Changes in the NPL orifice conductance on a transition from molecular gas flow to transitional flow

The conductance of an NPL orifice in the molecular regime is constant and can be exactly calculated from the geometric dimensions. For smaller Knudsen numbers the conductance increases and correction functions are employed to reduce the uncertainty in this range of pressures. The conductance is also constant in the viscous regime during flow into a vacuum and can also be calculated. A suitable function has been chosen with one free parameter, which is constant for both very low and sufficiently high pressures and the parameter was determined on the basis of the experimentally measured course of the conductance at the borderline between molecular and transitional flow. The function fits the experimental data very well and can be used to calculate the conductance of the orifice up to Knudsen number ≈1.