Abstract: "Discussion is directed toward eventual optimization of the largest diffusion-pump systems. Less than 100 diffusion pump fluid molecules per cm 2/sec are possible to detect using an accumulation method. Optimization discussed demands highest possible system speed compatible with the above contamination rate. Bakeable oil diffusion pump systems, with equal orifice valve-tap units, without conventional baffling, permit theoretical through-put speeds up to 0.3 of the system orifice. The average backstreaming rate of oil in two unconventionally baffled commercial pumps of 6-in. and 10-in. size is reduced to < 3 x 10-(-4) g/cm-2/24 hr, including heating and cooling the pump boiler. A Ho factor of >0.4 is maintained. Results on trapping with activated alumina, retained by a wire mesh, at both liquid-nitrogen and room temperature are included. A variety of pumping speed measurements and bakeable combination valve-trap units are discussed."