> Pretty sure it would simply equalize to the external pressure. External partial pressure of helium which is extremely low. > Consider it another way. If you

> Pretty sure it would simply equalize to the external pressure. 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.