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Vacuum pressure inside a 650 gallon tank

  1. Feb 19, 2013 #1
    Need to find out how much force would it take to pull up 650 gallons of water into a tank from the bottom by sucking out the air from the top of the tank? Also how much vacuum pressure would be inside tank once full? I am basing this off an upside down fish tank. Example on youtube or like the upside down dog waterers
  2. jcsd
  3. Feb 19, 2013 #2

    Simon Bridge

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    Welcome to PF;
    ... bottom of what? How?
    Vacuum does not exert a pressure, but the term "vacuum pressure" can be used as jargon term to refer to how much lower than atmospheric the pressure is or something like that.
    The needed pressure difference depends on the dimensions of what the water is in and how much water is being transferred at once.
    I'll have to go hunt - do you have a reference?
    A quick google shows up stuff like "water rover" spill-proof dog bowls.

    Basically, if you have a tank of water, with a vacuum above it, the water just stays there.
    You have to do something else.
  4. Feb 20, 2013 #3
    Well vacuum does not exert a pressure but the surrounding air does as it always tries to occupy the space which is with a lower pressure or almoust empty of air at all.
    Just like a vacuum cleaner works not by sucking in the dirt instead the motor creates a lower pressure in the apparatus so that the surrounding air with a higher pressure would push in taking the dirt with it.

    The problem in your case is that air is much more lighter and less dense than water, water is heavier , technically you can pull water up a sealed cylinder or container using vacuum only the vacuum has to be really powerful so you will have to create a large pressure difference.
    I had a old vacuum cleaner which I modified so that I could wash my carpet and then just suck the dirty water in but it could suck it slowly if I would move it faster the incoming water would get heavier and the pressure difference created by the spinning motor wouldn't huge enough to keep on sucking it further.
    I believe you could do what you say but the questions is would it be worth it maybe instead of sucking air get a water pump?
  5. Feb 20, 2013 #4
    If I understood correctly you are referring to an equivalent situation to submerging a glass in a tank of water, inverting the glass upside down, and lifting the bottom of the glass above water level? Except in this case you want to evacuate the air after the fact?

    If those presumptions are correct the total force needed is simply equivalent to the weight of the water lifted above the water level of the lower tank 1/2 the maximum height of the second tank, presuming the circumference of the upper tank is constant at all heights. Depending on your purpose there is likely other issues I can only guess. If the psi exceeds atmospheric pressure it also causes problems, but this has more to do with the surface area, or total height of the upper tank, than it does total volume of water.
  6. Feb 20, 2013 #5


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    You need to know the height of the tank.

    As others have said a vacuum doesn't "suck". You are relying on air (or water) pressure outside the tank pusing the water in at the bottom.

    Lets suppose you had a tank that was 2m tall and very nearly full of water, then the pressure at the bottom would be about 0.2 bar (2.48psi) above atmospheric pressure.

    Aside: There is a table here you can use for different height of tank..

    To force more water into the tank at the bottom (eg to actually fill it to 2m) the pressure at the top of the tank would need to be at least 0.2 bar lower than atmospheric. That way the pressure at the bottom is less than atmospheric and water will flow in.

    So in short for a 2m high tank you would need to reduce the pressure at the top to below -0.2 bar (relative to atmospheric pressure).

    If you want to know the absolute pressure at the top of the tank it would be atmospheric pressure - 0.2 bar. Atmospheric pressure is about 1 bar so the absolute pressure would be about 0.8 bar.

    You should check that the glass is strong enough! For example 0.2 bar = 2.48 psi = 357 lbs/square foot
  7. Feb 20, 2013 #6


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    If you have the height of the water in the tank, pressure is just mgh... or 1psi per 2.41 feet.

    I wouldn't be too worried about strength unless the tank is externally braced: if the tank can hold water in, it can hold air out...probably better.
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