INRIM continuous expansion system as high vacuum primary standard for gas pressure measurements below 9 × 10−2 Pa

The continuous expansion system can be considered the state of the art for pressure measurement in the ultra high vacuum range. In the last years, at INRIM, a new continuous expansion system was designed, assembled and characterized. The system is the high vacuum primary standard in the pressure range from 1 × 10−6 Pa to 9 × 10−2 Pa with relative standard uncertainty ranging from 2.1% at 1 × 10−6 Pa down to 0.4% at 9 × 10−2 Pa. The system is based on the passing of a measured gas flow through a fixed and known conductance. The gas flow is generated and measured by a primary gas flowmeter based on the constant-pressure–variable-volume method.In the first part of the paper both a correction for the effect of transitional flow through the orifice and a new analytical evaluation of orifice conductance are presented. In the second part the accuracy of the system and the pressure uncertainty evaluation are described.