Traditionally, the roughing pump is a rotary two stage vacuum pump by Welch, Precision, Leybold, etc. Most manufacturers want a minimum of 100 microns of vacuum maintained at the outlet of their diff pump.
Rotary vacuum pumps can be had new for about $600.00 for a small one and the better welch pumps are about $1500.00 fully rebuilt, but used (Duniway Stockroom). I have bought most of mine at hamfests in the $100.00 price range.
Richard Hull
Created on Tuesday, February 13, 2001 5:31 PM EDT by Richard Hull