[lesstutrey]

My guess as to why it doesn't, would be a very low level of entropy. Heat rises, cold sinks, so the gas that comes off the top is still colder than surrounding air, and is thus sinking below the air, and sort of creating a vacuum seal of it's own (like forcing an upside down cup into a aquarium).

My dewar operates at 12psi (the heater on it auto-shuts off once 12 psi is reached, not that i've gotten to use the dang thing) but that's just mine.. they make the same model with a slightly different manifold and it goes to whatever psi you dial in. More manifolds need a feature i didn't even realize i was taking for granted, a fill level indicator. Even after months (possibly years) of storage my dewar still has "something" making it read not quite empty and shoots out gas if i open the vent.