Re-using air from BCD in dire emergency?

[DareDevil]

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In any kind of situation where I am able to ascend directly to the surface, the CESA will do the job. If I am in any other kind of situation (like TS&M's example), breathing from the BCD will give only a marginal and short-lived advantage.
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At which speed are you surfacing? If you are at, say, 120 feet you will need 4 minutes to go up at 30 feet/min. Or are you prepared to go faster?

I am not sure I will not feel the need to breath for 4 minutes in a high stress scenario and I practiced CESA -a long time ago- only at shallow depth, IIRC 30 feet. I am not even sure I will face an OOA scenario with air in the lungs. It could well be the opposite.

However, I am pretty sure I would feel at least slightly safer by knowing I can suck some air from any available container, be it a regulator, a BCD or else.

The BCD is more tempting as I can/could reuse that air.

DareDevil

 

[Cave Diver]

Since I'm not very familiar with subs, can someone tell me if the escape hatch de-pressurizes slowly or rapidly?

I'm just thinking escaping from 600' in the contraption could lead to one HELL of a suit squeeze! :crazyeye:

 

[boulderjohn]

At which speed are you surfacing? If you are at, say, 120 feet you will need 4 minutes to go up at 30 feet/min. Or are you prepared to go faster?

I would certainly go faster than 30 FPM. When I learned to instruct CESA, we were told that 60 FPM is considered a normal ascent rate, and we would assume that a diver doing a CESA would do it somewhat faster, and that would be acceptable. I would expect a diver dong a CESA to do it in maybe 1.5 minutes from 120 feet.

There have been many threads on this topic over the years. A little searching and you will find them, and in them you will read many actual stories of people who have successfully done CESAs from that range, often with no particular difficulty. I knew several people personally who did some from the 75-100 feet level.

If you do indeed need a breath as you ascend, I find it really interesting that you would rather go for the air in the BCD rather than the air coming from your regulator. Your rationale that it is better because you can rebreathe it is also interesting--if you are doing a CESA and take a breath off your regulator, you should not need to rebreathe it--one full breath should be enough to get you to the surface. If indeed you need a second or third breath, you will continue to get them off your regulator as you ascend to lesser and lesser partial pressures.

 

[DareDevil]

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If you do indeed need a breath as you ascend, I find it really interesting that you would rather go for the air in the BCD rather than the air coming from your regulator. Your rationale that it is better because you can rebreathe it is also interesting--if you are doing a CESA and take a breath off your regulator, you should not need to rebreathe it--one full breath should be enough to get you to the surface. If indeed you need a second or third breath, you will continue to get them off your regulator as you ascend to lesser and lesser partial pressures
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Thanks for taking the time to answer. The answer is simply ascent speed.

As I said, I can't say whether I will have any air in the lungs when I begin to ascend. Maybe yes, maybe not. I would bet that if a diver finds himself/herself in an sudden (unpredictable) OOA scenario, his/her lungs are empty or almost empty.

Either way, if I can breath at least one (but possibly more) time either from the BCD or the regulator(s) -I never said I would avoid them like the plague- I do not see any reason why I shouldn't do that and therefore surface at a safer speed. If there is no immediate risk of drowning, why should I increase the risks of DCS?

You said you find the BCD choice interesting. I'll explain what I think, which is only my opinion of course: I have an OOA so no air is coming from (both) regulators for any reason whatsoever. I do not know whether there is any air left in them and/or in the tank which will be available at shallower depths. Maybe yes, maybe not. But I DO know however there must be some air left in the BCD since I know how much weight I carry when I dive, hence the BCD choice.

I totally agree with you that if you choose to go faster (60 fpm or more) then you'll need way less time and therefore one breath would be enough. It is the first thing I asked.

DareDevil

 

[*dave*]

My deepest free ascent was from 115'. There was no urge to breathe and the ascent took 2-3 minutes. You have all the gas you need for the ascent in your lungs. What happens when you take that first breath off your BCD and it's mostly water?

