Complete High Vacuum Time Table

Hi Everyone,

I did not really want to do two high vacuum topics in a row, but I had the time to do some extended testing on a high vacuum chamber last week, that I've always wanted to do, so I thought I would share...

First, a very brief intro to cryo-pumping as some may not be familiar. A mechanical (rotary-vane style) vacuum pump is limited to a vacuum level of between 0.5 mTorr and 50 mTorr. The oil and other molecules given off by the moving parts of the pump actually "back stream" into the chamber preventing it from reaching vacuums higher (lower pressure) than this range. To get to higher vacuums, a different technology must be used. A cryo-pump is nothing more than an extremely cold surface (just 15C above absolute zero) that will liquefy just about anything that comes in contact with it. When molecules liquefy on this surface, they stick to it, effectively removing it from the chamber volume, reducing the chamber pressure, taking it to higher vacuum.

In the graph above (click on it for a larger version), the chamber and cryo-pump pressures are shown on a logarithmic scale in milliTorr (on the left), and the cryo-pump temperature is shown in degrees Kelvin on a linear scale (on the right). It may seem confusing, but it is the only way to see everything all on the same graph.

At the start of the test, we see that both the chamber and cryo-pump are at atmospheric pressure (760 Torr), and the cryo-pump is at room temp (300K). First we need to "rough out" the cryo-pump, with our mechanical rotary-vane pump. If we started cooling the cryo-pump down at atmosphere, it would become over loaded with liquid water and air very quickly. The cryo-pump cavity is small and takes only 6 minutes to reach 16 mTorr. At this point we can begin to "rough out" the chamber while we start cooling the cryo-pump down.

The chamber itself is considerably larger than the cryo-pump cavity and takes about 30 minutes to reach 16 mTorr. At this time our cryo-pump is only at 223K (-58F), and we'll have to wait about an 1-1/2 hr for it to reach 15K, which is when we can open the gate valve isolating the cryo-pump from the chamber.

At about an hour into the test, we see that the cryo-pump temperature of 181K (-134F) is beginning to take the cryo-pump to vacuums beyond the capability of the mechanical roughing pump (below 16mTorr). Ten minutes later we are beyond the range of the convectron gauge and can no longer measure the vacuum level in the pump.

We finally reach 15K at 2 hours and 14 minutes into the test and we can finally expose the chamber to the cryo-pump. Starting almost immediately after the gate valve is opened, there is a period of a few minutes that we are out of range of both the convectron and ionization gauges. The ionization gauge starts reading the chamber pressure at 6 x 10-6 Torr, and the chamber pressure continues to decay to the -7 scale in approximately 40 minutes.

Unless otherwise requested by our customer, the majority of Bemco's high vacuum chambers will behave in this fashion. Although the overall time scale will depend on the chamber's size and the cryo-pumping speeds. For more information on our high vacuum systems visit our website at www.bemcoinc.com

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