The low-pressure regime where the mean free path exceeds the duct size, Knudsen number above about 0.5. Each molecule travels independently in straight lines between wall hits and leaves each wall in a random direction, so there is no suction and a pump only removes what wanders into its inlet. Conductance no longer depends on pressure: an orifice passes about 11.6 L/s per cm² of area for air at 20 °C, and long, narrow or bent lines become severe bottlenecks.
We're deep in molecular flow, so that 90-degree elbow in front of the turbo is costing us half the speed.
In molecular flow a pump cannot pull gas toward it, so the speed you get at the chamber depends mostly on how wide and short the path to the pump is.
Also heard as
- free molecular 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.
Conductance
How easily a pipe, orifice or valve lets gas through, expressed in litres per second.
Effective pumping speed
The pumping speed actually delivered at the chamber after losses in the connecting pipes and valves.
High vacuum
The range from about 10⁻³ down to 10⁻⁷ mbar, where molecules mostly hit the walls rather than each other.