Abstract: A device for delivering through a pipe a mass of a selected gas close to a selected predetermined mass value m, to a system at a predetermined working pressure P0 substantially greater than 100 bars according to the present invention incorporates a vessel of known volume V0, fitted with an inlet valve and with an outlet valve that is operatively connected to control passage through the pipe. A volume of a selected gas is injected into the vessel until a pressure of the gas in the vessel reaches a predetermined initial high pressure P1 with the initial high pressure P1 being greater than the working pressure P0. A temperature T of the gas in the vessel and the pressure of the gas in the vessel are measured, and a calculated pressure P2 is calculated using a formula: ##EQU1## wherein the volume of the vessel is substantially greater than a volume of the mass m at the working pressure P0 and the initial high pressure P1 is greater than the working pressure P0.