...
And besides, that pump that can pump so very much won't even work under viscous flow; it might even break. (too much detail?)
different pumps for different pressures:
...
A vacuum chamber filled with air at room pressure is to be pumped down into really low pressures:
Turn on the first pump, it pulls air and the pressure drops, through viscous flow and a bit into molecular flow, he pressure drop slows and then stops. The pump can't remove molecules any faster than they're leaking into the chamber. Turn on the second pump. It could probably viscous flow as it starts up, but as it's blades spin faster and faster (think jet engine)
Pressure, revisited: this time it's chambers.
...