Re-using air from BCD in dire emergency?

[Cave Diver]

Momsen Lung

Cutaway version showing the internals: http://www.descocorp.com/A Lung 2 Diagram.jpg

(Above image removed to return post formatting to proper ratio)

 

[Soggy_Diver]

Last year, I tried this in a pool under the watchful eye of my dive buddy. I needed some extra weights to get neutral with significant air in the BC. I tried it for about a minute on the bottom, then later tried a slow 16 foot ascent, again taking about one minute. The good news was that it was really easy to control my ascent rate by either exhaling back into the BC (rise), or exhaling through my nose (rise slower or stop). The bad news was that my BC (Sherwood Silhouette) leaks air around the inflator valve even when it is fully depressed, so after 5-8 breaths, it was useless. I could slow the leaking by twisting the inflator so that it was below the hose and pointing down, but that is too complicated. So not really a useful procedure, I decided.

 

[Smokin]

I've also tried breathing from my BC just as part of my emergency "bag of tricks." I thought about it in case of a "out of air" situation on a deco dive where all you had was your deco gas and needed to get to your first stop. Not that it has ever happened or will, but thinking about stuff like that and testing it out in controlled circumstances might save your ass later. I've also used my wetsuit to float me and also ascended upside down to model a BC failure. I may be testing for the "tail of the curve", but some dives are boring and I start getting weird just to entertain myself.

 

[DCBC]

From reading The Terrible Hours, IIRC, the rebreathing that was done by the Navy actually involved a sort of scrubber.

There are a lot of reasons not to breathe the air in your BC. There isn't (or shouldn't be) very much of it, it will build up CO2, and may contain organisms that could put you at risk for pulmonary infection. (Although I chuckle every time I read this, because anybody who has been in a dust storm has probably breathed as many fungal organisms as they'd get out of a couple of BC breaths -- normal lungs are pretty resistant to infection.) But the bottom line is that breathing ANYTHING that has oxygen in it is better than suffocating and drowning. And the gas in your BC has oxygen in it, and no CO2, and is therefore breathable.

I hope I never have to remember that.

Yes, scrubbers were used in some units. More recently, the submariner made a free ascent, or utilized a suit which was filled with compressed air in the escape chamber before it was released. The buoyancy took the submariner to the surface while he rebreathed the available air in the hood. No scrubbers or air source was otherwise available.

In this scenario (or in the case of rebreathing from a BC), although the CO2 would increase, the available air would be expanding on ascent and diluting the CO2 within the BC itself at the same time. This scenario would be quite similar to utilizing a hooded suit.

You make an excellent point about the microbes within the BC and your observation of having air available is equally on-target.

Sorry about the posting format a previous post seems to have changed the format somewhat and I'm unknowlegable as to how to reset it. :-(

 

[rstofer]

It is a rather interesting "safety feature".

If you're properly weighted, by the time you're OOA, you'll be about 1 Lb positive if diving an 80 cubic foot tank, although that's not much weight and is probably offset by wetsuit compression at depth.

Terry

I just can't get to these numbers. If I assume a 7mm Farmer John losing, say, 20# of buoyancy due to compression at depth and even if I am diving with an Al 80 getting 6# more positive, my wing should still have to support 14# at depth. So there is still about 1/4 cf of air.

I figure my 30# wing is about maxed out at 100' with a full HP 100 and a 7/8mm wetsuit. That's about 1/2 cf if my reg should quit. I've got to vent it anyway...

But I have to go with TSandM, if a diver survives this event, they should take up golf.

Richard

 

[Rick Inman]

Here's a fun story of what CO2 did to a guy on a Fenze PO-68: LINK

 

[AfterDark]

Thought I'd define BC breathing as I practice it, one, two lungfulls exhale into the water not the BC.
This would have made my 1st free ascent from 70" due to sudden regulator failure after exhaling
much more pleasant instead of the heart thumper it was. Having nothing to exhale as I started
my ascent I had to wait for what was left in my lungs to expand enough to have something to exhale.
That made me anxious. Would have been nice to have a half a breath or so to start the trip up.
All this talk of C02 is just noise if all you do take breath or two from it.

 

[Gilldiver]

 

[DaleC]

god for bid if you ever pick up a cool looking rock or grab a few lobsters or scallops if you get hungry. That will throw your "buoyancy rated pressure gauge" way off, and you might just find your self in an OOA situation while 10 lbs. negative.

Well, if they ever find me on the bottom with ten pounds of rocks in my pocket you can always say you told me so.
But of course, this would mean that I breathed one tank down to zero, switched regs, and then breathed the second tank down to zero all the while ignoring two spg's.

 

[Rick Inman]

This would have made my 1st free ascent from 70" due to sudden regulator failure after exhaling
much more pleasant instead of the heart thumper it was.

Tell us more! You had a sudden reg failure where the reg failed closed, not open? One breath was perfect then the next - nothing? And no buddy? This is so extremely rare for a reg to fail closed suddenly like that and be buddy-less at the same time... What brand of reg was it? What caused the failure? Were you solo diving?