A vacuum pumping system includes a primary vacuum pump having an inlet for coupling to a system, and a secondary vacuum pump having an inlet coupled to the exhaust of the primary vacuum pump. By providing the first pressure chamber in combination with the first valve, a space with substantially constant size is provided at each pumping cycle. Lift-off detection can be accomplished by monitoring for a signal that indicates a sustained touch on the touch plate, and correlating a change in such a signal with a lift-off event.