Question about water pressure inside a sealed tube.

[Low-Q]

If we have a >10 meter long vertical aligned tube which at first are open at the top. We put this under water in a >10m deep pool so the tube can be filled with water. Then, while the tube is under water, we seal the top. When we rise the tube above water level, the tube will still be filled with water. If we lift the tube more than 10 meters above the water, it will start to create a space of vacuum at the top.

So the question is: What are the water pressure inside the tube at water level in the pool?

So the next: If we then seal the bottom end of the tube also, and rise the whole tube in the air, above the water in the pool. What will then the water pressure be at the bottom inside the tube?

Vidar

[fritznien]

If we have a >10 meter long vertical aligned tube which at first are open at the top. We put this under water in a >10m deep pool so the tube can be filled with water. Then, while the tube is under water, we seal the top. When we rise the tube above water level, the tube will still be filled with water. If we lift the tube more than 10 meters above the water, it will start to create a space of vacuum at the top.

So the question is: What are the water pressure inside the tube at water level in the pool?
14.7 psi same as ambient air preasure.

So the next: If we then seal the bottom end of the tube also, and rise the whole tube in the air, above the water in the pool. What will then the water pressure be at the bottom inside the tube?

Vidar

asume the bottom of the tube was just below water surface when sealed 14.7 psi
air preasure supports the colume of water.
fritznien

[Low-Q]

Thanks for clearing this up. Sometime one doesn't see the obvious

Vidar

[tbird]

If we have a >10 meter long vertical aligned tube which at first are open at the top. We put this under water in a >10m deep pool so the tube can be filled with water. Then, while the tube is under water, we seal the top. When we rise the tube above water level, the tube will still be filled with water. If we lift the tube more than 10 meters above the water, it will start to create a space of vacuum at the top.

So the question is: What are the water pressure inside the tube at water level in the pool?

So the next: If we then seal the bottom end of the tube also, and rise the whole tube in the air, above the water in the pool. What will then the water pressure be at the bottom inside the tube?

Vidar

hi vidar,

sorry i couldn't get to you earlier but i had my finger in the dike.

i was a little confused by the previous answer so thought you might be too.  let me give it a try.

first we need to establish which pressure you are using, gauge or absolute.  the difference is 0 psi on a gauge unit (when not hooked to anything) doesn't reflect the atmosphere around you, but an absolute gauge will be showing the 14.7psi (+ or -).  it will read 0 in a perfect vacumn.  since the atmospheric pressure is often over looked when working with this kind of stuff, i suggest you use absolute psi.  it may help you see things more clearly.

second, what is the diameter of your tube.  it will make a difference in the ability of the water to stay in the tube when you have the top sealed and take it out of the water.  if it is small enough, like a straw, you will have no problem holding the water in the tube, unless the > 10m is enough to create cavitation.  that's where the weight of the water itself is able to pull the water apart and create a bubble.  if the diameter is too large, the surface tension of the water won't be able to prevent air from entering the tube, thus letting the water run out.  if you put a sheet of something (like the stuff a paper milk carton is made of is fine) over the end before taking the tube out of the water (with the top sealed), if it has a good seal between the paper and the tube, it will come right along with the tube, without holding it in place.  the atmospheric air pressure does the holding.

let's assume the diameter allows the water to stay in the tube.  now that it is out of the pool, with the top sealed,(btw, since you sealed the tube under water, there will not be any area void of water at the top, unless as i said before, the >10m was really more than 1 atmosphere) the weight of the water (we'll say the >10m = 1 atmosphere), the pressure at the bottom would equal 1 atmosphere (14.7 absolute psi).

on the other hand, with the bottom sealed and the top open, the pressure would be 29.4 absolute psi.  that's 1 atmosphere of air and 1 atmosphere of water, so our absolute gauge would read 29.4 apsi.  to figure gauge psi, just subtract the 1 atmosphere of air.  it will read 14.7 gpsi.

you mention raising the tube to a higher level out of the water.  the psi will stay the same except for the amount of air pressure (atmospheric) you lose with altitude.

hope this didn't confuse you more.

tom