Sorry for the delay...
So after playing with Ni-H I've achieved condition when behavior that I described at the first post I can confirm that this is happening also with high purity Alumina tubes.
This mean that pressure is going to a negative values too.
How can you explain this?
Normally I use SS tube AISI 316. Fully filled with powder -- at the end. Yes, it is touching the wall.
All materials are from Alfa Aesar. Nickel is around 80 - 150um in size.