Re-using air from BCD in dire emergency?

[diverdoug1]

Could you please give is the source for this data? I've always wondered if these calcs have been made and how different activities all size up. Thank!

Institute of actuaries 2004, Provident Ins. actuarial tables. (they are a bunch of blood suckers if you ask me!)

P.S. I also msiquoted the table
Cave diving is slightly higher mortality risk per hour than general aviaton, and general aviation is slightly riskier PER MILE than riding a street motorcycle. So both cave diving and general aviation are MUCH riskier per hour than motorcycle riding. I am not sure if this it stratified into VFR conditions or all general aviation conditions.

 

[Codyjp]

3ft3/3000psi=.001ft3/psi
100'=4ata(3ata ascent)
3ata+14.7psi=44.1psi
44.1*.001=.04ft3

Plenty, if you have really small lungs.

Post of the month. For sure.

 

[Codyjp]

Institute of actuaries 2004, Provident Ins. actuarial tables. (they are a bunch of blood suckers if you ask me!)

P.S. I also msiquoted the table
Cave diving is slightly higher mortality risk per hour than general aviaton, and general aviation is slightly riskier PER MILE than riding a street motorcycle. So both cave diving and general aviation are MUCH riskier per hour than motorcycle riding. I am not sure if this it stratified into VFR conditions or all general aviation conditions.

If they are breaking things down into a per mile basis then I'd guess that rock climbing is probably the safest sport in the world.

 

[DCBC]

I'd prefer to use any gain in pressure to keep water out of my cylinder or inflate my BCD on the surface. I've done hundreds of free ascents and have never had the desire to breathe during the ascent.

Why would you be concerned with water in your cylinder or power inflating a BC when you're on the surface when you're out-of-air at depth in an emergency situation?

Hundreds of free ascents... never had the desire to breath during the ascent? Were these free swimming ascents, or positive buoyant ascents? From what depths?

I've taught free ascents in the Navy and to recreational & commercial students. I can't say that I've never felt a desire to breath when I've had no air available to me...

 

[AfterDark]

Why would you be concerned with water in your cylinder or power inflating a BC when you're on the surface when you're out-of-air at depth in an emergency situation?

I've taught free ascents in the Navy and to recreational & commercial students. I can't say that I've never felt a desire to breath when I've had no air available to me...

+1000 on that!! I had a great desire for air! Thats why I rushed to the surface as fast as I could! DCS, lung injuries, treatable, dead ain't living.

 

[diverdoug1]

If they are breaking things down into a per mile basis then I'd guess that rock climbing is probably the safest sport in the world.

I would guess that they compare aviation and motorcycle data on the basis of distance because the NTSB would compile the data in this fashion. The comparison of cave diving is done with aviation is done on a time unit basis (which makes sense). So on a mortality per hour basis ; cave diving > general aviation > on road motorcyclling

 

[DareDevil]

...
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.
...

This is the basic issue. According to what I can read recommended speed is 30 fpm. When I started to dive and get training, 60 fpm was the rule and nobody raised an eyebrow about that. Maybe in practice 60 fpm is still perfectly safe as you say. If you have material about using 60 fpm, I would be grateful to know more about that topic.

The bottom line is that if I can manage to surface at 30 fpm (despite the OOA) I would be happier. Whether I do it with a CESA because I have my lungs full and/or because I can get some air from regulators or BCD seems an academic issue to me: while going up IF I would feel the need to breathe I'll try what I have available, otherwise not. In any case, the BCD can store air for later use, the regulators not.

Of course if I stll have air in my lungs I will vent it out as I ascend (and yes I know Boyle's law and yes I can't get air from the regulator if I am venting air out of mouth and yes I can't get air from the BCD if I have the regulator in my mouth and yes if all is lost, I'll dump the weights and skyrocket up... As moron as I am, I can figure that out and I also wrote that in a previous post).

The other starting hypothesis (maybe you are near the end of the dive, maybe you are over/under weighted, maybe you will understand you are going to run out of air within 2 breaths, ...): sorry I believe if we start to make hypothesis about how and when and where an unexpected emergency will happen, then it is not unexpected anymore and therefore the whole spectrum of how-to-dive-safely commandments apply: maybe you are monitoring air, maybe the computer is doing that for you, maybe you have a second (third, fourth, ... ) regulator, maybe you have a pony, maybe you have a buddy, maybe there is a line with a spare bottle waiting for you at 10m and so on. The options are unlimited but the final result is always the same: there won't be the need to do any CESA, ESA or else.

That does not mean I do not think the hypothesis you made are wrong or I do not apply those rules mentioned above to dive safely: as I said before, I dive following those principles. What I simply mean is that I do not know whether all those hypothesis you are making will be applicable in the unlucky scenario (that hopefully will NEVER happen) I described.

DareDevil

 

[DCBC]

...and yes if all is lost, I'll dump the weights and skyrocket up.

Good point DareDevil! An ascent isn't worth a dam if it gets you up to the last foot and you black-out (regardless if you do it in style or not). It's the surface or nothing. It doesn't matter what injuries you have on the surface, you still have a chance of survival. Unless you make the surface, you have none.

 

[*dave*]

Why would you be concerned with water in your cylinder or power inflating a BC when you're on the surface when you're out-of-air at depth in an emergency situation?

Because I was comfortable enough with the ascent and knew I wouldn't need any air for it.

Hundreds of free ascents... never had the desire to breath during the ascent? Were these free swimming ascents, or positive buoyant ascents? From what depths?

Free swimming, roughly....80% were from 60' and the other 20% were deeper. A 60' free ascent was required for SSI AOW certification in the 70's, so we got to do them a lot.

I've taught free ascents in the Navy and to recreational & commercial students. I can't say that I've never felt a desire to breath when I've had no air available to me...

I always thought that 95% of free ascent execution is mental. I can touch 80' when free diving, for a total of 160' traveled on a breath, under load. The act of doing a free ascent from 100' on a fresh breath that expands as I ascend is nearly trivial in comparison..... unless I get mental about the things that could go wrong.

As a disclaimer, I'll mention that I'm not promoting free ascents as viable option to any but the worst chain of cascading cfs. I was young and ignorant in the 70's and free ascents were a standard skill then. There is no acceptable reason anyone should need to perform a free ascent..... Yet, I'm still glad I have it in the bag.

 

[Teamcasa]

With a thick ass bulkhead that resists ambient pressure. They don't escape the law, they manage it.

Are you suggesting that submarine and a scuba cylinder aren't subject to Boyle's law? They are... Poke a hole in either one and you'll get different results, depending on depth. What happens when a burst disc lets go on the surface? What would happen if the disc magically disappeared at... 2000' depth? In one case, there would be a rather loud release of the tanks contents. In the other, water would rush into the tank as though it were a vacuum.

What law would you use to describe the different effects?

OK, Let's explore this a little.

A Scuba tank with 2000 psi inside presses out on the inside of the tank expanding it a little (very little). You take that same tank to 100' you have a whopping 44 psi pressing on the outside of the tank. It's not hard to imagine that the 2000 psi on the inside is not going to be effected much by the 44 psi.

Temperature (Charles Law) is going to have a much greater effect on the working pressure inside the tank but in the case of a Scuba tank, Boyle's law impact is so small, no standard or even a high end Scuba pressure gauge would be able to record it.

Even if the Scuba tank had 100 psi at 100' the pressure inside would still overcome the 44 psi exerted by the water, not to mention the strength of the tank walls.