Vacuum Chamber Pressure Sustainability

If you're talking about a perfectly sealed chamber there will not be any pressure decay / increase due to leakage....

There will be pressure changes due to temperature changes as well as volumetric changes....

John.

Hi

Vacuum can sustain as long as there is no way for atoms of molecules to enter the vacuum. Molecules of gas trapped in the inner walls of the chamber keep coming out from the wall slowly in time and also with greater temperatures. For all materials, whatever it may be it has a vapor pressure and evaporates at some rate and those solids also go into vapor phase to some extent. Then there is a leak from joints/seals.

Generally degasing is done in vacuum chambers by heating the chamber for several days and romoving the atoms and molecules released in the chamber using Ionic pumps and Helium cold traps. This makes things better. For remaining atoms and molecules to be trapped, a special electrode is evaporated inside the Photo Multiplier tubes to get to 10^-11 torr which can remain up to 10^-10 torr for about 10 years if it is a good work. More is the surface area of the closing lid, greater is the leak into the chamber. Hence small opening if closed properly then it may go for more than 10 years, if degas and surface trapped atoms and molecules are shucked out before sealing.

Material of the chamber may also play greater role. Hence, it should not start evaporating.

I agree with John that temperature plays a great role. In cold trap, molecules can not move and will fall at the bottom because of zero velocity and no kinetic energy. Also by heating, trapped atoms and molecules will come out and will become free. Heating is to be a past of the vacuum process before closing the chamber.

In vacuum chambers you have these virtual leaks :

outgassing of wall chamber

permeation of seals

outgassing of process.

Excluding welding leaks ,you need the adeguate pumping speed to equilibrate the above outgassing sources.

It will also depend on your seal materials, copper gasket or gold wire seals are pretty permanent unless disturbed but any elastomer seal will always have a leak rate. Many systems that maintain vacuum for extended periods will use a small ion pump to scavange out any outgassing.

I think good example is Photomultiplier tube 10^-11 torr and that is built to last many years. I have also used the reversed process, high pressure (25 atm) ionization chamber and these two designs last for nearly 10-20 years. Good enough for lots of work. Both of these technology rely on small opening which is sealed properly. There is glass to metal or ceramic to metal joints also in these and they don't affect pressure much.

Double wall mechanism will also work much better. Leak is pressure difference dependent hence if difference in pressure is kept to near zero then there will be almost no leak.