nitrox) and I have an OOA situation on a nitrox dive. Am I correct in assuming that it's safe to switch to the 20.9% backup for my ascent? I can't wrap my head around it at the moment.

VS...

All my cylinders are O2 cleaned...the default on my my computer is EAN 32...all my cylinders are filled with EAN 32...

My main cylinder for the NITROX rebreather is 32%...my three bailout cylinders...which can be used as primary cylinders...are also filled with 32%...for me it just makes more sense...

During an emergency bailout switch-over at recreational depths...and providing your bailout EAN mix is suitable for your depth...a little more oxygen is better than a little less...if you're excited and huffing gas...less nitrogen is always better...

W...

 

[johndiver999]

What does that have to do with carbon dioxide?

 

[Brian G]

...a little more oxygen is better than a little less...if you're excited and huffing gas...less carbon dioxide is always better...

Are you saying that your body produces less CO2 when you are breathing 32% than 21%? I don't think that can be true.

 

[Lorenzoid]

I THINK the implication may have been that breathing a higher ppO2 might help mitigate the harmful effects of CO2 retention, not directly reduce CO2 production or retention. But I have no idea whether that is true.

 

[W W Meixner]

Are you saying that your body produces less CO2 when you are breathing 32% than 21%? I don't think that can be true.

BG...

You're uptake of nitrogen is less breathing 32%...than it is breathing 21%...

Less uptake...less off gassing...shorter surface interval...slightly less impact on second dive as far as bottom time and depth...

Review your basic NITROX training manual...

W...

 

[broncobowsher]

If you want to split atom (finer than splitting hairs) that could be true. CO2 in air is 400 PPM now? You are displacing about 10% of your air with O2. So your uptake of CO2 drops from 400 PPM to 360 PPM.

If you have a basic Nitrox manual that states that something magical happens with CO2 while on Nitrox, please share. I can't find it.

 

[EFX]

Say I have Spare Air (the air model, not nitrox) and I have an OOA situation on a nitrox dive. Am I correct in assuming that it's safe to switch to the 20.9% backup for my ascent? I can't wrap my head around it at the moment.

So let's mock up a scenario with some numbers. Using my Excel tissue compartment spreadsheet (you're using it aren't you? ) let's put in 90 ft for 35 minutes using Nx36, a second depth at 89 ft for 5 minutes, with no safety stop (we're OOA remember), descent rate = 60 ft/min, ascent rate = 30 ft/min, GF = 95/95. To simulate an OOA situation switching to air, in the same profile, make the deep deco gas %O2 = 0 and the shallow deco gas %O2 = 21 (air). The deco gas (sDeco2) is our spare air. The switch to the spare air will occur at 89 ft after being level at 90 ft for 35 minutes. The spare air depth at 89 ft for 5 minutes is obviously suicide but let's use it as a worst case example.

After calculating (see image below), the ss shows the CTC (controlling tissue compartment) ongasssing through the 5 minute gas switch at 89 ft. At the end of the 3 minute ascent to the surface the CTC is offgassing at 93% of the surfacing m-value. So, while the 93% is high it is within limits. One thing to keep in mind when switching from Nx36 to Nx21 is you're going from 64% N2 to 79% N2. So expect plenty of ongassing. Using Nx32 in this example the CTC's surfacing m-value is 97% a more dangerous scenario. Also, the 90 ft 35 minute segment for Nx36 is at the limits of a no deco dive giving an NDL of only 3 minutes.

So, you'll be OK. I agree with others. Don't use a spare air on anything but shallow depths. Get at least a 19 ft3 pony.

 

[jeffsky29]

If this is tech diving, count me out - Way too much math for my enjoyment.