Understanding High Vacuum

High vacuums can be tricky. We're dealing with kinetic theory on a molecular level. One of the more striking facts to me has always been that even at high vacuums there are still millions upon millions of molecules present in just a few cubic feet of volume (in interplanetary space there are about 10 molecules per cubic centimeter). It is a reminder of just how complex our world is.

The process of achieving high vacuums can be difficult for someone new to the subject to grasp. When I first started working with vacuum systems someone gave me this helpful analogy:

Pulling a vacuum is like waiting for mice to find their way out of a maze. When you begin, imagine the maze is packed full of many mice (lets say 100 mice) with no empty space between them. When the exit of the maze is opened, within the first minute the majority (say 70) of the mice have already found their way out simply due to how many mice were in there to begin with. These mice were statistically bound to find the exit. Within the second minute the mice that remain are working harder to find the exit, and only 15 of them find it. The 15 mice that remain after the second minute take between one and three more minutes to find the exit, and some of them may never find it.

The mice in this analogy are obviously gas molecules or atoms. A mouse that is stuck in a corner and takes a long time to find the exit (or doesn't) may represent an oil molecule embedded in an o-ring seal, or a residue on the chamber wall from someone's hand (wear your gloves).

Feel free to share this with any vacuum industry newbies. For more information vacuum chambers as well as a helpful altitude versus pressure table (up to 250,000 ft) visit the Bemco website at www.bemcoinc.com