Pressure Depth in a Cave

[Benthic]

This is a very interesting question. I can see how some people think the answer is 4 ata. or even 1 ata. But even for a dive instructor this is not surprising. I had an instructor tell me that your regulator could free flow from looking up at the surface. Makes since doesn’t it? You have all the weight of the water column pressing down on your purge valve. But I would hope that a cave diver would know the answer from experience

Okay, this is absolutely shocking too, and I really hope you're kidding us with this story. The beauty of a regulator is that it delivers air to your mouth, at ambient pressure. So (assuming you held your breath, which we all know is a no-no) the pressure in the airspace behind the 2nd stage diaphragm would be the same as the water in front of it--regardless of which way your head is oriented. The instructor you speak of should be, as Reefnet stated, "flogged with a C-Card." Then his/her instructor rating should be yanked. Unbelievable.

Brian

 

[Thalassamania]

But all this raises a question that, frankly, I can not answer intuitively. Let's say we have two vertically identical, very large aquaria tanks that are open on the top and are 33 ft deep. These tanks are connected together by two "true siphons," one at just below the surface of the water and at the bottom of the tanks. Each of these siphons go up above the surface of the aquaria by 33 feet. What is the pressure at the top of the siphons? Why? My first guess would be that the shallow origin siphon is approx 1 ATA all the way through (ignoring the fine point of siphon's diameter) and the deep one would be approx 2 ATA all the way through (with the same stipulation) but my knowledge of fluid dynamics is inadequate to either be confident of that answer or to explain it. Can anyone help?

 

[markr]

But all this raises a question that, frankly, I can not answer intuitively. Let's say we have two vertically identical, very large aquaria tanks that are open on the top and are 33 ft deep. These tanks are connected together by two "true siphons," one at just below the surface of the water and at the bottom of the tanks. Each of these siphons go up above the surface of the aquaria by 33 feet. What is the pressure at the top of the siphons? Why? My first guess would be that the shallow origin siphon is approx 1 ATA all the way through (ignoring the fine point of siphon's diameter) and the deep one would be approx 2 ATA all the way through (with the same stipulation) but my knowledge of fluid dynamics is inadequate to either be confident of that answer or to explain it. Can anyone help?

The pressure at the top of both your siphons would be the same, slightly higher then 0 ATA. If you pulled the siphons any further up you'd end up with a vacuum at the top. One atmosphere of pressure is equivalent to around 33.5' of water or 29.9" of mercury. If the siphon tube is taller then 33.5' there won't be any water at the top of the tube there will be a vacuum.

 

[victor]

The pressure at the top of both your siphons would be the same, slightly higher then 0 ATA. If you pulled the siphons any further up you'd end up with a vacuum at the top. One atmosphere of pressure is equivalent to around 33.5' of water or 29.9" of mercury. If the siphon tube is taller then 33.5' there won't be any water at the top of the tube there will be a vacuum.

At that point the water boils, evaporates, and you get a pocket of clear gas at the top of the siphon that is not air. :11:

 

[Thalassamania]

The pressure at the top of both your siphons would be the same, slightly higher then 0 ATA. If you pulled the siphons any further up you'd end up with a vacuum at the top. One atmosphere of pressure is equivalent to around 33.5' of water or 29.9" of mercury. If the siphon tube is taller then 33.5' there won't be any water at the top of the tube there will be a vacuum.

I don't think this is right, I've seen teleost fish swim through such a siphon and they did not have their swim bladders expand, that is why I suspect that the pressure stays constant, though I can't explain why from first principles

 

[Peter Guy]

Thanks everyone and I hope that no one will take away my C-card just because I was confused by the notion that B and D "sealed" the loop and would keep the same amount of pressure throughout the whole system.

As far as I can tell, my two mistakes are:

a. Ignoring A and E which make it an open system; and

b. Ignoring the weight of the water column from B to C which would have the effect of decreasing the pressure the same amount as the weight (at least that is my take away of this)

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?

 

[Daner]

If the pressure at C was 4 ATA wouldn't your depth gauge read 99 feet and you wouldn't know you were actually 33 feet below sea level?

Like if a tree falls in the forest does it actually make a sound?

I vote for 2 ATA.

 

[Blackwood]

thnx Blackwood

my pleasure.

Since diving discussions are so often characterized by "this is my opinion, and it's better than yours," it's fun to through something objective into the mix now and again.

 

[markr]

I don't think this is right, I've seen teleost fish swim through such a siphon and they did not have their swim bladders expand, that is why I suspect that the pressure stays constant, though I can't explain why from first principles

It is right. Do a google search on siphon. The following quote is from Wikipedia but any decent explanation will have the same information:

"When the pressure exerted by the weight of the height of the column of liquid equals that of atmospheric pressure, a partial vacuum will form at the high point and the siphon effect is ended. For water at standard pressure, the maximum height is approximately 10 m (33 feet); for mercury it is 76 cm (30 inches)[FONT=&quot]"

If you saw a fish swim through a 34' tall siphon, it was breathing vacuum at the top.


[/FONT]

 

[Charlie99]

As far as I can tell, my two mistakes are:

a. Ignoring A and E which make it an open system; and

b. Ignoring the weight of the water column from B to C which would have the effect of decreasing the pressure the same amount as the weight (at least that is my take away of this)

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?

Item B is the important one.

That's the same reason why the pressure will be less than 1ata in a siphon that goes above the level of water open to the atmosphere.

I've seen teleost fish swim through such a siphon and they did not have their swim bladders expand, that is why I suspect that the pressure stays constant, though I can't explain why from first principles

Was it a siphon that actually went up above sea level (or the level of a lake). A siphon-like structure that is completely below the water level is like the problem described by the original poster. In that case, it was 2ata at the top of the "siphon".