I am trying to design a pressure vessel for testing purposes that is 12 inches long and must have an inner diameter of 1.5 inches. 1) What is the minimum thickness the walls can be in order to attain "vacuum" pressure (10E-6 torr)? I want to make the walls as thin as possible and still be able to pull a vacuum. Is there an equation for this somewhere? 2) Sealing of the pressure vessel -- I would like to use O-rings to seal the end caps. I need guidelines for O-ring design, since I am entirely inexperienced in this design area. Anyone have a good website for a crash course on O-rings or can give me some general advice on design? I can certainly provide more information if you need it. Thanks!