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Maybe I posted in wrong forum: Fluid Mechanics help! Chasing a deadlin

  1. Mar 10, 2013 #1
    1. The problem statement, all variables and given/known data
    A tank consists of two chambers, one open to the atmosphere and another closed and filled with a gas. The two chambers are connected by a sluice opening. What is the pressure in the gas, in gauge and absolute terms if the barometric pressure = 742 mm Hg. Difference in height of the chambers is 700mm, with the level of the open side being 700 mm lower than the closed side. Thanks in advance :)

    2. Relevant equations

    3. The attempt at a solution
    I'm not sure, I feel like there isn't enough information at least for me to work it out without using a formula. I'm not looking for completed answers but nudges in the right direction. It's been two years since I've done any physics and I've thrown myself in the deep end. Thanks
  2. jcsd
  3. Mar 10, 2013 #2
    Throw me a rope!
  4. Mar 10, 2013 #3
    Is the answer 92.065 kPa absolute pressure and -6.86 kPa gauge pressure?
  5. Mar 10, 2013 #4
    Please someone tell me if I'm right or wrong
  6. Mar 10, 2013 #5
    Just became aware that I needed to attempt a solution. What I've got is:

    deltaPressure (gauge) = Pressure - PressureAtomsphere = density x height x gravity

    Therefore gauge pressure = -1,000 kg/m3 * 9.8m/s2 * 0.7m = -6,860 Pa
    Absolute pressure = Atmospheric Pressure + Gauge pressure = 92,065 Pa Absolute Pressure
  7. Mar 11, 2013 #6

    rude man

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    Gold Member

    Hello Dylan - sorry but I'm not sure about this problem. Sorry too no one has responded. I think the problem may be the word "sluice", the meaning of which in this context being somewhat obscure, to me anyway.

    Did you also post on the Engineering forum?

    If you can't get any responses try to send a personal message to ChesterMiller. He has a lot of expertise in this area.

    rudy d.
  8. Mar 11, 2013 #7
    Seems straight forward. It astounds me that no one replied to such simple question.

    You are given difference in height. You are given barometric pressure, that is pressure of atmosphere.

    So, as per my thinking,

    Patm-Pgas = Difference in pressure
  9. Mar 11, 2013 #8
    Thanks for the replies. The sluice in this case is like a U bend in a manometer, say you have two chambers the sluice is a part at the bottom of the chambers where the water can flow between the two chambers - much like a manometer. I ended up approaching the question the same way I'd approach pressures in a manometer, and submitted my assignment. Not 100% sure if I'm correct but the answers seem plausible so I'm hoping for the best
  10. Mar 12, 2013 #9
    You have followed the approach which I mentioned but shouldn't the atmospheric pressure be more than the pressure of the gas ?
  11. Mar 12, 2013 #10
    If I'm interpreting the question correctly the atmospheric pressure is more than the pressure in the gas. The barometer reads 742 mm Hg which is 98,925 Pascals which is the air pressure of the side of the joined chambers that is open to the air. Then using the formula pressure = density*height*g I calculate the gauge pressure in the gas to be -6,860 Pascals and then add the gauge reading to the barometer reading to find the absolute pressure inside the chamber.

    Although it just occurred to me that the reading of the Barometer is given in gauge pressure (is it? it wouldn't really make sense to me if it was but I've submitted it and have since been picking holes in all my work) so maybe I need to adjust my absolute gas pressure accordingly. I am guessing I will lose marks if this is the case. I'm not too sure on this. It seems logical that atmospheric pressure has to be absolute as there is no other reference to adjust it by, I've tried looking up Barometer's but it isn't 100% clear to me.
  12. Mar 12, 2013 #11
    There is a sluice opening. Fine. And why did you approximate 0.742m to 0.7m in your work ? You should not do approximation unless you get the final answer. And also I think it is straight forward.

    If the one end would not be opened then you could have expected the equilibrium to be established and applied Boyle's law accordingly.

    Also what you did is same as what I did, except,

    You did : Patm+Gauge Pressure

    And you calculated Gauge pressure = -x say.

    What I suggested was,

    Patm-Gauge Pressure

    And Gauge Pressure = x

    Does not matter. I think its just as simple.
  13. Mar 12, 2013 #12
    742 mm Hg is the barometer reading which equates to 98,925 Pascals, the height difference between the two sides is 700 mm. I did not approximate I see what you're saying, I think that our working is the same from looking at it so hopefully this question is in the bag. Thanks again I really appreciate you taking the time to assist me on both questions
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