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Re: Re; diffusion pumps
For many years I used a pyrex diff pump backed with a direct drive 300 lpm
rotary vane pump. For a while we had access to a cold cathode gauge. The
best reading (taken on the maniflod body) was just cracking 10 to the minus 6
power after pulling down all night (actually I leave all my pumps on all of
the time except for the newer metal diff pump). This is pretty damn low.
What I found most interesting was that when pumping two 6' units at once,
we measured the vacuum (same place on the manifold) at mid 10 to the minus
5s. I was very suprised that these two numbers were so close. The glass pump
is speced at about 8 lps. As long as we don't do anything stupid all units
come off beautiful. About a year ago, we installed a second mech pump,
second main, and a stainless diff pump rated at 50 lps. Although we no
longer have that cold cathode gauge I cannot visually tell any difference in
the units regardless of thier length, diameter, or religious affiliation.
The only case where I see a difference is when we are out gassing some piece
of equipment for those guys that own my shop building.
In short, I do believe there some point of decreasing returns.
Morgan