A system for delivering a desired mass of gas, including a chamber, a first valve controlling flow into the chamber, a second valve controlling flow out of the chamber, a pressure transducer connected to the chamber, an input device for providing a desired mass to be delivered, and a controller connected to the valves, the pressure transducer and the input device. The controller is programmed to receive the desired mass from the input device, close the second valve and open the first valve, receive chamber pressure measurements from the pressure transducer, and close the inlet valve when pressure within the chamber reaches a predetermined level. The controller is then programmed to wait a predetermined waiting period to allow the gas inside the chamber to approach a state of equilibrium, then open the outlet valve at time = t0, and close the outlet valve at time = t* when the mass of gas discharged equals the desired mass.