The employment of the continuous expansion method for vacuum gauge calibration in the medium vacuum range is restricted by the condition of free molecular flow through an orifice. In the present work it is proposed to replace the first orifice by a long cylindrical capillary, where the gas flow regime varies from viscous at the entrance to free molecular flow at the exit. An equation describing the real capillary conductance in this transition gas flow regime is derived. Accurate measurements of the internal diameter along the capillary are carried out. The employment of the capillary allows us to expand the range of primary pressure p1 up to 105 Pa, to avoid the exit pressure p2 measurements (p2 < p1) and to use fewer reference vacuum gauges due to the fact that the gas throughput scales with the square of the pressure p1. The second orifice was substituted by a thin plate with small circular holes. It was possible to maintain free molecular flow in the holes at a pressure up to 1 Pa. A comparison between the developed vacuum reference apparatus and the reference standard of the “D.I.Mendeleyev Institute for Metrology” (Russia) is made.
