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School project about PSI

  1. Jan 27, 2017 #1
    I need help finding out how fast 120 psi will travel though a 3 inch tube in mph. My partners are counting on me and thanks to anyone that can help me.
  2. jcsd
  3. Jan 27, 2017 #2
    What do you know and what part are you having trouble with?
  4. Jan 27, 2017 #3
    When you say 120 PSI you are talking about a pressure. Flow is a different variable. For instance you could say I have a tube with 120 PSI in it, say 3 inch diameter and 4 feet long and it is enclosed so that is the total volume and we open a valve quickly to another tube say also 3 inch diameter and 4 feet long and at a vacuum level of 1 millitorr, you could do a speed calculation but just a tube with 120 PSI in it does not indicate flow so the speed would be zero there.

    Also, for there to be a flow, there has to be a place with say 120 PSI and another place with say 100PSI, that would flow from high to low PSI and the difference of 20 PSI would be the variable to use to calculate air velocity, or water or oil, whatever.
    Last edited: Jan 27, 2017
  5. Jan 28, 2017 #4
    "Flow" may not be the proper term, although the air in the pipe does move as it is compressed. Maybe "pressure gradient travel" would be a better term. Not a big deal in any case. First, be aware that pressure gradients travel at roughly sonic speeds so the time intervals will be quite short. And there will be a slight error due to the time it takes for enough air at 120 PSI to fill into the pipe to pressure it up. This can be minimized by using a relatively large reservoir and valve vs. the diameter of the pipe. For example, 1/2" tubing and a 2" ball valve on a 60 gallon reservoir. That's all I can come up with right now.
  6. Jan 29, 2017 #5
    Potato gun?
  7. Feb 2, 2017 #6


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    Science Advisor

    It's probably going to depend on details of your valve and container, but for a ballpark estimate, you could start with the speed of sound.
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