Pressure Depth in a Cave

[Blackwood]

The cave is open to and in equilibrium with the rest of the water.

 

[Dan Gibson]

A & E are exposed. If C is then exposed in this sytem, the water level will equalize with A and E (i.e. C can't be 33' below A or E). In this example, whereever that surface is located, A, C, and E would all then be exposed to 1 ATA. Ignore any elevation differences for atmospheric pressure since the change is insignificant in this case.

The height of the water column determines the pressure exerted by the water.

I do not agree, if the area between A&E were excavated to expose C to the surface, and thus the atmosphere, the 4ATA water pressure between A&B and E&D respectively would force water out of the newly exposed area C, and probably flood the excavated basin; the water levels in A&B and E&D would drop to the same level as the water level above the now flooded area C thereby decreasing the pressure at B and D from 4ATA to somewhere slightly over 2ATA depending on the depth of the water above C. The water surface above C would then be at 1ATA.

 

[Melicertes]

Note my edited comment at post

I was looking for a suitable source to refer you all to.

 

[Melicertes]

A & E are exposed. If C is then exposed in this sytem, the water level will equalize with A and E (i.e. C can't be 33' below A or E). In this example, whereever that surface is located, A, C, and E would all then be exposed to 1 ATA. Ignore any elevation differences for atmospheric pressure since the change is insignificant in this case.

The height of the water column determines the pressure exerted by the water.

I agree with you 100%

 

[Dan Gibson]

But do you believe the original anwer was 2 or 4 ATA?

I agree with you 100%

 

[Melicertes]

But do you believe the original anwer was 2 or 4 ATA?

In the case of a closed cave system where C is not exposed to the atmosphere my answer is bubbled in at 4ATA.

 

[Brewone0to]

This is actually a real problem for cavers,as when exiting certain caves you must do deco and then return to deeper depths befor your next stop.
Really messes with most computers.
OTOH, If you really believe that there is no pressure difference between 99' and 33'
in a cave. Why not challenge your self by taking a deep breath at 99' and hold it while swimming up as fast as you can to the 33' level.
Then come back to this post and let us all know what you've learned.

 

[boat sju]

Man this is looking complicated (and I am and Engineer). The pressure at any point in the water column is (density of the water) X (depth below the "free" water surface) so for freshwater 62.4 lb/cu ft x your depth. Theoretically, if the cavern is below sea level and the air is trapped regulator exhaust it would be under pressure. In a sense that's how a dive bell works.

 

[Melicertes]

This is actually a real problem for cavers,as when exiting certain caves you must do deco and then return to deeper depths befor your next stop.
Really messes with most computers.
OTOH, If you really believe that there is no pressure difference between 99' and 33'
in a cave. Why not challenge your self by taking a deep breath at 99' and hold it while swimming up as fast as you can to the 33' level.
Then come back to this post and let us all know what you've learned.

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 - this is not the case with a cave in real life and compromises my conclusion. So in a way I can accept a decrease in pressure at point C because the water pressure will force water through the cave sides to equalize with other pressures not indicated or discussed in the OP's original post; trapped air spaces in the system will likewise compress to play havoc with the water pressures in certain areas of the system etc.

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.

 

[lamont]

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

If it did apply a pressure and that exceeded the pressure of the water column "above" the water (and down, sideways and up is still "above" here), then the water would move.

Pressure is just force per unit area, and if forces are unbalanced then the matter that is being acted upon by that force will move according to Newton's laws. So generally the rock acts upon the water with a force that exactly opposes the pressure of the water (just like the floor under you acts with a force that exactly opposes your weight). If the rock acts upon the water with a force that exceeds the pressure of the water at that depth then both the rock and the water will move until the forces are balanced. As the water in contact with the rock moves it transmits a force to all the rest of the water around it, and the rock will act like a piston, raising the level of the water throughout the column.

Generally, though, you just ignore all the rock and look at the height of the water column...

So, the correct answer at C is 2 ATA. And we know that C is not exposed to the atmosphere, otherwise there would be a net force at C (air would push down with 1 ATA and the water would push up with 2 ATA) so the water and air would move and water would push upwards until it reached a zero water column height (if the displacement of water caused the level of water at other areas in the system to drop, the water would rise until those heights were equal).

It is definitely *not* 4 ATA at C, because you take that 4 ATA and add another 66 fsw column of water as you head towards B so that there would be 6 ATA pushing on the water at B from the direction of C, while there is only a 100 fsw column of water (4 ATA) pushing on B from the direction of A so 4 ATA != 6 ATA and the water would move until equilibrium was reached...