Pressure Depth in a Cave

[phunk]

well, that's not the scenario presented

the scenario presented was a flooded tube

as for the "air pocket" scenario, an air pocket could only exist at 1 ATM or "above" 1 ATM. at 1.1ATM, the water would "push up" the air back to 1 ATM.

In an air pocket scenario, the air pressure in the pocket will match the water pressure at that depth. It will never be 1atm unless the pocket is at sea level. In the original question the cave at point C, 33 feet down, will be 2atm.

so, even with an air pocket above you, you still calculate as i indicated:

[water depth/33] + 1 = ATM

Assuming you mean the depth from the surface exposed to outside air and not the depth from the surface inside the air pocket, then that's correct.

 

[Peter Guy]

Drewski -- I thought I made that clear at the beginning. After watching the "G. I. Show" where he swims into a huge room, it got me to thinking what happens to the pressure?

My problem, as I set it up, was in thinking the B to D "tube" was a perfectly closed and pressurized system AND that gravity would not have an effect on the lifted water at C.

 

[H2Andy]

In the original question the cave at point C, 33 feet down, will be 2atm.

but it is completely flooded, there is no air pocket

(are you trying to confuse me? you're doing a great job, btw ... )

 

[phunk]

Drewski -- I thought I made that clear at the beginning. After watching the "G. I. Show" where he swims into a huge room, it got me to thinking what happens to the pressure?

My problem, as I set it up, was in thinking the B to D "tube" was a perfectly closed and pressurized system AND that gravity would not have an effect on the lifted water at C.

Quite simply, like any other dive, the pressure is relative to the depth from the surface that's exposed to outside air. In your example, 33ft = 2atm.

 

[phunk]

but it is completely flooded, there is no air pocket

(are you trying to confuse me? you're doing a great job, btw ... )

Sorry.

I was just pointing out that it doesn't matter if there's an air pocket or not, the pressure is the same.

 

[markr]

Wiki is considering siphons that have reservoirs at two different height with "final end of the tube lower than the liquid surface in the (first) reservoir" as well as with the final end open to atmospheric. Give that special case, everything said is correct, however, that was not the special case I was asking about. In the case that I'm interested in the two reservoirs are at the same level and both ends of the tube are at the same level, but there is no flow, just water held above the height of the surface of the two reservoirs.

Permit me to describe again: Two identical reservoirs 33 feet deep at the same level. Two siphons each connecting the two reservoirs. One siphon just slightly below the surface of each reservoir and another siphon that extends to almost the bottom (33 feet) of both reservoirs.

If the pressure in a siphon tube could be varied by changing the depth of the ends of the tube, you could pump water from a lower surface level to a higher one by having the tube in the lower container deeper in the container then the end of the tube in the higher container.

A simple experiment to try at home: take two buckets and fill them so their surface levels are the same. Stick the ends of a tube full of water in both buckets. If the pressure in the tube is controlled by the depth of the end of the tube, pushing one end of the tube deeper into it's bucket should cause water to flow into the other bucket. The water would flow from the higher pressure to the lower pressure.

The end of the tube isn't what controls the pressure, it's the surface level of the water.

 

[Drewski]

Drewski -- I thought I made that clear at the beginning. After watching the "G. I. Show" where he swims into a huge room, it got me to thinking what happens to the pressure?

My problem, as I set it up, was in thinking the B to D "tube" was a perfectly closed and pressurized system AND that gravity would not have an effect on the lifted water at C.

Hi Peter:

My response to you would be that your depth gauge is an ABSOLUTE measurement of pressure. It is zeroed to sea level or 1 ATM. If you read some of the gauge product literature, you will see that manufacturers warn you about using it at altitude without re-calibration.

A bourdon tube measures pressure in BOTH air or water. Take it down to your LDS. Ask them to put it in that black pressure testing pot that most of them have. Watch what happens when they pressurize it. Compare the pressure in PSI to depth indication on the gauge. Do the PSI to depth conversion.

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

I won't get into the physics argument here, except to say this. Outflow from a cave relieves internal pressure. Just like the water pressure at your house will drop if a bunch of fire hydrants in your neighborhood are opened, water pressure in a cave will also vary. BUT, regardless if there is a "residual pressure" in the system, your depth gauge measures it accurately.

If it didn't, don't you think that some of the popular caves with deep entrances and then long, shallow galleries would bend a LOT more people than they do?

Peace...

 

[boat sju]

If you do find an air pocket at depth it's probably co2, so I wouldn't want to breath it anyway. Actually, I'm curious if anyone's encountered this situation? I could see pockets of regulator exhaust (for lack of a better term) accumulating in small overhang areas in tourist spots.

Sorry - not trying to sidetrack this thread any further.

 

[PaulSmithTek]

Talking to three engineer's in the office it would be 2 ata Because we are talking about absolute pressure the pressure does not change in the water column. 33' is 33' in a cave or not. That is was I was told. I still dont know for sure.

 

[Drewski]

If you do find an air pocket at depth it's probably co2, so I wouldn't want to breath it anyway. Actually, I'm curious if anyone's encountered this situation? I could see pockets of regulator exhaust (for lack of a better term) accumulating in small overhang areas in tourist spots.

Sorry - not trying to sidetrack this thread any further.

If memory serves, there is a nice little pocket back in Telford you can put you head and shoulders into. BTW, it DOES smell BAD.