Question 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!
Answer You have pressure, therefore height, in the tubes because you have air in the tubes. Even if you got all of the air out there will be a residual from water vapor. The height of water for a perfect vacuun is about 34 feet, but the height attainable is only about 30 feet because of water vapor pressure.
The same phenomenon ossurrs in a mercury barometer, but the vaporpressure of mercury is far less than water.
