The regime where the mean free path and the duct size are of the same order, Knudsen number roughly 0.01 to 0.5, which in typical lab plumbing falls in the medium vacuum range. Neither the fluid equations nor the molecular formulas fit cleanly, so engineers use interpolation formulas or design for the worse case. It is also called Knudsen flow.
Throttle valve sits right in transitional flow at our process pressure, so its conductance curve is nowhere near linear.
Also heard as
- Knudsen flow
- intermediate 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.
Viscous flow
Flow regime in which molecules collide mostly with each other, so the gas moves as a continuous fluid.
Molecular flow
Flow regime in which molecules bounce between the walls almost without meeting each other.
Medium vacuum
The band between about 1 and 10⁻³ mbar, where gas flow shifts from fluid-like to molecule-by-molecule.