text: For a Knudsen gas, the Knudsen number must be greater than 1. The Knudsen number can be defined as:
  where
   is the mean free path [m]
   is the diameter of the receptacle [m].
  When , the flow regime of the gas is transitional flow. In this regime the intermolecular collisions between gas particles are not yet negligible compared to collisions with the wall. However when , the flow regime is free molecular flow, so the intermolecular collisions between the particles are negligible compared to the collisions with the wall.