Re-using air from BCD in dire emergency?

[DareDevil]

Hi All,

I am posting this due to a discussion among scuba friends. The topic was whether it was possible or not, in an emergency situation and without any other source of air use the air which can be found in the BCD more than one time: that is inspire the air, expire back in the BCD and repeat the sequence.

Aside from any health consideration about the germs that can be there, the basic issue was the max level of Carbon Dioxide which can be tolerated and for how long.

Air coming out of our lungs has between 3-4% CO2 level compared to ~0.04% CO2 level of "fresh" air.

Now, according to what I have found, air is considered toxic when the CO2 level is above 5%, so the conclusion is that you could re-use the air of the BCD no more than two times due to the CO2 exchange ratio.

Is this correct or too simple? Or the toxicity is somehow gradual? Please remember the context: the diver has no other sources of air and he/she is ascending as quickly as possible but needs air to breathe.

Thanks,

DareDevil

 

[Cave Diver]

Check here: http://www.scubaboard.com/forums/advanced-scuba-discussions/270711-breathing-bc.html

 

[DareDevil]

Check here: http://www.scubaboard.com/forums/advanced-scuba-discussions/270711-breathing-bc.html

Thanks - I had seen that thread, it was very interesting. I have discovered there I am not the only one to have tried to breathe BCD air "to understand how does it feel".

CO2 increase in mentioned there as well, in the case you try to reuse air, alongside the inherent health issues associated to using that air, but no info about a tolerable CO2 level (maybe I have missed it).

DareDevil

 

[Diver0001]

in an emergency situation and without any other source of air

^^ well that's the crux isn't it. I'm pretty sure you could survive for a short time like this before you fainted but I can't imagine getting into a situation that was *so* bad that this was my only option.

I guess part of me wants to say "sure, if you've made enough mistakes that this is your only option then why not pile this one on top of the heap too ...."

It would probably work but why would you need it?

R..

 

[AfterDark]

We did that back when BC's inflated by LP air 1st came out. It's one of the things I'll be
refreshing myself on before I start diving for real again. A little tainted air in the lungs is better than water!

 

[TSandM]

CO2 toxicity is gradual. The first symptoms are air hunger and anxiety, and as CO2 builds, you get lethargic and eventually lose consciousness. One of the nasty toxicities of CO2 is acidosis, which builds very quickly and can lead to cardiac arrhythmias. However, if I were 100 feet from the exit to the cave and completely out of gas, I'd try it. I'd try anything; wouldn't you? And if I made it out, I'd sell all my gear because I was clearly much too stupid to continue to dive, for having gotten myself into that bad a situation.

 

[Gilldiver]

Unless you are in an overhead like a wreck or a cave you should be on your way to the surface quickly if you have to resort to BC breathing (there is a very long discussion we need to have if you run out of air in either, like what are you doing there with low air) . You will not be doing a safety stop in a last resort situation like this.

As you are going up quickly, the air in the BC is expanding and needs to be vented, so the idea of breathing back into a BC is a non-starter. You only breath out of a BC in effect venting it through your lungs.

So, unless you are going down or doing a latteral move at depth, CO2 build up and low O2% will never happen.

 

[IceBergSlim]

I can't link directly but if you go to Frogkick.nl - De Online Gids voor DIR Duiken - Doing It Right Duiken and click "Bibliotheek" then "Ademen uit inflator" you can see an example of this astonishing activity.

Bouyancy is a bitch but it's an entertaining practice.

Note: in this video the model is breathing off the inflator rather than rebreathing the wing itself.

 

[SteveAD]

I can't link directly but if you go to Frogkick.nl - De Online Gids voor DIR Duiken - Doing It Right Duiken and click "Bibliotheek" then "Ademen uit inflator" you can see an example of this astonishing activity.

Bouyancy is a bitch but it's an entertaining practice.

Note: in this video the model is breathing off the inflator rather than rebreathing the wing itself.

This really falls into the category of "stupid human tricks". I defy anyone to provide an example of a situation where an lpi was functional without a functional (even if freeflowing) second stage. I would much rather breathe off a freeflowing second than try to double button an inflator.

Sure, it's possible to do it....But why would you?

 

[IceBergSlim]

This really falls into the category of "stupid human tricks". I defy anyone to provide an example of a situation where an lpi was functional without a functional (even if freeflowing) second stage. I would much rather breathe off a freeflowing second than try to double button an inflator.

Sure, it's possible to do it....But why would you?

Agreed but the question was about using the wing to rebreathe air (presumably from a second stage) rather than using the lpi as a 3rd regulator.

Sorry if my example was not the best to illustrate the question.