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Stagnation pressure

  1. Feb 8, 2015 #1

    We use a manometer to measure the pressure of an air tunnel. The connection of the manometer has to be changed as sometimes water enters the manometer and contaminates the fluid inside.

    I have attached a picture in which I am showing the now and before of the connection. My question is: will this change in connection affect the reading in my manometer? (the black rectangles). If so, what will change in the equations? Now, the fluid is literally pushed inside the manometer, in the past it wasn't as the connection was attached the bottom of the tunnel. I have tried relating it to a Pitot tube reading, but I do not know how accurate this is, and some doubts have really gotten to me with the parameters.

    Please let me know if I did not explain myself very well.

    Thank you so much in advance.

    Attached Files:

  2. jcsd
  3. Feb 9, 2015 #2

    Quantum Defect

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    Why not go to an electronic pressure transducer?
  4. Feb 9, 2015 #3
    I only have a manometer which gives me a pressure Reading in mmH20. I can connect it the way I desire, but I have to calculate the speed of air in the tunnel. It currently reads the static pressure, but I do not know how to go from there to a determination of the velocity of the air.

    Maybe if I connect it in a way one of the ends is in the same direction of the fluid and the other determines the static pressure, I´ll be able to calculate this simulating a pitot tuve. What do you think?
  5. Feb 9, 2015 #4
  6. Feb 9, 2015 #5
    I think this will reduce moisture into manometer to some extent. But if the air has too much moisture content and its temperature is not less then dew point of moisture your problem may persist. To measure air velocity you may need a venturi/ orifice or some other flow measuring instruments. Also to measure air flow correctly you may also have to give attention to requirement of any flow measuring instrument you will use.
  7. Feb 9, 2015 #6


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    Staff: Mentor

    You answered your own question in the title: the new configuration gives the stagnation pressure whereas the old gave you the static pressure.
  8. Feb 11, 2015 #7
    You can homebrew a pitot tube with manometer connected for air speed and flow rate measurement. Russ is to the point...
  9. Feb 12, 2015 #8
  10. Feb 12, 2015 #9
    You new configuration will be measuring dynamic pressure (well, stagnation pressure since the air will stagnate at the fluid column). The old configuration would have been reading hydrostatic pressure minus whatever pressure drop you'd get due to the venturi effect. Since hydrostatic pressure is negligible for your old configuration you should get similar readings.

  11. Feb 12, 2015 #10
    Based on your picture the "Now" setup is measuring the difference between the tunnel total pressure (assuming you are out of the boundary layer) and the atmospheric pressure outside the tunnel. If you want to get the tunnel speed you can connect your reference end of the tube (the end open to atmosphere) to the wall of the wind tunnel so that it measures the static pressure in the wind tunnel. This way your manometer will give you the dynamic pressure which you can use to calculated velocity.
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