Notes from an email:

Tested the chamber with the leak checker.  We got 3.7 x 10^-6 mbar
liters / sec.  I estimate the chamber's leak rate around 1 mbar / 4
hours or so, and I randomly guess its volume is 100 ml.  If I smash
those numbers I get ~10^-5 mbar liters / sec which is a similar order.
I turned off the pumps for the weekend but left the He gas in the
cell.