Uncertainty evaluation of the IMGC-CNR static expansion system

A primary system based on the so-called static expansion of gases has been made available at IMGC-CNR for gauge calibration in the range 0.1 Pa to 1000 Pa. As is known, in such systems the generated pressure is calculated by application of the ideal gas law and is a function of the inlet pressure, the temperatures of the various volumes and their ratios. The uncertainty evaluation of the pressure measured with such system is the main subject of this paper. At present, the system is operated with three volumes (approximate to 0.01 litre, approximate to 0.5 litre and 50 litre) and two expansion ratios (R-4 congruent to 4761 and R-3 congruent to 95) determined by application of the multiple expansion method through pressure measurements performed with the best transfer gauges available at IMGC-CNR. The characteristics of such transfer gauges are discussed with respect to two different conditions of use: that is, for determination of the expansion ratio, R-j, and for measurements of the inlet pressure. The combined expanded uncertainty of the generated pressure (p(xi)) is then given by U(p(xi))/Pa 1.0 x 10(-4) + 1.7 x 10(-3) p(xi)/Pa in the pressure range 0.1 Pa to 10 Pa and U(p(xi))/Pa 1.7 x 10(-2) + 8.2 x 10(-4) p(xi)/Pa in the range 10 Pa to 1000 Pa. Considerations on participation in comparisons are also presented.