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Thread: Helium balloon deflated in hot truck

  1. #1 Helium balloon deflated in hot truck 
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    My daughter bought a Helium balloon and put it protected in the trunk, it was a hot day.

    When she had driven about 90 mins, she got it out and it had deflated significantly. She said "it's deflated because it's too hot"

    Her younger sister and a friend burst out laughing at her and said "everyone knows gases expand when heated"

    I intervened and said (in Mom-mode) something like "well yes generally, but this has gone down because Helium has escaped through the balloon's skin, this always happens and they always go down eventually but in the trunk it would have been so hot that the atoms were moving much faster, hitting the skin at a higher rate and with greater force, therefore would escape at a much higher rate"

    Younger daughter and friend were not convinced but my older daughter, rather uncharacteristically, agreed with what I'd said straight away, as it stopped their laughter.

    However I would like a second opinion to know if I was right.
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  2. #2  
    KJW
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    I don't think so. If we assume the daytime temperature is about 25C and the temperature inside the trunk is about 50C, then it may seem that the trunk is much hotter than the outside air, but one really needs to consider the absolute temperatures and not the degrees Celsius or degrees Fahrenheit temperature. Thus, 325K is not really that much hotter than 300K, and thus the helium atoms will not be travelling much faster in the trunk compared to outside. I'm inclined to think that the reason for the much greater deflation rate is due to the change in the balloon rubber rather than the helium itself, though the small increase in the speed of the helium atoms will no doubt have some effect even if it's not the full effect.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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  3. #3  
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    Thank you, I agree the temperature increase couldn't have accounted for all the deflation. Not sure what change in the balloon rubber you think attributed to the deflation?
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  4. #4  
    KJW
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    Quote Originally Posted by Chrissy52 View Post
    Not sure what change in the balloon rubber you think attributed to the deflation?
    I was thinking the elevated temperature would cause the stretched rubber to become softer and more porous, especially to the rather small helium atoms.
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  5. #5  
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    I'm not sure about that, doesn't rubber contract when heated so the 'pores' would get smaller. However I was thinking that as the gas inside gets warmer, it expands and so the balloon rubber is more stretched and the 'pores' larger to begin with - then more helium would escape until the ballon got smaller due to loss of helium and the rate of loss would decrease.

    Or maybe it's simply that there was a small puncture in the balloon?
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  6. #6  
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    Quote Originally Posted by Chrissy52 View Post
    I'm not sure about that, doesn't rubber contract when heated so the 'pores' would get smaller.
    Rubber (at least, ordinary, natural rubber) has a large-ish positive temperature coefficient of expansion (I just looked it up -- it's about 80ppm/K), and thus expands when heated, just as most materials do. So, one would expect the porosity to increase with temperature. Now, whether that by itself suffices to explain the observations is another thing (and think) altogether. A naive model would predict a linear expansion of about 0.1% for a 12-degree C rise in temperature. The area of the pores would thus increase 0.2%.

    This calculation neglects the fact that the balloon is filled with a gas under pressure, but it gives us a rough idea of the contribution by this postulated mechanism.

    However I was thinking that as the gas inside gets warmer, it expands and so the balloon rubber is more stretched and the 'pores' larger to begin with - then more helium would escape until the ballon got smaller due to loss of helium and the rate of loss would decrease.
    The amount of expansion is small, so one would not expect a large effect, but it certainly compounds things. Note that, as some gas escapes, the leakage rate associated with that postulated mechanism diminishes, and thus the leakage self-limits.

    Or maybe it's simply that there was a small puncture in the balloon?
    Given the foregoing, that's not looking too bad as a hypothesis!
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  7. #7  
    KJW
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    Quote Originally Posted by Chrissy52 View Post
    I was thinking that as the gas inside gets warmer, it expands and so the balloon rubber is more stretched and the 'pores' larger to begin with - then more helium would escape
    This would certainly contribute to the faster rate of escape. Actually, not only would the diameter of the pores increase due to the increased surface area, but the length of the pores would also decrease due to the thinning of the rubber. Individually, both of these would act to increase the rate of escape of helium from the balloon.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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