A method for delivering vaporized precursor in a substrate processing system using a vapor delivery system includes (a) selectively supplying push gas to an inlet of an ampoule storing liquid and vaporized precursor during a deposition period of a substrate; (b) measuring a pressure of the push gas and the vaporized precursor at an outlet of the ampoule during the deposition period; (c) determining a maximum pressure during the deposition period; (d) determining an integrated area for the deposition period based on a sampling interval and the maximum pressure during the sampling interval; and (e) repeating (a), (b), (c) and (d) for a plurality of the deposition periods for the substrate.