The pumpdown proceeds at a modest rate until about 50-100 torr, then it accelerates
dramatically as the turbo pump becomes effective at the lower pressures.
The transition in behavior in the range 50-100 torr is expected because the turbo
pump is ineffective until the mech pump reduces reaction chamber and turbo pump
pressures from the fluid (viscous) flow regime to the molecular flow regime. As
pressure drops below about 100 torr, the turbo gains speed and begins to contribute
noticeably to the overall pumping capability of the turbo pump system; and as it
helps pump down the system, it also reduces its own inlet pressure, and thereby
becomes even more effective in pumping.
The rapid onset of effective turbo pumping is accompanied by a brief increase in mech
pump pressure, followed by a decrease to mech pump base pressure. This
transient increase in mech pump pressure is associated with the onset of turbo
pumping, which delivers a large gas load from the turbo inlet port to its exhaust port.