On Friday, January 24, 2020 at 1:04:01 PM UTC-5, Mike Coon wrote:

That is if you reduce the pressure by pumping until the temperature
drops to below about 2°A (see
https://en.wikipedia.org/wiki/Superfluid_helium-4). I remember doing
this and seeing the helium stop boiling energetically and go unnaturally
calm. It was still boiling, of course, because heat was still leaking
in...

Helium will leak even in its gaseous state. Metalized Mylar balloons last much longer than latex, but they still fail far more quickly filled with helium than with air. The helium diffuses right through the material.

Peter Wieck
Melrose Park, PA