Mathematical model applied to the experimental calibration results of a capillary standard leak

Gas leaks supplied through metal capillaries are nowadays largely used in industry to calibrate "in situ" leak detectors (necessary in tightness tests, versus both vacuum and atmospheric pressure) or in whatever applications in which well-known (and very low) gas flows are necessary. If the capillaries are calibrated with a primary device or a reference one, the traceability of flow measurements to the national primary standards is guaranteed. At IMGC-CNR, standard leaks are calibrated by means of two primary flowmeters, operating respectively with reference to vacuum and to atmospheric pressure. In this paper, with the aim of developing a general calculus model for a unique calibration curve in both fields of use of a capillary leak, results relative to the calibration of a particular type of an all metal, crimped-capillary leak are presented and discussed. This capillary has been calibrated for helium, sulfur hexafluoride and nitrogen with reference both to vacuum and to atmospheric pressure and for molar flows ranging from about 1 x 10-(11) mol/s to 5 x 10(-7) mol/s.