Focus on ascent rate and keeping an open airway. Boyle's law will take care of the rest.

Now, if I found myself in an overhead, with a gas supply that would not get me to the surface, I would likely use my BCD as a rebreather. But, for that to take place, I would have had to make several very bad mistakes.

 

[boulderjohn]

Either way, if I can breath at least one (but possibly more) time either from the BCD or the regulator(s) -I never said I would avoid them like the plague- I do not see any reason why I shouldn't do that and therefore surface at a safer speed. If there is no immediate risk of drowning, why should I increase the risks of DCS?

You said you find the BCD choice interesting. I'll explain what I think, which is only my opinion of course: I have an OOA so no air is coming from (both) regulators for any reason whatsoever. I do not know whether there is any air left in them and/or in the tank which will be available at shallower depths. Maybe yes, maybe not. But I DO know however there must be some air left in the BCD since I know how much weight I carry when I dive, hence the BCD choice.

Good that you are thinking this through. A couple of points to consider:

1. Unless you are very much over-weighted, you will get very little benefit in time from the air in your BCD. There really should not be very much in there, especially at the end of your dive with a (near) empty tank. Under those circumstances, your BCD should be very nearly empty. The amount of extra time you buy will certainly make no difference for DCS.

2. You will certainly get air from your tank as you ascend--it's simple physics. But you have to have your regulator in your moth to get it.

3. The 60 FPM ascent rate used to be the industry standard until relatively recently. Studies by DAN and others indicated that 30 FPM is better, but 60 FPM is still a perfectly acceptable ascent rate for normal diving. The PADI tables were calculated on that ascent rate, and if you are within recreational limits, you would be doing a normal ascent with a 60 FPM CESA according to the PADI tables. You will have very little DCS risk at normal recreational depths and times.

4. If you are as deep as we are discussing, you should normally have a tough breath or two before you are OOA, giving you time to realize what is happening and begin your ascent with a breath. With modern regulators this is less likely to be true than in the past, but you still should have some warning. (At shallower depths it is more likely to be a sudden loss of air.)

5. Just a reminder that you have O2 in your blood and won't actually need to breathe for a while. I have been told 1-1 1/2 minutes worth.

6. Finally, if you are really in a situation where you are as desperately out of air as you suggest, then the ascent rate and the requirement to exhale become less important. If you have absolutely no air in your lungs, you don't have to worry about lung over-expansion. You are now a candidate for a buoyant ascent, which means dropping your weights and allowing the little bit of air in your BCD to help propel you to the surface.

 

[*dave*]

If you have absolutely no air in your lungs, you don't have to worry about lung over-expansion.

That's not true. You won't have to worry about as soon, but residual volume will expand to the point that you will need to vent to avoid an over expansion injury. We never have "absolutely no air in our lungs".

 

[Teamcasa]

2. You will certainly get air from your tank as you ascend--it's simple physics. But you have to have your regulator in your moth to get it.

Just how does a tank, with only a few PSI in it gain air as you ascend? The tank is sealed and ridged, and as such, not susceptible to outside pressure changes.

 

[boulderjohn]

Just how does a tank, with only a few PSI in it gain air as you ascend? The tank is sealed and ridged, and as such, not susceptible to outside pressure changes.

It does not gain any air at all.

The regulator has to be able to deliver the air at ambient pressure. If the diver is at 4 ATA, it can't. Go shallower, and it can.

 

[Teamcasa]

Just how does a tank, with only a few PSI in it gain air as you ascend? The tank is sealed and ridged, and as such, not susceptible to outside pressure changes.

It does not gain any air at all.

The regulator has to be able to deliver the air at ambient pressure. If the diver is at 4 ATA, it can't. Go shallower, and it can.

I got that part but in the context of this thread, (have zero air and deciding to resort to the air in your BC) I doubt there would be enough for a single breath but I'll leave that to the divers who have actually had a deep OOA experience and no buddy or pony to answer.