INRIM continuous expansion system as high vacuum primary standard for gas pressure measurements below 9 x 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 x 10(-6) Pa to 9 x 10(-2) Pa with relative standard uncertainty ranging from 2.1% at 1 x 10(-6) Pa down to 0.4% at 9 x 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. (C) 2009 Elsevier Ltd. All rights reserved.