Re-using air from BCD in dire emergency?

[boulderjohn]

If you have material about using 60 fpm, I would be grateful to know more about that topic.

As I said, it is still the recommended ascent rate for at least one agency. It is the basis of the most widely used tables. It was the standard for about 40 years, during which time the DCS risk was so low it rounded off to 0%. If I am out of air at 100 feet, I am thinking that a DCS risk that rounds off to 0% is acceptable to me.

DAN's study of ascent rates found taht a 30 fpm ascent rate was marginally better than a 60 fpm ascent rate, but that safety stops were more important than either. Are you planning on taking a safety stop while OOA?

 

[Cave Diver]

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.

I'll tell you the same thing I told Rick earlier in another thread.

You're not going deep enough.

 

[Gilldiver]

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:

The escape suits are self venting, so there is no squeeze on compression or accent.

The thing to remember is that this is an ESCAPE SUIT, you don't escape from a submarine if it is safe to stay in it.

How this particular suit works is that:
4 of 5 guys get into the escape chamber and the bottom hatch is closed
In about 30 seconds the chamber is pressurized to outside ambient pressure
At anything over 200 feet or so, most people will have just passed out
The top hatch is opened from inside the boat and the 4 or 5 guys take a ride to the surface with the bubble. How fast is this, I don't know 200-250 feet per minute, maybe more.

Having the men escaping pass out helps the whole process as being unconscious they don't hold their breath out of fear etc. and will vent on the way up. If all goes well, the submarine "survivor" now wakes up on the surface wondering why his ears hurt.

Now what does the last guy do when there is no one left in the boat to open the top escape hatch for him? I guess the wife/mother get the Navy Cross at the funeral.

As has been mentioned, one of the biggest problems is the time from the sinking till the escape can cause problems. In the above, everything goes well as long as the internal pressure in the boat is below 2 ATM as the decompression obligation is nonexistent. But if the boat has been filling, the pressure may be above 2 ATM and if the men have been at that pressure for a period of time there may be a real DECO obligation and any rescue ships would need to get them into a chamber quickly.

As you can see, this is last resort stuff and gives you a chance if your sub is on the bottom at 600 feet or less, is fairly upright, there is someone to pick you up on the surface, and if you can get to the escape chamber. The sad fact is that the escape suits are more for moral than applicable to an actual escape.

 

[DareDevil]

Are you planning on taking a safety stop while OOA?

Sorry, is *that* a question? If you really feel compelled to ask, then there must be a serious communication breakdown...

DareDevil

 

[DCBC]

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.....

I'm less comfortable undertaking a compressed air cold-water swimming ascent from 100 feet (Doff SCUBA and mask) than free-diving with FMS. Mental stress caused by the lack of SCUBA and the inability to see clearly are certainly factors that increase the pucker factor. I've even practiced these from 200 feet (regulator in the mouth, just-in-case), but have only been successful in about 50% of the attempts.

In a FMS free dive you hyperventilate, hold your breath and go for it. In the cold-water compressed air ascent, you worry about air embolism. If you blow-out too much on the ascent, you don't have the air in your lungs to expand. Too much or too little, you're screwed. It takes practice to get it right. You don't always have a visual of the surface to use as a reference point to judge your depth.

Having been in several underwater emergencies, I know how much my heart-rate can increase and the demand it has on my breathing sensors. Cold-water on the face is another factor, as are fitness, fat content, effort, etc.

As the Diving Officer at DCIEM, I've been a test subject in a study to measure the decrease of a Diver's breath-hold ability under stress. I wouldn't be cocky enough to say that there's anything trivial about a 100 foot emergency ascent, so I can only conclude that your skills must be superior to mine.

 

[AbyssalPlains]

I heard about an incident where a diver became entangled or entrapped and was able to stretch his air supply long enough to be rescued by breathing his gas supply through his bc instead of his reg. He used his bc in a semi-closed rebreather kind of way, I guess. No idea whether the story is true, but to me, the scenario is very plausible.

 

[boulderjohn]

Sorry, is *that* a question? If you really feel compelled to ask, then there must be a serious communication breakdown...

DareDevil

It was rhetorical.

DAN's research indicated that although the 30 fpm rate was better, the 60 fpm rate worked, and a bigger difference was found in the safety stop. Since there was not a significant difference between a 60 fpm ascent rate and a 30 fpm ascent rate, and since you are obviously not going to do a safety stop (which is what you would need to do to ake a real difference), there is no real point in worrying about the ascent rate difference when your real problem is that you are out of air and are primarily interested in coming up with some more.

 

[Rick Murchison]

... What would happen if the disc magically disappeared at... 2000' depth? ... water would rush into the tank as though it were a vacuum.

Uh, almost maybe but not quite plumb... a full AL 80 has a bit over 200 Atmospheres in it, or about 6800 FSW (you'd have been ok with "over 2080M" )
Rick

 

[*dave*]

Uh, almost maybe but not quite plumb... a full AL 80 has a bit over 200 Atmospheres in it, or about 6800 FSW (you'd have been ok with "over 2080M" )
Rick

Thanks, that's what happens when I pull numbers out of my ass.

 

[pittyyofool]

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.

an AL80 displaces about 6700ML of water. When hydro pressures are applied (5000psi) a tank usually displaces about an extra 65ML. that is a less than 0.1% stretch of the tank volume under full load. Adding 44psi of external pressure to a tank would create such a small unmeasurable amount of shrink it would be in the ball park of 0.0000000000000001%.

Charles Law is null and void in this case, we are talking about a tank that has already been in the water for some time

I believe scuba gauges show the apparent pressure in a tank. (inside tank pressure - outside depth pressure= gauge pressure) at 220fsw the gauge pressure of a tank (not breathed on) should read 100psi less than it did at the surface.
to get a true pressure reading the gauge needs to be in a vacuum.

This leaves "100psi" of (unusable) air in an "empty" tank at 220fsw. If you ascend to the surface that 100psi is now usable, and 100psi is plenty of air for about 2min of breathing.
That is why in an OOA you can still "kinda" breath on the way up.