air pocket underwater remains constant size??

[deadmau5]

hey all

 

i was listening to my uncle talk about a dive he did off the coast of australia not too long ago.

 

He said in an underwater cave below sea level there was a chimney shaped area which had an air pocket at the top of it. as the divers passed through the cave the expelled air rose up the "chimney" and into the pocket. Having dived in this cave many times over the years, and after speaking to many other older divers, my uncle found that the air level in the cave remained constant. Again, the cave is under sea level.

 

Does this have something to do with pressure and the oxygen re-entering the water?

[CaptainPanic]

I can come up with an answer, and because your info is limited, and because the problem is quite complex, I don't know it is true or not:

 

If the divers expel air, and it enters the air-pocket, then this should initially push the water level down. This means that the pressure inside that cave is increased (you can say that the water is now pushing up even harder, which results in increased pressure). When a gas is at higher pressure, the driving force to dissolve is higher. This means that the gas from the pocket will dissolve again until it reaches a certain equilibrium value.

 

What I don't understand is why it exists at all. Since the pressure in an underwater cave is most likely higher than atmospheric pressure, I'd expect that over time, the pocket of air disappears completely.

 

The cave is under sea level, but is it possible that the water level in the cave is the same level as the sea surface? That would change the whole answer.

 

p.s. I'm not sure that this belongs in the chemistry forum. It deals with dissolution of gases in water, and pressure - perhaps physics is better?

[Sisyphus]

Questions that I'd like to know the answers to: How big is the air pocket? How precisely is the "constant" water level measured? At what depth is the surface in the air pocket relative to the outside surface level?