TESTING OF TURBOMOLECULAR PUMPS

Turbomolecular pumps (TMPs) are used to provide clean pumping to attain pressures down into the 10(-10) Torr range. Some of our data indicated that the pumping speeds and compression ratios claimed by some manufacturers of TMPs were greatly overstated. To measure these parameters, an apparatus was constructed following AVS recommended practices. Hydrogen, helium, nitrogen, and argon were used as test gases, with their flow rates controlled to provide inlet pressures over the range 10(-6)-10(-2) Torr for pumping speed measurements and exhaust pressures over the range 0-1 Torr for compression ratio measurements. Ten pumps from eight manufacturers were tested, all with 6 in. o.d. Conflat flanges at their inlets. Wide variations were found between the pumps, with the specifications of the manufacturers met by only a few pumps. Pumping speeds varied from 0.3 to 1.1 times those specified by the manufacturers. Compression ratios were frequently exaggerated by a factor of 10 or more. As expected, four of the pumps, equipped with coaxial molecular drag pumping sections, had much larger compression ratios than conventional pumps, particularly for the light gases.