The regime at relatively high pressure, where the mean free path is much shorter than the duct (Knudsen number below about 0.01). Gas behaves like a fluid, momentum passes from molecule to molecule, and flow can be laminar or turbulent; in the first seconds of a pump-down it can even choke at sonic speed in a valve orifice. Conductance in this regime rises with average pressure, so a line that looks generous at 100 mbar becomes restrictive at 0.01 mbar.
Up near atmosphere we're in viscous flow, so crack the roughing valve slowly or you'll blow every particle off the walls.
Also heard as
- continuum flow
Related terms
Knudsen number
Ratio of the mean free path to a characteristic size such as pipe diameter, used to name the flow regime.
Transitional flow
Intermediate flow regime between viscous and molecular, where wall and intermolecular collisions both matter.
Molecular flow
Flow regime in which molecules bounce between the walls almost without meeting each other.
Soft pump
Slow, throttled roughing from atmosphere that avoids stirring up particles and condensing moisture.