Some of the older vacuum texts have a number of the critical dimensions for a few specific diffusion pumps. One set of material that I have (dating from pre-1950 I believe) has dimensions for a couple or three pumps.
For home-lab use I would recommend a 2-stage 2" diffusion pump. Bigger pumps (4" and up) use lots of heater power and cooling water. Definitely bad news if you are living in California. And, you just don't need anything bigger for a home neutron generator.
That said, I would not recommend going with the u-bild-it approach unless your goal is to make a diffusion pump. 2" diff pumps are fairly available on the surplus (and for Richard, the Hamfest) market. If I wanted another 2" diff pump tomorrow I could spend a couple hundred with a commercial surplus company and get one. If you are willing to wait a bit longer you can certainly get one for much less through other channels. I have long since stopped actively searching for diff pumps but I end up buying one or so a year just because it's not that hard to part with $30 or so for another nifty piece of hardware.
As far as the pump's working condition goes, if the thing hasn't been stomped on and the chimney is complete and intact, you are ok. Heaters can die but replacements aren't that hard to find and in many cases you can improvise a bit.
Diff pumps made in the last quarter century or so do have a number of improvements (thanks to folks like Mars Hablanian of Varian) over the earlier pumps.
So, I heartily recommend building a pump if you want to understand how to build a diff pump. But if your goal is to make vacuum, get a used pump.
As an added note, the small glass oil pumps that were so frequently mentioned in the old AmSci articles (e.g. Hickman single stage up-jet pumps), it's been pretty well proven that these pumps don't really pump. They are really a trap that prevents backstreaming from the mech pump to the chamber. Same as the zeolite or micromaze traps. That's why you can get just about as good a level of vacuum with a molecular sieve trap as you can with a Hickman up-jet pump.
Steve
Created on Friday, March 30, 2001 10:27 AM EDT by Steve Hansen