You sure about that? Everything I've ever heard says that balloon gas is generally grade 4, which is 99.99% pure. Not good enough for MRI, but quite a lot better than 80%.

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Jblx2 an hour ago | parent | next [-]

>99.99% pure. Not good enough for MRI

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?

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pfdietz an hour ago | parent | prev [-]

Economically I expect it wouldn't be that pure, since it doesn't have to be that pure to provide lift, and party balloons are not trying to maximize lift.

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rootusrootus an hour ago | parent [-]

Out of curiosity I did a minor amount of research to get an idea.

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.

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pfdietz an hour ago | parent [-]

Perhaps "balloon grade" here is not "party balloon grade". Weather balloons? Research balloons?

	▲	rootusrootus an hour ago \| parent [-]
		My guess is that places like AirGas aren't really supplying many weather or research balloons. I suspect the easier answer is 'Balloon Time is low grade crap aimed at people who don't know any better and just want to pick up some balloon gas while grocery shopping.' It's like the difference between people who go to a gas station to refill propane tanks, and people who swap them at Home Depot. (though the smart fellers do swap at Home Depot occasionally, if they need a fresher tank...) 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.