Re[2]: Gas Ballasting
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If you keep on hand a drying cartridge of some type of dessicant - Drierite, Silica Gel, Mole Sieve, etc.- just vent your chamber through it. Or, if the chamber is going to be open for a while, wait till you are ready to pump down, and then rough it to say below 0.5 Torr, and then vent the chamber through the drying cartridge. Leave it at atm pressure filled with dry air for as long as you can, and then pump down. If you can warm the chamber while it is filled with dry air (or, dry Nitrogen) you will desorb more rapidly. The concept is this: A dry gas, at 1 ATM pressure, bombards the walls many more times per second than the residual gas molecules do at low pressure. The gas molecules colliding with the walls, mechanically remove the adsorbed water molecules. A second or even a third flush will remove still more. What could be an all day process, will be reduced to a couple hrs.

A note on gas ballasting, I have an old Welch pump that does not have a gas ballast valve. So I flush the entire bell jar and pump, using a Mathiesen Hydrox unit to remove the O2,Water and Hydrocarbons. I got the unit surplus for about $10. With a needle valve to control the gas bleed, it works wonders at drying the pump oil. With a 12" glass bell jar, I can usually rough to about 10 microns in about 5 mins, and if the diff pump is on and ready to pump, reach the low minus 6 torr range in about 20 - 30 mins.

Dave Cooper



Created on Thursday, March 01, 2001 2:43 AM EDT by David Cooper