Pressure Depth in a Cave

[Blackwood]

I guess I am arguing this from an academic point of view and not so much a real life point of view. I have yet to see how others justify their 2ATA answer though if it's not for the reasons I have mentioned above. If it is calculated simply because it is 33' below the surface and seen as analogous to an open water environment then I cannot agree.

Again: The cave is open to and in equilibrium with the rest of the water. It's one body of water.

If you want, you can prove it to yourself. Go fill up the sink. Take a styrofoam cup and put it face down into the water. Position it so that it's halfway into the water. Poke a hole in the top. See if water fills the cup or if it just fills halfway (so that it's at the same level as the rest of the water in the sink).

 

[Peter Guy]

I'm glad to see there IS disagreement! I was just hoping it wasn't a really stupid question with an obvious answer!

And yes, my thought is that the passage from B to D is a "sealed" passage -- that is, no access to the outside.

 

[Blackwood]

If the water at C is at 4ATA because of the water at B and D, then picture B is correct (B and D push the water up at C).

Any takers?

 

[DaFishy]

ok i never went cave diving befor so can some1 explaine to me what it's like and is there alot to see??

 

[Peter Guy]

Lamont -- IF you have a BIG bottle of water (cap on, sealed vessel) are you saying the pressure at the bottom of the bottle is greater than the pressure at the top? I thought that one of the characteristics of a fluid was that it exerted pressure equally in an enclosed vessel? What am I missing here?

BTW, I thought the answer was that C was at 4 ATA because it was 4 ATA at both "ends" of the "vessel."

 

[Charlie99]

And yes, my thought is that the passage from B to D is a "sealed" passage -- that is, no access to the outside.

A thought experiment to run is to think what would happen if someone drilled a shaft from the surface down to point C. It's pretty easy to see that water would run up the shaft until it got to the same level as points A and E. Then it would be very obvious that there is 2ata of pressure at point C. Then seal off the shaft at point C. Nothing has changed. The pressure will stay at 2ata.

Charlie Allen

edit: replace B&D with A&E -- I meant the atomospheric points.

 

[Charlie99]

Lamont -- IF you have a BIG bottle of water (cap on, sealed vessel) are you saying the pressure at the bottom of the bottle is greater than the pressure at the top? I thought that one of the characteristics of a fluid was that it exerted pressure equally in an enclosed vessel? What am I missing here?

What you missed is the weight of the water. A 1 inch x 1 inch column of water 33 feet tall weighs 14.7 pounds.

Imagine that 1"x1" x 33' high column of water. At the bottom, you have 14.7 pounds of water pushing down. At the top, you don't. That's why one set of units for measuring pressure is "feet of salt water".

Or in the HVAC business, you will find pressure expressed in "inches of H2O". The sort of instrument originally used to meaure the small pressures used in heating and ventilation systems was simply a u-shaped tube. Any pressure differential between the two sides would make the water level in the two sides of the u unequal.

 

[Dan Gibson]

Which is it? Sealed or unsealed? You just contradicted yourself in the same sentence.

I have to mention here that I am arguing the point based on the concept of a perfectly sealed environment with only A and E open

 

[captain]

Lamont -- IF you have a BIG bottle of water (cap on, sealed vessel) are you saying the pressure at the bottom of the bottle is greater than the pressure at the top? I thought that one of the characteristics of a fluid was that it exerted pressure equally in an enclosed vessel? What am I missing here?

BTW, I thought the answer was that C was at 4 ATA because it was 4 ATA at both "ends" of the "vessel."

You are confusing a liquid with a gas. Gas in a closed vessel exerts an equal pressure in all directions.

The answer is 2 ATA. Doesn't matter what the configuration of the cave is, pressure will always be equal the actual depth below the water's surface.

 

[lamont]

Any takers?

If its not vacuum sealed, then the water will move. If it is vacuum sealed, then you've got a water barometer there and you'll be at less than one atmospheric pressure.

Somewhere recently in the past few days I read an article about diving in a water barometer... I can't remember where I came across it though...