Air backup for a nitrox dive?

[EFX]

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.

Using my ss simulating a dive to 90 ft and switching to a spare air at 89 ft for 0.1 min, with stops at 60 ft and 30 ft for 0.1 min (the stops simulate roughly a continuous ascent to the surface), and an ascent rate of 30 ft/min will take 3.3 minutes to get to the surface and use 3.9 ft3 at an RMV of 0.5 ft3/min. We got a dead diver. Changing the ascent rate to 60 ft/min gets her to the surface in 2.3 minutes and uses 2.3 ft3 at the same RMV. Conclusion: use a spare air only for shallow depths.

 

[Esprise Me]

Using my ss simulating a dive to 90 ft and switching to a spare air at 89 ft for 0.1 min, with stops at 60 ft and 30 ft for 0.1 min (the stops simulate roughly a continuous ascent to the surface), and an ascent rate of 30 ft/min will take 3.3 minutes to get to the surface and use 3.9 ft3 at an RMV of 0.5 ft3/min. We got a dead diver. Changing the ascent rate to 60 ft/min gets her to the surface in 2.3 minutes and uses 2.3 ft3 at the same RMV. Conclusion: use a spare air only for shallow depths.

I get what you're saying (I think!). Does that mean you carry a larger pony bottle, or exercise more care in selecting buddies, for those deeper rec dives? And, steering back a little ways toward the OP, if you're diving Nitrox, do you feel strongly that your emergency air supply (pony or buddy) must be the same mix?

 

[Bob DBF]

Nothing more and certainly not worth 5 pages of bickering...

New in town?


Bob

 

[broncobowsher]

Who would calculate a RMV of .5 on someone who just did an out of air? Some people can pull that number, many can't, even when calm and collected. Throw in a panic and that needs to be doubled or tripled. Maybe more.

 

[broncobowsher]

BG...

You're uptake of CO2 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...

You are confusing gasses. Nitrox reduces the uptake of NITROGEN, not Carbon Dioxide. By displacing Nitrogen with Oxygen there is less Nitrogen to get into the body. Oxygen doesn't matter as it doesn't saturate tissues, it is consumed by tissues.

If you can find something in your Nitrox training about the on-gassing of CO2 and how more oxygen makes any difference, please copy that page and post it. You say it exists, I don't think anyone else can find that page because I don't think it exists. If you can't cut and past, at least reference what page in what manual (probably a version number as well since the manuals do change over time)

 

[TrimixToo]

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 carbon dioxide is always better...

W...

OK, I'm confused. What relationship are you suggesting between O2 and CO2?

The CO2 limit for OCA is 1000 ppm, or .1%. (I'll note that that is the maximum allowed. My compressor is more like 200 ppm, or .02%, which is likely typical where I live. YMMV. I didn't look up Grade E but I suspect it's the same.)

Are you suggesting that:

- Reducing the maximum inspired CO2 .066% rather than .1% or so makes any significant difference?
- Breathing a higher percentage of O2 changes the rate at which your body makes CO2?
- One is less likely to pant when breathing more O2?
- If you're huffing gas that CO2 retention is lessened when more O2 is in the mix?

If it's one of these, I am ready to be convinced, and I'd be fascinated to understand what led you to that conclusion. If it's something else, please explain.

 

[EFX]

I get what you're saying (I think!). Does that mean you carry a larger pony bottle, or exercise more care in selecting buddies, for those deeper rec dives? And, steering back a little ways toward the OP, if you're diving Nitrox, do you feel strongly that your emergency air supply (pony or buddy) must be the same mix?

The OP didn't mention if he were diving solo. If diving solo beyond 30 ft I would not go with anything smaller than a 19 ft3. IMO, diving with a trusted buddy is better than diving solo although I dive solo right now mainly because I don't have a regular buddy and I want to dive. I recommend carrying air in the pony even when diving nitrox as the main gas so the mixes do not need to be same.

 

[AfterDark]

Well, it beats drowning.

Seven pages, what is there to post after this? I'd breath 18%O2 if it'd get me to the surface!

 

[EFX]

Who would calculate a RMV of .5 on someone who just did an out of air? Some people can pull that number, many can't, even when calm and collected. Throw in a panic and that needs to be doubled or tripled. Maybe more.

Only experienced or small divers would have an RMV of 0.5 in the best of conditions. If we up the RMV to 1.0 ft3/min then using the 90 ft scenario taking only 0.3 minutes to go on the spare air at 89 ft then usage is: 8.1 ft3 at a 30 ft/min ascent and 4.6 ft3 at a 60 ft/min ascent. You're not going to make it if the spare air is only 3.0 ft3. You would have just enough gas to break the surface assuming you could suck the spare air completely dry at a 110 ft/min ascent.

 

[RayfromTX]

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.

If in fact, in an out of air situation in which you must deploy and use a spare air, you are able to maintain the low RMV of .5 then you will need to surface at 50 fpm. That is a rate that is not trivial to maintain while controlling your breathing rate. I suspect that most would either run out of air or exceed even the rather quick ascension rate of 50fpm. I would take the 3cuft over none but would not choose it as a bailout strategy. I would consider it more of a hybrid CESA where you actually got to breathe if you had to. The oxygen content would be rather trivial at that point.