Performance of mechanical vacuum pumps in the molecular flow range

Abstract

An important and remarkable property of oil-sealed rotary vane mechanical vacuum pumps has been demonstrated. Such pumps retain their positive displacement pumping mechanism at partial pressures approaching the ultrahigh vacuum region. Pumping speed measurements for helium, based on partial pressure, have been made at an inlet pressure as low as 10−6 Pa with an ultimate partial pressure of helium near 10−8 Pa. Experiments conducted with a nominal 3 l/s two-stage pump (Varian 200) demonstrate that a constant pumping speed is maintained at very low helium concentrations as long as the presence of helium at the discharge of the pump is minimal, i.e., as long as the maximum compression ratio for helium is not exceeded. Similar results may be expected with other gases except the pump oil vapor or species outgassing from the pump.