Pressure measurements using a closed tub manometer (barometer)

[WESO1]

We have fluid mechanics question regarding pressure measurements using a closed tube manometer (barometer). My daughter is doing the following science experiment:

We filled 2 closed end tubes with different levels of water and then turned them over and supported them in a bucket of water. Inside each tube at the top should be a near perfect vacuum created which means no pressure is exerted on the top of our water column. One tube is supporting a 12 inch water column while the second tube supports an 18 inch water column. We know that an atmospheric pressure of 14.7 psi will support a 34 foot column of water.

Our questions are: Why do our 2 closed end tubes maintain the 12 & 18 inch column of water? We thought the atmospheric pressure acting on the open bucket of water would cause each tube to support the same level of water. Why not and what is our actual pressure in each tube ?

And lastly, since we have a pressure of 14.7 psi acting on the open bucket of water, we actually expected this pressure to “push” water into each tube completely filling them since there is a vacuum above each closed tube and because the atmosphere can support a column of water 34 feet high. Why did the tubes not completely fill with water?

Please help !
Thank you!

 

[Yorkman]

If the two tubes were not completely filled to the top before turning over and placing into the resevoir, the gas inside the tube would travel to the enclosed end and would actually be under atmospheric pressure acting on the resevoir, less the static pressure due to the column of water.

To create a vacuum the tube would need to be over 34 feet long and filled to the very top before inverting into the bucket/resevoir. The amount of "vacuum" created would be proportional to the height of the tube you started with. The longer the tube the deeper the theoretical vacuum. At some point the liquid in the tube would be begin to flash reducing the vacuum affect "in theory".

Man that was fun hope that answers some of your questions

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI