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further explanation of Holohan's demonstration

steamfitter Member Posts: 156
At Holohan's dead men's school seminar (which I have on video), he conducts a quick demonstration using two balloons and a ball valve, explaining how higher pressure goes to lower pressure. He blows up one balloon, attaches it to a pipe connected to one side of the closed ball valve and then blows up another balloon with less breath making the balloon smaller than the first one and connects it to the other side of the ball valve. When he opens the ball valve (much to the audience's suprise) the smaller balloon empties into the larger one.

His explanation is that the smaller balloon had more pressure, that the molecules were closer together and the higher pressure always goes to lower pressure.

Although I think I understand what is happening( having done the demonstration myself several times), I find it difficult to explain to others. The smaller balloon must have the higher pressure, but exactly why? Can anyone help explain in more detail ( and maybe in more scientific terms that can be translated to laymen's terms) how this experiment/demonstration works?

Questions I get are along the lines of:

 Is the same amount of air(breath) going into each balloon?

If you've seen the experiment/demonstration and can explain what is happening in detail I would appreciate it. It's a great demo, but for some reason I can't seem to find the right words to totally explain it. A little help please!

Thanks guys!



  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Never seen that one before...

    And am just guessing here, but my guess would be that the smaller ballon has thicker walls, and is therefore capable of holding a higher pressure even though it is smaller. I am sure that everyone there thought the two balloons would equalize in in size. I would have thought so. That Holohan guy really makes you think some times, don't he ;-)

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • SpeyFitter
    SpeyFitter Member Posts: 422
    Blowing up a Balloon

    You ever notice when you are blowing up a balloon you have to give it a good puff to get it started but once you get to a certain point it begins to get little easier to add air? Now why exactly that is no one knows but I guess the point is sometimes things in hydronics and steam sometimes defy our thinking but once you understand what is happening it makes sense.
    Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,093
    edited July 2012
    Thanks for remembering that!

    The larger balloon has less pressure inside of it because the balloon is stretched. When you stretch a balloon the bond between the molecules weakens and they have less ability to pull themselves back into the original shape. If you remember from the seminar, I explained that this is why we stretch a balloon before blowing it up.

    The point I make at the seminar is that we can understand a principle (high-pressure goes to low-pressure in this case) but not fully understand the components of the system (such as what we find on old steam jobs). If you go thinking that the BIG balloon's air will flow into the SMALLER balloon when I open the valve set between the two balloons you'll never be able to figure out what's wrong with that job.

    I'm glad you enjoyed it and that it still has you thinking. I've done my job.
    Retired and loving it.
  • Hmmmmm..

    Dan:  I thought that's what actually happens: The small balloon empties into the larger balloon, no?
    Often wrong, never in doubt.
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,093
    Thank you, Alan.

    Having just spent a week in Cape May with the entire family my brain has turned to Victorian mush.

    Yes, of course, the air in the small balloon empties into the big balloon and not the other way around (as I wrote). I will now go back to catching up, and this time, monotasking rather than multitasking. ;-)

    Thanks, pal.
    Retired and loving it.
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