I've seen similar advice in the Veeco operating manual for a diff pumped evap chamber. They recommended installing a small diameter( ID ~ 1/4" )valved transfer line from the diffusion pump to the large chamber - in parallel with the main HiVac valve. In the preferred mode of operation, the roughing pump and diff pump are started at the same time. The chamber is roughed to about 100 microns and valved off, while the diffusion gets going. When it is operational, and before the main HiVac valve is to be opened, the valve on the transfer line is opened, allowing the diff pump to begin pulling the chamber down below 100 microns pressure.
The small diameter allows for a quite large pressure difference to develop from the diff pump to the chamber. The diff pump stays at or below 1 micron which keeps backstreaming from it in hand, and the mech pump stays at or above 100 microns while the diff pumps completes the roughing. Once the pressure in the large chamber gets in the range of 1 micron or less, the HiVac valve can be opened.
I have never actually used this exact setup, but the manual claims it allows faster pumpdown, and lower overall backstreaming.
Dave Cooper
Created on Sunday, May 06, 2001 1:41 AM EDT by David Cooper