Helium doesn't have that much lift

What kind of lift would this have compared to hydrogen?

It's not much of a difference. Hydrogen only provides about 8% more lift than helium.

Helium or even hydrogen would be needed in huge quantities to provide any lift where the medium has a density close to air at sea level and with Earthlike gravity, but those are not the only options. You can go two ways with it. One is to consider being on a planet or moon with much lower gravity than here. I worked out that Enceladus, a satellite of Saturn, might be an appropriate environment given an environment with sea level type air pressure and hydrogen was used.

The other way to go would be to have a denser medium. Being underwater is an obvious example but rather less than fun because human bodies float in that anyway. However, particularly dense gases or high pressure outside the body, such as xenon or sulphur hexafluoride in the air, would help swing it. Provided at least 200 millibars of that pressure is provided by oxygen the atmosphere would still be breathable. A combination of low gravity and dense atmosphere would make it less extreme.

So i would say it can be done in special circumstances. The closest you could get to them would probably be on Titan or possibly Venus (though Venus is ridiculously hot!). A malfunction of a hydrogen or helium cylinder on the surface of Venus is a possible scenario, provided the person concerned was wearing a REALLY effective environmental suit which was also very stretchy.

Needs to be a bigger volume. :)

I went through the math a while a go - Helium provides something like 0.03lbs of lift per cubic foot of volume @ STP. To lift myself, and the weight of the envelope, would require a balloon around 30' in diameter (which, if the 0.03 number is correct, would lift 424#).

If one balloon is approximately 1 cu. ft., then to lift one pound would require 34 of them. For 110#, that would be 3,740 balloons. Feeding 3740 into 4/3 * PI * R^3, and solving for R comes up with a radius of 9.6 feet, or a balloon 19 feet in diameter.

I fully admit my math may be faulty, and those numbers might be completely off base, so YMMV.

Kryslin

Interesting find, BlankSlate!

I can imagine now how big the inflation subject would have to be to be able to fly. And thinking about it makes me feel all warm and fuzzy inside.

Unfortunately air pressure will be proportionally (to g) less there. Then same volume of helium will be needed.

By the way:

Helium is 4g per mole, while air is 29g per mole. Difference is 25g per mole. One mole of gas takes 22.4 liters of volume in normal atmospheric pressure.
If we are going to lift a 50kg girl (rather skinny), it will take 2000 moles or 44800 liters, or 44.8 cubic meters of helium. Thats sphere with radius 2.2 meters (or aprox 7 feet).

I imagined an enclosed habitat containing a breathable atmosphere, which is why i didn't consider the density of the real atmosphere in those places except for Venus and Titan.

A human filled with gas won't have that gas at the same molarity as the surrounding atmosphere. It will be under pressure. If the person in question weighed fifty kilos and also had extremely baggy skin or internal organs initially, they could be filled with a lighter than air gas at comparable pressure to the surroundings. The same would, interestingly, apply to a large, baggy garment, though it would have to be huge for that to make a difference.

However, we are elastic and our organs, body walls or skins therefore applies pressure to the gas inside us. This means that any inflating gas will be under pressure pretty soon, before it causes any distension. Incidentally, that also means it would cut off the blood supply after a certain point, somewhat above atmospheric pressure and further above it than physiologically due to adrenalin. When Denise Nickerson wore the inflation suit in 'Willy Wonka', she had to be shifted about because it was cutting off her circulation.

Thanks for that. I knew about the two separate costumes but not that much detail. It's very interesting. So was it the polystyrene one which was weighted or the inflatable suit?

I've thought about this rather a lot!

I still think inflation would impair blood flow. Suppose that with the adrenalin the systolic blood pressure is 250 mm Hg. That's equivalent to 1.3 bars. That means that once pressure within the organs or body cavities got beyond 1.3 bars, which isn't very high, the heart wouldn't be able to push the blood through the arteries to the rest of the body. So you would first of all get a throbbing sensation, then pain from poor blood supply, then tingling like pins and needles when an arm has been slept on, then numbness as the blood supply gets completely cut off. That would also mean the musculature would relax, because the ability to release energy to maintain muscle tone would be lost. That relaxation would lower the pressure slightly and maybe allow blood to flow again, so i think there'd then be a relaxation allowing more inflation, then a stage of tension, then more relaxation and so on. Eventually the pressure would just be too high all the time.

Oh, good question, i never noticed that before. Isn't it amazing how you can watch something hundreds of times without noticing a detail like that?

It's pleated but looks to me like it's got something quite thick underneath it. It's not like it's just a binliner and i think it would've been quite elastic and resilient just by the look of it.

Completely agree with the blood pressure thing, which brings up another point: would the pressure force blood into the extremities, for instance the genitals?

I completely agree. All ordinary matter has weight. Hydrogen and helium are simply the lightest forms of matter. Air is heavier than them. This would only work if physics were Aristotelian, with certain substances exhibiting levity and others having gravity. There are ways round it but there's no way it could happen inside a normal human body in normal conditions.