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Related to this previous question: Could a city be built out of Balloons?

Remember in Oz where the witch is riding in on a bubble. enter image description here I began to think how could I make this as real as possible.

You can breathe helium in place of nitrogen to have an 85% helium to 15% oxygen ratio. Could a person be put inside a clear balloon with enough buoyancy to ride in a bubble?

Would there be any practicle use for this?

enter image description here

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    $\begingroup$ so basically a blimp with added oxygen. $\endgroup$
    – John
    Apr 23, 2018 at 0:58
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    $\begingroup$ Question doesn't appear to have anything to do with World Building per se. This is basically a math / physics question and might best be moved to one of those forums. $\endgroup$
    – elemtilas
    Apr 23, 2018 at 1:11
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    $\begingroup$ @elemtilas OP is often banned from physics, as is currently the case, which may be one reason for the question being posted here instead. $\endgroup$
    – pipe
    Apr 23, 2018 at 11:26
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    $\begingroup$ @pipe I really don't see how the OP being banned from another SE forum makes it okay to ask a purely physics / science question here. Might as well try the luck over on SE.Music! Also, one wonders why the OP is banned from SE.Physics in the first place... $\endgroup$
    – elemtilas
    Apr 23, 2018 at 23:21
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    $\begingroup$ It sounds like the only objection to this question is that it isn't connected enough to WB. I don't see that as a problem. It's as connected as many other (still open) questions about what could or could not happen in a particular world, real or not. The edit is a lot clearer as well. I VTR. $\endgroup$
    – Cyn
    Apr 23, 2019 at 2:18

1 Answer 1

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A cubic meter of air weighs about 1.2kg. A cubic meter of 15% O2 and 85% He would weigh about 20% of that or about .25 kg. so a cubic meter of the O2/He mixture would lift about 1kg.

Ignoring the weight of the bubble itself, a 70kg person would need a 70 m3 bubble, which would be about 5 meters in diameter. Since the bubble material would have to be pretty strong to support an adult standing on it, let's call it as needing a 6 meter diameter bubble to also lift the bubble's weight.

Note: While floating in a bubble will attract media attention, it would be most unkind to ask the floatee to do interviews...

According to hosstuffworks:

The average adult at rest inhales and exhales something like 7 or 8 liters (about one-fourth of a cubic foot) of air per minute. That totals something like 11,000 liters of air (388 cubic feet) in a day.

The air that is inhaled is about 20-percent oxygen, and the air that is exhaled is about 15-percent oxygen, so about 5-percent of the volume of air is consumed in each breath and converted to carbon dioxide. Therefore, a human being uses about 550 liters of pure oxygen (19 cubic feet) per day.

So a human needs about 1 cubic meter of pure oxygen/day or about 5 cubic meters of the O2/He mixture. This website says that people can comfortably breath air that is 25% depleted of O2 (equivalent to 10,000 feet which by experience is no particular problem for healthy people.)

Given that, an adult needs about 20 cubic meters of the O2/He mixture/day so as to not deplete it below 75% of its original oxygen. The 6 meter sphere has about 120 cubic meters of the gas mixture, so it will last about six days without any difficulty and would probably work for double that in a pinch.

Hydrogen works the same as helium within the accuracy of these calculations, but a no smoking policy is advised.

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    $\begingroup$ Build your "bubble" from a selectively permeable material which allows carbon dioxide to pass one way, and oxygen to pass the other way, while not allowing hydrogen to pass the same direction as carbon dioxide. $\endgroup$
    – Bob Jarvis - Слава Україні
    Apr 23, 2018 at 2:41
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    $\begingroup$ @BobJarvis : that won't work because hydrogen is much smaller than carbon and oxygen. $\endgroup$
    – vsz
    Apr 23, 2018 at 4:06
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    $\begingroup$ @jamesqf : then it won't just be a thin and light membrane, but a heavy and complex machinery. Or do you know of any membrane which can do this? $\endgroup$
    – vsz
    Apr 23, 2018 at 6:06
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    $\begingroup$ @vsz just add a small chemical scrubber that captures CO2. No need for heavy or complex. $\endgroup$
    – tucuxi
    Apr 23, 2018 at 9:16
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    $\begingroup$ @vsz: this is perfectly simple to handle. You use a microcrystalline honeycomb of handwavium filled with liquid unobtanium. Not ONLY does this handle the "selectively permeable" thing, but it ALSO sharpens razors, causes hair to grow WHERE YOU WANT IT, cures warts, makes you smarter, better looking, taller (or shorter - your option), AND if you ACT NOW, it comes with a LIFETIME SUPPLY of GINSU-KNIFIUM!!! AS SEEN ON TV!!!!! :-) $\endgroup$
    – Bob Jarvis - Слава Україні
    Apr 23, 2018 at 11:09

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