A system for supplying precursor gas to a substrate processing chamber includes a first mass flow controller including an inlet to receive a carrier gas and an outlet. An ampoule is configured to supply a precursor gas. A valve system, in fluid communication with the first mass flow controller and the ampoule, is configured to supply the precursor gas and the carrier gas to a momentum-based flow restriction member. A pressure sensing system is configured to sense an inlet pressure at an inlet of the momentum-based flow restriction member and an outlet pressure at an outlet of the momentum-based flow restriction member. A controller is configured to determine a flow rate of the precursor gas at the outlet of the momentum-based flow restriction member based on a difference between the inlet pressure and the outlet pressure.