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Exploring water pressure

  1. Jul 25, 2017 #1
    I'd like to explore water pressure by having students build water towers out of both 2-liter bottles and thinner 1 liter bottles.
    1. Assuming they build towers of the same height and fill them with the same type of water, we should see the same water pressure for identical heights within the towers correct?
    2. Looking for some ideas on how to read the pressure. Are there gauges available to read the pressure at the bottom of each tower? I read somewhere that 1 psi can be generated with each 2.3 or so feet of water depth. I had initially thought to attach the valve stem of an old inner tube to the bottom of each tower so that pressure could be read using easily available tire pressure gauges, but they don't appear to be sensitive enough.
     
  2. jcsd
  3. Jul 25, 2017 #2

    scottdave

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    I think for the valve stem gauges, you need air. So if you could make a small "air trap" which would expose to pressure, then it might work.
     
  4. Jul 25, 2017 #3

    scottdave

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    Maybe you could do kind of the reverse. Create a kind of expansion tank, with valve stem on top. Pump up with bicycle pump, and see how the column of water responds.
     
  5. Jul 25, 2017 #4
    Interesting. That was sort of another extension of the same unit. I wanted to put a chunk of styrofoam into a water filled vessel and then increase the pressure till the styrofoam deformed/crushed. All to demonstrate the effect of hydrostatic pressure at "crush depth". Still trying to work out how much pressure would be needed to accomplish this and then figure out if it could be done safely, ie: without blowing up the vessel which would need to be glass or plastic so the whole thing is visible.
    I'm working to create sort of hands-on components to a museum installation aboard a WW2 submarine that will allow visitors to explore the physics of pressure among other things.
     
  6. Jul 25, 2017 #5

    scottdave

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    That's awesome. If I think of any other ideas, I'll pass them along.
     
  7. Jul 25, 2017 #6

    scottdave

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    Also, I have seen water pressure gauges, which can screw onto a fitting like on the exterior water faucet (for a garden hose). I remember my dad used to have one at home. It had a dial and a needle. It would read the pressure when you opened the faucet. I found a couple, online. But now looking at these, I don't know if that will help you. The range on a couple are 0-200 psi. You need about a 33 foot tall column of water for 1 atm (14 psi), which will not move the needle very much.
     
  8. Jul 27, 2017 #7
    What about using the height of a fluid column to measure pressure? You could link the 2 liter and 1 liter bottles together with a valve and then open it. The levels will equalize when the pressures are equal (which is for equal heights). It would be more dramatic to use a thinner piece of tubing so that you could show that, say 100 ml of water could balance 4000 ml of water as long as the heights are equal. I definitely recommend checking out Pascal's treatise on fluids: The Equilibrium of Liquids. If you have access to a university library (or library loan) you can find it in The Physical Treatises of Pascal.
     
  9. Jul 27, 2017 #8

    scottdave

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    I am just thinking out loud here. Maybe something like an expansion tank (possibly clear) with some water in it. Hook in a valve with bicycle air pump to add pressure to the tank. Have some clear tubing for the water to move through, and have it run up, vertically.

    Another thing: on the Science Channel show Impossible Engineering, they did one about the new Virginia class submarines. They had made a "mini sub" in an aquarium tank, and would pump air out and in to it, to make it submerge and surface. That show has several episodes, which are ship or maritime related, this season.
     
  10. Jul 27, 2017 #9
    Here's to thinking out loud:
    How about measuring pressure by using the projectile motion of a stream a-la Torricelli's law? Fill both vessels to the same height and measure the distance each jet travels before hitting the ground.
     
  11. Jul 31, 2017 #10
    Thanks for all the input so far.
     
  12. Jul 31, 2017 #11

    berkeman

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    You might be able to make a demo/gauge with a large-size syringe body, to form a graduated volume of air under a piston-type arrangement. Start with the syringe almost full of air at atmospheric pressure and seal the tip. Then lower it down into the column and keep reading off the changing volume of the air in the syringe at each depth. You might need to pre-lube the walls of the syringe to keep stiction from being a problem...

    http://image.11st.my/g3/2/9/4/9/7/0/35294970_B_V1.jpg
    [​IMG]
     
  13. Aug 1, 2017 #12
    Great idea!! Something that could possibly be done in a classroom setting with a number of kids. Need to do some calculations. How tall of a column of water would be needed to produce results in the syringe that would be visible to students?
     
  14. Aug 1, 2017 #13

    berkeman

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    Water pressure increases by 1atm for about every 10m of depth...

    https://dtmag.com/thelibrary/water-world/
     
  15. Aug 1, 2017 #14

    berkeman

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    Water pressure increases by 1atm for about every 10m of depth...

    https://dtmag.com/thelibrary/water-world/
     
  16. Aug 1, 2017 #15
    How about Boyle's Law:

    P1V1 = P2V2

    P1 = 101.325 kPa (STP)

    V1 = 20 mL

    V2 = 10 mL

    P2 = P1V1 / V2 = 20 mL (101.325 kPa) / 10 mL = 202.65 kPa
    As a practical matter, it remains to be seen how much of a water column we can build and whether or not it will be sufficient to depress the plunger in the syringe enough to be measured by the hawk-like eyes of some high school students. (sarcasm)
     
  17. Aug 1, 2017 #16
    BTW: the V1 and V2 in the previous example were numbers I generated to do the math as an example. :)
     
  18. Aug 1, 2017 #17

    berkeman

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    So you'll get about 1mL per meter. 2-3 meters worth of water column should give you a visible change, assuming you can lube the inside of the syringe wall with something slippery...
     
  19. Aug 8, 2017 #18

    jim hardy

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    Tire gages go to about 60 psi. You need an inexpensive one that's more sensitive.
    Check thrift shops for cast off blood pressure cuffs. The gage on them is 0 to 300 mm of mercury. That's about 1 atmosphere, roughly 4X the sensitivity of an everyday tire gage.


    But for more exactly what you asked

    try a search on "inches of water pressure gage"
    my first hit
    upload_2017-8-8_21-6-5.png
     
  20. Aug 22, 2017 #19

    scottdave

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    I don't know where you are with this project. I was at Academy sporting goods, and purchased this pump with pressure gauge for $6.99, and I was reminded of your project, as the ball that I need to inflate recommends 4 to 6 psi, and this gauge seems to do the trick.
    RpmQnUS.png
     
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