physics question, air pressure in cave

[RaceBannon]

I'm not a diver but I am interested.
I'm working on a project and need info.
It concerns pressures in underwater caves.
ok say a cave is 20 feet down opens into a small area where there's air above, the air layer is right at the level of the water that goes into the cave, now if a person was in here exhaling for awhile or for sake of argument any extra air is added would the air above the diver keep compressing or would it break the barrier and rise up the exterior of the cave?
Would there be any compression of the air space?
Personally I'd think that the excess air/exhailing would freely rise up the outside of the cave but fluid mechanics is not my strong point.
picture, forum question pic - ScubaBoard Gallery

 

[DandyDon]

I wasn't sure about the situation until I looked at the pic. It would help to post it here.

Extra air added to the cave would leave the cave. You can create a similar situation with a bowl upside down in a sink and a small air hose.

 

[OkByMe]

The trapped air pressure would be equal to the "at depth" water pressure. Adding air to the trapped air in the pocket would displace more water, equilibrium will occur. Chances are that since the air is already trapped and has not dispersed, there is no where for it go except in solution in the surrounding water column.

 

[pisoiu]

I would add that it is a very bad idea for a person to breathe from that trapped air. You don't know what it contains, it may not have oxygen at all or it may have toxic gases.

 

[Hickdive]

The result of the picture is that the amount of air in the cave would increase until it eventually bubbled out.

 

[tursiops]

When I first saw the OP, I didn't understand the scenario/question. Now, with the picture added, all is clear. And all the answers are spot on.
Actually, I really DO have a PhD in fluid mechanics.....and rarely ever find a question that it helps me answer. Even this one.

 

[tech_diver]

Something else, I've never tested this in vivo so it may or may not be a big issue. ..

It is possible to damage or compromise the pressure transducer on a dive computer by exposing to elevated ambient air pressure. Many computers have a warning that explains the transducer must be completely submerged in water during chamber testing.

I don't know if it would be a problem but I wouldn't want to test it on my computer. My buddy and I surfaced in an air pocket 30-40 feet down. The water was up to our necks and he was about to raise his hand out of the water to give me an OK when I saw his wrist computer and stopped him. I don't know if it would have actually hurt his computer but I sure it wouldn't be good for it.



By the way, never breath in the air pockets. You never know what gases, farts or exhalations are there...

 

[pisoiu]

I tested few days ago 2 vr3's and one midland xtc280 camera down to -50 meters in hyperbaric chamber, of course none of them were in water. Nothing bad happened. But I also heard that some computers will leak air inside (but not water) at elevated pressures.

 

[Bob DBF]

The example and answers change once a storm comes through and the surface is not a flat line.



Bob

 

[Diver0001]

I'm not a diver but I am interested.
I'm working on a project and need info.
It concerns pressures in underwater caves.
ok say a cave is 20 feet down opens into a small area where there's air above, the air layer is right at the level of the water that goes into the cave, now if a person was in here exhaling for awhile or for sake of argument any extra air is added would the air above the diver keep compressing or would it break the barrier and rise up the exterior of the cave?
Would there be any compression of the air space?
Personally I'd think that the excess air/exhailing would freely rise up the outside of the cave but fluid mechanics is not my strong point.
picture, forum question pic - ScubaBoard Gallery

I think I understand your question. You're actually asking 2 questions. First, "if an air bubble is under water, is it compressed?" and as a second question, "could it be dangerous to a diver?"

Right?

The answer to the first question is "yes". Suppose you suspended an upside down bucket in 10m of water an filled it with air. That air would be "compressed" to 2 ATA (the ambient pressure of the surroundings). So yes, "submerged" air is compressed.

Your second question is if it could be dangerous to a diver. The answer is more involved.

You already know that air expands when you go up. If you were to take a full breath of air in the under water bubble and then go up then the air would expand and it could expand to the point that it causes a lung injury if you hold your breath. How much it expands will depend on how far under the water the "bubble" was.

The other potential danger to a person could be decompression sickness. If you spent "long enough" breathing air out of a bubble "deep enough" under water then you could run a risk. Depth and Time are the two main factors that determine this risk.

Due to factors outside the scope of this answer, if the depth of the bubble was not deeper than 6m then there is no risk. However, risk increases notably in relation to the depth of the bubble and the length of the exposure. If you were to breathe for an hour on a bubble at 10m under the water, there would be no real risk. If you were to breathe for an hour from a bubble at 20m under the water then there would be significant risk of decompression sickness if you just swam to the surface. If you were to breathe for an hour from a bubble 30m under water then you would almost certainly be injured if you just swam to the surface.

R..