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Effects of different types of vacuum at equal pressure

  1. Aug 23, 2011 #1
    Hi everyobdy.

    First, forgive me for mistakes, since English is not my native language.

    I'm going to ask a kinda strange question, so let me add a short premise. I'm a first grade MD, andrology will be my area of expertise. I have to give an advice about a patient who has Peyronie' disease (penis with a pronounced curve, basically) . This disease has been treaten with chemicals and mechanical therapy; more specifically, I'm going to prescribe a vacuum device.

    One of my fellows argues that water-based pumps will provide a better outcome. His argument is that, for any given level of negative pressure, say 5hg, the expansion in the penis will be certainly more noticeable if the vacuum is generated by water than if generated by air.

    I do understand that, being air not expandable at that pressure (differently than air) the space created, so to speak, by the water which is pulled out, will be filled entirely with the penis; where I differ, is the final outcome, after about 20 minutes : my reasoning is that, slowly (let's suppose the pressure has dropped to 3hg in both scenarios), the density of water will posite a limit to the expansion of the penis; since water is hard to compress, where air is not, after a while the penis will have to fight an higher compressive force if imersed in water than if in a air-generated vacuum.

    I do believe also that the same higher compressive force of water, in hand with the bouyancy, will cause more length expansion in the water vacuum than in air and more girth expansion in air than in water (this is a secondary question, though).

    Hope my question is clear.

    I'd like to hear a very simple explanation and if possible an opinion from somebody whith an operative, professional experience in a related field.

    Thanks to everybody in advance for the help.
  2. jcsd
  3. Aug 23, 2011 #2
    I wouldn't have the slightest idea how to help you with the biological aspect, but generally speaking, if something experiences a pressure gradient then that is independent of the medium in which it is immersed. In other words, if you maintain a negative pressure gradient (pressure in container is lower than outside) then a fluid or gas will want to expand to fill this space, in your case blood.

    As such I don't believe the intermediary fluid matters whatsoever from a pressure standpoint; pressure is pressure. If you picture a water balloon, then that can be our model of the human body. Now you lop off the front part of a cringe so you only have the back plunger part; sealing this up against the balloon and drawing it back would cause the balloon to expand out. If the cringe was full of fluid then you would actually be able to achieve a higher pressure gradient than with air BECAUSE the fluid is incompressible.

    The biggest thing to understand is that there's no such thing as negative pressure. A fluid always pushes against the walls of its container. A vacuum pump works because the air outside pushes against the balloon or the body harder than the air from the pump, thus the balloon or body is pushed into the pump by the outside air.

    So in conclusion, I think the water or other liquid would work better, assuming that getting a larger pressure difference is the goal. Because there is a pressure applied the water should be sucked out as the organ in question expands, so the incompressibility of water is a benefit.

    Edit: and by all means don't take my word for it, try the balloon experiment yourself. You *should* see that drawing the plunger back a given amount causes more deformation if the cringe is liquid filled rather than air filled, due to air being incompressible.
  4. Aug 24, 2011 #3
    Yes I do know that 'negative pressure' is technically not correct Firestorm, I used it like an abbrevation, kinda a slang to save words.

    Thanks for your answer.

    Any other reply?
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