Cave-diving and water pressure question

[Mudd]

I'm trying to wrap my head around pressure experienced while in a cave. Now my confusion may be based on overly simplified information forming my base understanding. So first off here are my beginning thoughts:

1) Pressure is due to the amount of matter above you and the make-up of said matter (be it air or water)

2) Pressure increases as you descend in open water because there is more water above you - more water more pressure.

3) Swim at the same level in open water and the pressure will remain relatively equal (imagine a calm surface) because the amount of water above you has not changed.

4) Dive computers calculate your depth by registering this pressure and calculating the depth based on an average mass of the water (change from fresh to salt requires changes in "constants" being used in the calculations)

I concede that some of this may be wrong or too simple...

Now for the situation...

A cave has an opening that is at 60' below the surface. The cave itself has a maximum internal height of say 15'. When you enter the cave, what happens to the water pressure? There is less water above your head, so does the pressure decrease? Does this also throw off your depth gauges? Does the rock above you "offset" the weight of the difference of the water?

I can't for the life of me wrap my mind around the reasons, for either answer actually...

Let me also say that I have no intention of doing cave(rn) diving until adequately trained... but this doesn't stop me from thinking about the principles involved...

 

[Cave Diver]

The hydrostatic pressure is the same. It doesnt matter if you are in a passageway that is only 5' high with 60' of rock above you, you are still at 65' of depth.

 

[tc]

Think of it this way- the pressure at 100 FSW (for example) is the same if you are in the middle of the ocean or 100 feet down in a well (Assuming a salt water filled well) 100 feet down.

The cave simply goes sideways- the water column above you is still 100 feet deep even if it is not directly above you- the weight of the water column remains the same.

 

[Brewone0to]

It's not a problem at all, no real pressure changes involved.
See above posts.
That being said, the real pressure is on your shoulder
holding up the ceiling so it wont fall in on you.

 

[knotical]

Good question. There was a lengthy discussion on just this topic a few years back here on ScubaBoard: http://www.scubaboard.com/forums/basic-scuba-discussions/164988-pressure-depth-cave.html
The pressure is based on how far below the water’s surface you are.

 

[Gombessa]

If you take an open tube down to 60ft and hold it horizontally, you've got a cave. If you go down a wall at 60ft and drill a hole into it, that's a cave too. If you have an air pocket at the ceiling of a cave, that air is pressurized to the level of the water column around it, the fact that there's rock overhead doesn't "reset" it to 1ATA.

 

[don Francisco]

The folks above are right. Hydrostatic pressure within a liquid is a constant at any given level. It's determined by the depth from the open surface. (or more precisely the depth below an open surface plus the pressure on that surface)

Imagine, for a moment you swim down 60' under an open diving bell, and up through 2 feet of water into a trapped air space. The air pressure inside the bell would be equal to water pressure at 58 feet (60 down - 2 feet up) If that weren't so there'd be 2 adjacent areas of different pressure in the water under the bell, one based on 2' of depth and one at 60', which in nature would be an unsustainable condition.

In thinking about this remember that water pushes in all directions at once, so it can transmit forces sideways as readily as downwards.

 

[Mudd]

That being said, the real pressure is on your shoulder
holding up the ceiling so it wont fall in on you.

HA!

So the column is sideways or more diagonal... and the pressure is still due to the water. Sort of like taking a deck of playing cards and shifting it...

Let me preface this by saying I'm not trying to be a deek, I just want to wrap my brain around this...

What throws me is the possibility of there being a pocket of air or gas in the middle of the cave system Technically there is no water above you at that point, even diagonally...

No water above for increased mass to cause the pressure... So why? Is it purely compression of molecules at this point coupled with the tendency of molecules to spread "uniformly" at that row of water? I may have just answered my own question...

 

[Mudd]

Holy crud you guys reply fast... lemme read before I reply any more...

 

[jaybombs25]

what about if there was hypothetically a pocket of water just a bubble under 60ft of rock completely enclosed and surrounding it was the sea. think of a pillar sticking out of the sea with a bubble of water trapped in the middle.... What would the pressure be?