What is the reason that MRI needs grade 6 vs grade 4 helium? I'm imagining that the superconducting wire is within a cryostat filled with liquid helium. Doesn't seem like there would be any appreciably partial pressure of things like nitrogen or oxygen at 4 Kelvin. I imagine the reactivity of oxygen is pretty low at 4 K as well. How much dissolved oxygen or nitrogen can liquid helium support? And how much solidifies out and sinks to the bottom of the cryostat?
Turns out that you are right, some balloon gas is 80%. Specifically, the "Balloon Time" tanks you can buy at places like Target say "not less than 80%" helium.
On the other hand, I went to AirGas and a few other suppliers and they seemed to have 95%-97.0% helium gas as their definition for balloon grade.
Definitely worth knowing what you're getting, in any case, so you don't get ripped off, and so you can actually get that lawn chair contraption into the sky.