Would most of the helium exit, until it was balanced with just the partial pressure of helium in the atmosphere? That would be nearly a vacuum, wouldn't it?
Consider it another way. If you have such a device with a vacuum inside, would it not pull in the external helium over time to reduce the vacuum?
External partial pressure of helium which is extremely low.
> Consider it another way. If you have such a device with a vacuum inside, would it not pull in the external helium over time to reduce the vacuum?
It would, but only until the partial pressure of helium inside is equal to the partial pressure of helium outside (assuming the membrane is permeable only to helium). After that point the same amount of helium will traverse both ways, establishing the equilibrium.
Since this hollow core would be much larger than the crystalline structures in a metal would this not therefore be a very weak comparison?
Ignoring the fact that what you're describing sounds suspiciously like a Maxwell's demon, I think the equilibrium would be at a higher pressure because helium escaping against an overall pressure gradient would be doing work.
In essence, at the boundary I think the rate at which helium escapes would not simply be proportional to the gradient created by the internal pressure and the exterior partial pressure, but I think would include a term involving the whole exterior pressure.
The difference in this situation will be very small in my opinion, broadly because the gas molecules in the atmosphere still have comparatively high mean free paths and therefore won’t interact with the “escaping” helium molecules.
I don't know the answer, but this does make me think of atomic sieves like the ones used in oxygen concentrators.
This is an armchair scientist explanation of them, but they "concentrate" oxygen by first having atmospheric air pumped in and then pressurized. The microsieves have holes so small that mostly only oxygen can fit through, the larger CO2 and Nitrogen atoms simply won't fit.
Then, pressure is let off and fresh air brought in. The fresh air scrubs out the oxygen depleted air and refreshes it with standard air.
Then, the pressure decrease allows the oxygen to leak back out of the sieves, leaving you with oxygen enriched air.
I don't know if there are any atomic helium sieves, but if you can find one it might be a start to testing the question.
For what it's worth, this is done industrially to separate gasses using porous membranes.