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Homework Help: Pressure difference in two pipelines

  1. May 15, 2010 #1
    1. The problem statement, all variables and given/known data

    A water manometer is connected to the wall of two pipelines in order to measure pressure difference between the pipes. Manometer shows a 0.5m height difference, pipe B is 3m higher than pipe A. Find pressure difference between pipes.
    [PLAIN]http://img191.imageshack.us/img191/9528/fluid.jpg [Broken]

    Density water: 1000kg/m3
    Density Oil: 850kg/m3

    2. Relevant equations
    Pressure difference = density of fluid x height difference x 9.8

    3. The attempt at a solution
    From the illustration it is evident that pipe B will have lower pressure than pipe A since the oil pressure in pipe A can depress the water in manometer to a certain height even as Pipe B is mounted higher than pipe A.

    If the two pipes were located at the same height, the manometer alone would show the true pressure difference:
    Pressure difference manometer = 1000 x 0.5 x 9.8 = 4900 Pa

    Since pipe B is mounted 3m higher than pipe A we need to add this pressure difference:
    Pressure difference altitude = 850 x 3 x 9.8 = 24990 Pa

    Total pressure difference is the sum of these two: 24990+4900 = 29890 Pa.

    The correct answer is 25751 Pa.

    My reasoning must be wrong somewhere?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 19, 2010 #2
    V.quickly I get 25725 Pa as the height difference is actually 2.5m due to pipe B having more water in it than pipe A (0.5m more). If you made the difference 2.50312 you'd get the answer given, maybe the error is due to assuming a flat fluid boundary instead of a meniscus, but that's jsut a wild guess sorry.
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