If process conditions permit I would first look at an O-ring in lieu of a gasket. One caveat in using a O-ring is no to push the temperature limits of the selected material.
If conditions exist that prohibits the use of an O-ring I would look at the Helicoflex line of gaskets especially their Delta gasket. These gaskets generally require far less gasket seating stress than the normal gaskets
2 torr is nothing. I would use a gasket. Gaskets are cheap, custom fit to the application, and easy to use and design around. For a vessel that large, o-rings would be difficult to purchase and the vessel would require special machining using gigantic machine tools. For a pressure that minuscule, just pick out a material with the right permeability characteristics and design a simple way to clamp the joint shut. The force on the interface is only about 40 lbs.
Don,
I'm guessing your 40lb figure came from assuming that 2 torr means 2 torr below atmospheric. I'm about 98% sure that the OP means 2 torr absolute. That makes for about 30. Thousand.
Perhaps you're right. I don't often use torr for vacuum. Generally vacuum is expressed in a negative fashion, e.g. a "25 inHg vacuum" means 25 inches of mercury below atmospheric, not 25 inches absolute. 2 torr would make more sense to mean absolute.
"Normal" expression of vacuum depends a lot on your context. If you work around HV/UHV equipment, pressure is usually expressed as absolute, especially when you get into the single digits of pascals. Daily barometric variations are greater than these pressures, so expressing them as a "negative" value doesn't really make sense.
I used to work in the refrigeration industry and "inches of vacuum" always means negative inches of mercury, below atmospheric. That's how it is verbally communicated and that's how the gauges are numbered:
