kf2qd:
Most likely leakage. The Helium Molecule is larger than the Hydrogen molecule so it leaks less. Pump any other gas into a container and it stays there. Pump hydrogen in and if there is ANY thought of a crack it will all leak out.Another possibility is the amount of expansion of hydrogen over helium.
And for the volumn it might just be too dangerous...
Hyrogen will burn in almost any mixture with oxygen. Some gases can actually self smother because they need a critical mixture to burn.
I mentioned the permeation of hydrogen in my original post. I know that is a problem with hydrogen gas. It's also a problem with helium. I wonder which is worse. Hydrogen gas is diatomic. Helium is monoatomic. The ratio of molecular weights is slightly less than 1:2 not 1:4. I wonder what the molecular sizes are.