Time a properly sealed chamber should hold a high vacuum?

[Jacke]

My chamber tends to lose its vacuum pretty fast when I turn off the pump. About how long should a properly sealed fusor chamber hold a high vacuum when you turn off the pump?

[Jerry Biehler]

How high is it rising? A newly assembled chamber that has never been pumped down or baked out will outgas for a while and when you turn off the pump the pressure will rise.

[Richard Hull]

I assume you seal off the chamber with a valve rather than just turn the pump off. If a mechanical pump only is used it will not hold the maximum vacuum for very long at all. You just can't stop out gassing and virtual leaks for a good while after commissioning the device.
If gets even harder when you valve off the secondary pump to hold a deep vacuum.

Richard Hull

[prestonbarrows]

There won't be a universal answer since it depends on a number of factors.

There will always be a finite gas load on a vacuum chamber. The two main sources are outgassing and permeation.

Outgassing is basically microscopic bits of water or other crud boiling off the surface or sub-surface of your chamber walls or anything inside the chamber. Some materials that are not vacuum rated (plastics and rubbers generally) will boil away themselves and need to be avoided. Outgassing tends to improve the longer your hold vacuum and/or run plasma as the contaminates get flushed out of the system. It also tends to depend on pressure, so will be non-linear in time after you seal off the chamber; initially the pressure rises fast then rolls off to a slower rate over time.

All materials are slightly permeable at the atomic scale and gas will slowly diffuse through the bulk material. Hydrogen is particularly good at doing this. Stuff from the atmosphere side will diffuse through the material and pop out into your vacuum, this happens even with thick metal plates. Rubber o-rings tend to allow a relatively high amount of permeation. This is improved by minimizing the length of o-rings or using metal gaskets. UHV rated rubber o-rings are fine for fusor work. You only have to worry about permeation if you are going for insane vacuum levels for electron microscopes and such.

An outright leak, a physical hole in the vacuum chamber wall, will be in a choked flow mode and allow a constant flow rate into the chamber. This is pretty independent of the pressure inside the chamber so you tend to get a linear pressure rise in time in this case. Looking at the slope of pressure vs. time is a good way to tell if you have a real leak or just a virtual leak from outgassing.

So, depending on the above sources you will have particles entering your chamber at some rate. Depending on the volume of the chamber, this directly relates to a rise in pressure over time.

Attached is a quick whitepaper on related topics.

Just to throw some very ballpark numbers at you, a decent chamber a few liters in volume will take about 15 min to rise 1 mTorr. Again, this can be a few orders of magnitude higher or lower depending on the quality and condition of your vacuum chamber. It is more useful to establish how your system behaves when working well and then notice when it changes rather than trying to directly compare it to anyone else's system.