Pressure Depth in a Cave

[Drewski]

So, regardless of what else is going on, the depth on your gauge IS the depth you are AT.

Just for further clarification, I should have said "the depth on your gauge IS the relative depth you are AT," because if a "residual pressure" DOES exist, your gauge would STILL measure it. What I mean by this is if some "residual pressure" added say 3 PSI to the water column, your gauge would read approximately 7 FT deeper than your ACTUAL depth below sea level.

Peace...

 

[jponline77]

Great discussion... I'm not sure that chastizing people who didn't get it right away was appropriate. It is a little counter-intuitive to understand, otherwise pascal wouldn't have needed to create a "principle" to explain it. He could have explained to the people asking for an answer "it's simple simple stupid don't you see"

I'm assuming, because of "b" above, that if I had "sealed" the water at D, C would still be 2 ATA -- am I getting closer to understanding?

*edit* I assume you mean sealed at B and D, right?

Peter, you are right. Interestingly, if the seal was perfect and it was a perfectly rigid container, when you took that container out of the water above the surface it would remain at 2 ATA. This is because there is nowhere for the pressure to be released from the container to get equalized with the outside air. You probably already figured that out as well.

JP

 

[Drewski]

Great discussion... I'm not sure that chastizing people who didn't get it right away was appropriate. It is a little counter-intuitive to understand, otherwise pascal wouldn't have needed to create a "principle" to explain it. He could have explained to the people asking for an answer "it's simple simple stupid don't you see"

*edit* I assume you mean sealed at B and D, right?

Peter, you are right. Interestingly, if the seal was perfect and it was a perfectly rigid container, when you took that container out of the water above the surface it would remain at 2 ATA. This is because there is nowhere for the pressure to be released from the container to get equalized with the outside air. You probably already figured that out as well. JP

Huh? So back to his original post, are you saying that point "C" is MORE than 2 atmospheres?

 

[jponline77]

Huh? So back to his original post, are you saying that point "C" is MORE than 2 atmospheres?

No, for the original post the pressure is 2 ATA at C. I am assuming that B,D are being sealed off at depth when C is at 2 ATA. When you now come to the surface it remains at 2 ATA. Why, well 1 ATA air takes up twice the space and although the air in C would love to be at 1 ATA it can't because there is no space for it to go. Why does the air in your tank remain at hundreds of ATA of pressure?

 

[Blackwood]

No, for the original post the pressure is 2 ATA at C. I am assuming that B,D are being sealed off at depth when C is at 2 ATA. When you now come to the surface it remains at 2 ATA. Why, well 1 ATA air takes up twice the space and although the air in C would love to be at 1 ATA it can't because there is no space for it to go. Why does the air in your tank remain at hundreds of ATA of pressure?

We're talking about water which, for all intents and purposes, is incompressible.

 

[jponline77]

We're talking about water which, for all intents and purposes, is incompressible.

Are you agreeing with me or disagreeing as this could be an arguement that strengthens what I was proposing?

If you disagree:

1) Do you see that the air would remain at 2 ATA as it is compressable and would not have anywhere to expand?
2) Now if we are just below the surface of the water in C, can it be at 1 ATA and the air just next to it be at 2 ATA?

The answer to 2 is no.

JP

*edit* also, if you remove the air pocket the answer remains the same... that will be the homework question to answer why.

 

[diebeste]

Check out the following link and pay attention to the second paragraph: "Pascal's law states that when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container."

I'd say B and D are subjected to increased pressure relative to A and E, and since it's a closed container even if we just prefer to call it a cave, weeeell....

It is not a closed container: A and E are open to the atmosphere.

 

[ortoman]

air pressure in the cave is 4 ata, which is equal to the pressure of the water..because water compresses the air , as the compresor forces air to dive cylinder.. the compressor here is the water...

 

[Blackwood]

Are you agreeing with me or disagreeing as this could be an arguement that strengthens what I was proposing?

I'm guess I'm not really sure what you were proposing. I was replying only to post 112.

It seems like you said that if you were to seal water at 33' in a rigid container and ascend, the container would hold water at 2ATA at the surface.

...if I had "sealed" the water...
___________________________REPLY___________________________

if the seal was perfect and it was a perfectly rigid container, when you took that container out of the water above the surface it would remain at 2 ATA.

If that container holds water, I disagree.

 

[Charlie99]

I'm guess I'm not really sure what you were proposing. I was replying only to post 112.

It seems like you said that if you were to seal water at 33' in a rigid container and ascend, the container would hold water at 2ATA at the surface.



If that container holds water, I disagree.

My answer is "indeterminate", because water is neither truly incompressible, nor is the container truly rigid.

This isn't just semantics, it really is one of those cases where small 2nd order effects start to dominate. In that sort of system the pressure can vary widely from things like expansion of the water due to temperature changes. Things like gases dissolved in the water going into or out of solution also have big effects.

If the container is very close to rigid, then just a tiny amount of dissolved gas coming out of solution will keep the average pressure of the water at the same average pressure it was when sealed up.

Yes. I deliberately used the word "average". When sealing up a bunch of water, I'm pretty sure what is going to dominate is the pressure of the gases dissolved in the water, so the average depth is what counts (assuming a constant liter/ata solubility with pressure changes).