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Pressure difference in a hydraulic system?

  1. Mar 17, 2005 #1
    Is there any difference in pressure in a pressurized tank/piston filled with a liquid between the bottom of the tank/piston and the top of the tank? My problem is that Pascal’s law states that the pressure exerted on an enclosed fluid is transmitted undiminished throughout the fluid and acts equally in all directions. But this law is not taking in account the pressure exerted by the fluid itself (density * Gravity * Height) (Bernoulli). I believe that the pressure exerted in the bottom of the fluid should be grater than the pressure exerted at the top. Maybe the difference is negligible, but the fact is that it is there. Can someone help me with this.

  2. jcsd
  3. Mar 17, 2005 #2


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    You are correct. That being said, you are correct that in most cases that difference is negligible, especially in hydraulic systems. If we are talking hydraulic systems, then another thing to consider is that the reservoirs in a hydraulic system are usually, relatively small. That would give you a tank size that would result in a pressure variation from top to bottom due solely to [tex]\rho gh[/tex] that is extremely small. Consider that with the fact that usually an operating system pressure is in the range of 1000-3000 psi and you can see that the number truly is not worth considering.

    Is there something else about this that really bugs you?
  4. Mar 17, 2005 #3
    Thank for your reply. All this discussin began when I read this page: http://www.fraserker.com/heli/uniflow/how_uniflow_works.htm

    In my opinion all the theories used to justify the use of this product are wrong. Please read it and comment.

    As I see it, it doesn't matter if the return from the muffler is at the bottom or at the top of the tank, the pressure of the fuel exiting the tank will be the pressure supplied by the muffler + the pressure exerted by the fluid itself. I'm planning to write to these guys end tell them how wrong they are but I need a second, expert opinion.

    Thanks for your help.
  5. Mar 18, 2005 #4


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    I read the page and the ideas are sound. You can prove them to yourself at any time pretty easily. I must admit, when I see the words "hydraulic system" I automatically think of power systems where you pull off a vented tank at the bottom and return at the top. Since this guy likes to think in terms of head, pressure in terms of feet or inches, then just look at the tank set up. He is pulling off the bottom of the tank, but the exit lines for the tank are at the top. That means that the pressure acting at the bottom of the tank is rho*g*h, but once the fuel travels back to the top of the tube, that part of the pressure head is gone. By sealing the tank (removing the vent) he is making sure that the pressure differential pushing the fuel to the engine is the pressure of the muffler output minus the pressure at the carbuerator. However, that is for the situation of normal flight. In the event of inverted flight, the fuel is now being pulled off the bottom of the tank and the outlet of the tank is at the bottom as well. In that case there is a VERY SLIGHT increase in pressure due to the fuel weight.

    If we used alcohol as a rough approximation for the fuel, it has a specific gravity of about .8. That means that it's density is 80% that of water, which is 1.94 sl/ft^3. Using an approximate height of 2 inches for the height of fuel in a small tank, that equates to a gravity pressure head of .072 psi in addition to the pressure supplied by the engine. I'm not quite sure if that is enough to squabble about.
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