Pressure Depth in a Cave

[Peter Guy]

I watched the G.I. "show" on the Florida caves the other day and it got me to thinking about the "depth" when diving in a cave. Pretty obviously when you are in the water column that has direct access to the surface, your "depth" is your "depth" -- that is, if you are 99 feet below the surface you'll be at 4 ATA's. But what happens when you go down, through a tunnel and then come up -- as they did into some huge rooms and swim up to the top of the room (perhaps ending up just a few feet "underground") but the only water exit is through another tunnel at the bottom of the room. Is the whole room at the same "depth pressure" even though you might be much higher in the water column (or closer to the ultimate surface)?

I hope this makes sense -- it just made me curious.

- - - - Trying to create a picture of this - - - -


A.............................Ground Level................................ E
*..............................................................................*..
..*..............................33' C.....................................*...
....*....................... * * * * * *..............................*.....
.......*..................*..................*.........................*........
..........* 99' B....*......................*...99" D.........*..........
.............* * * *............................... * * * * * *............

Points A and E are 1 ATA

Points B and D are 4 ATA

What is the ATA of C (assuming the water course "*" is open only at A and E) -- 2 or 4?

 

[Blackwood]

C is at 2ATA.

 

[Coldiver1]

After viewing the diagram (and I am not a physics or hydrologist major) it seems to me that location C would be ATA 4 due to the fact that although it is bent the water column is still being forced by ATA 4 water pressure on either side. Like a garden hose originating at the spigot, traveling up to a bedroom window, and into a waterbed. Should be the same pressure as the spigot????

 

[Rec Diver]

C is at 2 ATA. Gold star for blackwood, he rememberd his high school physics.

 

[amascuba]

I have wondered about this as well. I'm not sure of the answer. We need to remember that the "water column" is a measure of vertical pressure starting from the top of the atmosphere to the depth that you want to measure. In Peter's example the cave raises in elevation to 33'. One thing that you have to consider is the pressure of the atmosphere on the ground and on the ground the the water at point C. Now does the 33' take into account the weight exerted on the ground from the atmosphere? Or does the 33' come from the pressure of the atmosphere and the top of the cave to the bottom of the cave at point C, exempting the ground between the atmosphere and the cave at point C?

 

[JeffG]

I have wondered about this as well. I'm not sure of the answer. We need to remember that the "water column" is a measure of vertical pressure starting from the top of the atmosphere to the depth that you want to measure. In Peter's example the cave raises in elevation to 33'. One thing that you have to consider is the pressure of the atmosphere on the ground and on the ground the the water at point C. Now does the 33' take into account the weight exerted on the ground from the atmosphere? Or does the 33' come from the pressure of the atmosphere and the top of the cave to the bottom of the cave at point C, exempting the ground between the atmosphere and the cave at point C?

The cave ceiling is not resting on the water. If it was, I don't think the water would be able to hold it

 

[BSea]

C=2 ATA. Think of a glass jar that is upside down. If you take it down to 33' will the air still go to the bottom of the jar? Of course not. it's the pressure of the water forcing it up and compressing it. The same would be true in a cave.

 

[amascuba]

The cave ceiling is not resting on the water. If it was, I don't think the water would be able to hold it

I know that, but does the earth between the atmosphere and the submerged cave have any role of applying pressure on the water column?

 

[JeffG]

I know that, but does the earth between the atmosphere and the submerged cave have any role of applying pressure on the water column?

take a weigh scale and stand in a doorway. Does it weigh the house?

 

[amascuba]

Point taken.