Re-using air from BCD in dire emergency?

[*dave*]

Depending on the initial depth of the OOA event, there can be a considerable gain in pressure on ascent. IIRC, Shek Exley's tanks were retrieved with over 900psi in them. Of course, at the depth he went OOA, gauge pressure was zero.

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.

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.

 

[Teamcasa]

Depending on the initial depth of the OOA event, there can be a considerable gain in pressure on ascent. IIRC, Shek Exley's tanks were retrieved with over 900psi in them. Of course, at the depth he went OOA, gauge pressure was zero.

Come on Dave, he was at 800 feet! Hardly a fair example of air gain from depth.

 

[*dave*]

Come on Dave, he was at 800 feet! Hardly a fair example of air gain from depth.

Extrapolate Dave...

For every 33', there will be a reduction in ambient of 14.7psi. Using an AL80, each pound of pressure equals .027ft3, so usable gas increases by .4ft3 for every 33' ascended.

.4ft3 will buy me about a minute on the surface, about 30 seconds at 33'.

 

[bleeb]

How much air is actually in a BCD? I don't remember anyone offering a number in this thread.

Of course, conditions vary, but if you're diving in deep cold water with a 7 mm wetsuit, there's compression of the neoprene and loss of buoyancy, but quantitatively how much is this? If it's half of the 20 lbs of lead I'm wearing, that's about 4.5 litres/0.16 cu ft, which would be enough for several breaths.

Just wondering.

 

[Cave Diver]

How much air is actually in a BCD? I don't remember anyone offering a number in this thread.

Of course, conditions vary, but if you're diving in deep cold water with a 7 mm wetsuit, there's compression of the neoprene and loss of buoyancy, but quantitatively how much is this? If it's half of the 20 lbs of lead I'm wearing, that's about 4.5 litres/0.16 cu ft, which would be enough for several breaths.

Just wondering.

One cft of air provides about 64lbs of buoyancy. Assuming a recreational BC, it would have less than half of that amount of buoyancy. So, somewhere in the neighborhood of about .45 cubic feet of air in a recreational BC if it's fully inflated.

 

[Teamcasa]

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.

Extrapolate Dave...

For every 33', there will be a reduction in ambient of 14.7psi. Using an AL80, each pound of pressure equals .027ft3, so usable gas increases by .4ft3 for every 33' ascended.

.4ft3 will buy me about a minute on the surface, about 30 seconds at 33'.

So, if I'm at 80' and my SPG indicates zero psi, you think that I will gain .95ft3 by the time I reach the surface, right?

But the question remains, at 33'ft will I have generated enough of a PSI increase to open the first stage to pull any of that .6ft3?

 

[Codyjp]

So, if I'm at 80' and my SPG indicates zero psi, you think that I will gain .95ft3 by the time I reach the surface, right?

But the question remains, at 33'ft will I have generated enough of a PSI increase to open the first stage to pull any of that .6ft3?

So what would be gain be on a spare air during an ascent from 100'?

 

[*dave*]

So, if I'm at 80' and my SPG indicates zero psi, you think that I will gain .95ft3 by the time I reach the surface, right?

That sounds about right, not because that's what I think, but because that's what Boyle's law tells us.

But the question remains, at 33'ft will I have generated enough of a PSI increase to open the first stage to pull any of that .6ft3?

Depends on your first stage, but in most cases: absolutely. It's not like a switch that gets flipped to off once tank pressure equalizes with ambient.

 

[Teamcasa]

That sounds about right, not because that's what I think, but because that's what Boyle's law tells us.


Depends on your first stage, but in most cases: absolutely. It's not like a switch that gets flipped to off once tank pressure equalizes with ambient.

Does Boyle's law apply to the air inside of a cylinder that will not change shape at those depths?

 

[*dave*]

Does Boyle's law apply to the air inside of a cylinder that will not change shape at those depths?

Yes, it does.