Implementation of the continuous expansion method with a variable conductance in the pressure range 1 × 10−6 to 1 × 10−3 Pa

•The continuous expansion method (pressure ratio technique) has been carried out with a non calculable conductance.•The conductance has a crescent-shaped area.•Experiments show the conductance remains pressure independent up to 5E-3 Pa.•The final reference pressure depends on the pressure ratio and the conductance determined at high pressure.

The continuous expansion method is widely used by National metrology institutes for traceability of vacuum measurements below 10−3 Pa. It is usually implemented using a fixed conductance such as a small circular orifice. This article describes how LCM-LNE has used the method with a variable conductance and includes a detailed uncertainty budget.