Dave Cooper mentioned gas ballasting of the roughing pump in regards to the water load in a system. I have touched on this lightly on the old BBS, but his comments made me want to elaborate. Many on this list probably don't know what gas ballasting is, or how to do it or why it is done.
Gas ballasts are valves which open the first stage of a two stage pump to expel water vapor created normally at the short beginning of a pump down from air. This is normal as the moisture laden air passes the first stage in the pump and is volatized to vapor. If a pump is unballasted, (as some few models are), or if the user doesn't ballast at the start of pump down, the moisture from the vacuum chamber winds up in the oil!! This can ruin the oil and ultimately damage and ill maintained pump via rust and sludge deposition.
The ballast valve should be opened all the way prior to the pump being activated. Most quick and dirty pump operators cock the valve open until the system is down to 100mtorr This often lasts only about 30 seconds to 1 minute. I tend to leave my ballast wide open until a base pressure is reached on my TC gauge. Often 5 minutes! I then close the ballast and let the pump bottom out from there.
The ballast being open turns your two stage pump into a single stage unit and the base pressure might be about 200 microns or even more. I let the thing pump until the base pressure has been reached for about 3 minutes and then close off. The close off should see a steady and continuous further falling of pressure. If I see pressure fluctuations back upwards after closing the ballast valve, I open it all the way up again as there are still slugs of air or moisture trying to be pumped.
Make sure you acquire a ballasted pump!! Change your oil often, and your pump down times will stay short and the water load via back streaming will stay low.
Richard Hull
Created on Tuesday, February 27, 2001 5:25 PM EDT by Richard Hull