High Vacuum Pumpdown: What’s in Your Chamber and How to Remove It
It may seem a simple matter to hook up a vacuum pump to a vacuum chamber and remove the air. Some days the chamber achieves the desired vacuum pressure in a short time and other days it takes much longer. Why the difference?
First it is important to understand the composition of clean, dry air at atmospheric pressure.
|
Component
|
Symbol
|
Volume
|
|
Nitrogen
|
N2
|
78.084% |
|
(99.998%)
|
|
Oxygen
|
O2
|
20.947% |
|
|
Argon
|
Ar
|
0.934% |
|
|
Carbon Dioxide
|
CO2
|
0.033% |
|
|
Neon
|
Ne
|
18.2 parts per million
|
|
Helium
|
He
|
5.2 parts per million
|
|
Krypton
|
Kr
|
1.1 parts per million
|
|
Sulfur dioxide
|
SO2
|
1.0 parts per million
|
|
Methane
|
CH4
|
2.0 parts per million
|
|
Hydrogen
|
H2
|
0.5 parts per million
|
|
Nitrous Oxide
|
N2O
|
0.5 parts per million
|
|
Xenon
|
Xe
|
0.09 parts per million
|
|
Ozone
|
O3
|
0.07 parts per million
|
|
Nitrogen Dioxide
|
NO2
|
0.02 parts per million
|
|
Iodine
|
I2
|
0.01 parts per million
|
|
Carbon Monoxide
|
CO
|
Trace
|
|
Ammonia
|
NH3
|
Trace
|
From a practical perspective, one need only be concerned about the Nitrogen, Oxygen, Argon, Carbon Dioxide, Neon and Helium in the chamber. Further, for vacuum levels not lower than 1x10-3 Torr, the Neon, Helium and Hydrogen are not significant. There is however a wild card that must be considered: water vapor.
Removing the water vapor from a system is often the largest impediment to reaching a good vacuum. When we begin pumping on a chamber that has been exposed to atmospheric moisture for an extended period of time, it is as if we are pumping on bulk liquid. When the vapor pressure of water is reached (24 torr at room temperature), water molecules begin to desorb from the surface. Water molecules adhere to the walls of the chamber more tightly than they hold on to each other.
Further, water also finds its way into the molecular structure of non metals such as plastics and elastomers. It is this variation in water vapor content that very likely is the difference in pump down times, particularly if the ambient humidity level varies significantly day to day or season to season and your chamber is not in a very well controlled environment.
A few simple precautions can minimize the difference in pump down time from day to day. First, minimize the use of elastomer o-rings and gaskets in the chamber. O-rings can out-gas more water vapor than the surfaces of the chamber walls. Plastic tooling or fixtures inside the chamber will also lengthen the time to pump down the chamber.
Second, the temperature of the chamber will have a dramatic effect on the time to remove the water vapor on the walls. For every 10° C the rate of evaporation approximately doubles, greatly reducing the time to evolve the water off the walls. So if possible, elevate the chamber temperature and you will notice a dramatic decrease in pump down time.
Third, the longer the chamber is open to the atmosphere, the greater the amount of water vapor attached to the chamber walls. Minimize the time the chamber is exposed to the atmosphere by closing the chamber door asap, and if possible leave the chamber under vacuum between experiments.
If you would like help optimizing high vacuum pumpdowns, please contact us at your convenience.
This article was authored by Michael A. Grandinetti (MikeG@iestechsales.com)
IES Technical Sales is a value added technical sales, distribution, and solutions provider serving the high technology vacuum, plasma/thin film, temperature, fluid handling and metrology markets.