Aviation: lifting power of helium vs hydrogen

Self-tutoring about airships: the tutor compares the lifting power of helium and hydrogen.

Helium’s atomic mass is 4.0, while hydrogen’s is 1.0. However, hydrogen is diatomic, meaning it will only exist, by itself, as H₂, bringing its mass to 2.0 as a gas particle. Helium stays single: it won’t react with itself or anything else, so its effective mass remains 4.0.

One might think, therefore, that hydrogen, being only half helium’s mass, has twice its lifting power – not true. Apparently, if they’re both pure, helium’s lifting power is about 93% that of hydrogen’s. It’s virtually impossible to get pure gases; impurities included, helium’s effective buoyancy falls to about 88% of hydrogen’s. Since hydrogen is flammable and helium isn’t, helium is the clear choice for filling a balloon you want to rise.

Why, though, is helium so similar to hydrogen’s buoyancy, given its mass is double? I don’t know for sure, but suspect it’s because hydrogen’s molecules are attracted to each other, while helium atoms are not attracted to each other. Therefore, the idea would continue that more hydrogen molecules naturally occupy the same space compared with helium. That doesn’t follow the ideal gas law, but perhaps hydrogen isn’t an ideal gas, while helium is. This is just an idea I have – possibly not true.

BTW: for operational reasons, the practical lifting power of a helium airship might be around 50% that of a hydrogen-filled one of the same size – exactly as one might expect by their masses. However, the reasons are not what I would have expected. About that, as well, I may follow up.

Source:

airships.net