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Bouyancy of a contained helium gas ?


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A helium filled mylar balloon, loses pressure as the small He atoms sieves trough any envelope, as far as I understand.  Sent aloft, when the He pressure inside the balloon equals the atmospheric, the balloon stops rising, as far as I understand.

At certain altitude the differential pressures of gas filling the balloon and the atmospheric outside its envelope, nearly equalizes, reducing the He leak.  But there is still some weight of the balloon.  Do helium balloons last much, much longer aloft at high altitudes than at low altitudes or there is a factor am not considering ?

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Considering temperature at very high altitudes, affecting density.  How does it work ?

And other than helium, its convenience or not for 'ballooning' use is not only an atom/molecule size but its specific gravity in function of temperature too, right ?  So a larger molecule that wont leak trough the envelope + low density at low temperature is convenient ?

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  • 3 weeks later...

Yes, your understanding of how helium-filled balloons work is mostly correct. As helium atoms are smaller than the molecules in air, they can slowly leak through the walls of the balloon. This results in the balloon losing some of its lifting power over time. When the pressure of the helium inside the balloon is equal to the pressure of the surrounding air, the balloon stops rising and reaches a state of equilibrium.

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On 2/18/2023 at 6:28 PM, Externet said:

Considering temperature at very high altitudes, affecting density.  How does it work ?

To a very good approximation, the effect of temperature  on the density of air and helium is the same (in percentage terms).

The balloon floats because its average density (including the walls of the balloon)is less than that of the air.
As the balloon rises the air gets less dense.
There comes a point where the density of the balloon and the surrounding air are the same and, at that level, it stops rising.

It's made slightly more complicated by the fact that the balloon changes volume (and thus density) as it rises.|
With mylar balloons, once the envelope is "full", the volume stays pretty much constant (until they burst).

High altitude balloons are launched looking rather empty, to allow for expansion as they rise.


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