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Question about fluid flow and pressure in pipes

  1. Feb 5, 2013 #1
    Does a pipe have to be completely filled with a fluid to measure its pressure? For instance if I have a small diameter pipe that diverges to a bigger pipe. The small pipe will be completely filled with water, but will the bigger pipe be? If the bigger pipe is only half full of flowing water, will a pressure gauge still return a reading even though there isn't direct contact between the gauge and the water?
  2. jcsd
  3. Feb 5, 2013 #2
    If I pipe isn't filled, flow is dependent on gravity (like, say, sewer gravity mains). You will not see any measurable pressure (i.e. any head pressure) due to flow, but that has little at all to do with the location of the pressure gauge.

    However, with that said, it is important to understand your setup. Simply going from small bore to larger diameter does not necessarily mean you will have a partially filled pipe. It all depends on the setup (i.e. open to atmosphere? Is the small bore force fed? etc)
  4. Feb 5, 2013 #3
    Thanks for your response Travis. What I'm trying to do is measure the pressure in a 1/8" tube. The place that I want to measure the pressure won't be open to the atmosphere. I'm concerned that If I break the the 1/8" tubing and put inline a bigger pvc fitting (3/4") with an attached gauge then I won't get accurate readings.
  5. Feb 5, 2013 #4


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    Staff: Mentor

    If the system is closed, there need-not be any air in it.
  6. Feb 5, 2013 #5
    If I understand your meaning, you want to break a 1/8" line and install a 3/4" fitting that supports the gauge?

    If the system is not open to atmosphere and there are no air pockets (i.e. the system is primed) then you will not notice a significant change in the pressure.

    In a flat system (i.e. no elevation changes) the static pressure in the pipe will remain constant regardless of the pipe diameter.

    This is a nice explanation here.
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