Just a note for folks using mechanical pumps: O'Hanlon's book on vacuum techinque warns strongly against using an unassisted mechanical pump to try and get below around 15 Pascals (1 Pa = 7.5 microns), as backstreaming of the mechanical pump oil starts to get significant below this pressure. This is the recommended crossover pressure for a mechanical/diffusion pumped system. The point is also made that the crossover should be accomplished quickly. A trap is also recommended to help get rid of backstreaming pump oil vapor. Perhaps one possible way of using a trap to get around the oil backstreaming problem and to assist in the pumpdown without too much wear and tear on the trap is to bypass it for the initial roughing phase to 15 Pa, then do the rest of the pumpdown through the trap. For up-to-air, the trap is isolated with 2 valves as earlier recommended. Unfortunately, this would require three valves, but nothing about ths business is cheap....
I highly recommend the O'Hanlon book(A User's Guide to Vacuum Technology, 2nd Ed., Wiley-Interscience). At first reading, I thought it was a bit too academic, but careful digging reveals a lot of nuggets of information and vacuum-style horse sense.
Created on Wednesday, May 02, 2001 12:32 AM EDT by Richard L. Hester