Abstract: A pair of mass flow sensors are used to separately monitor the mass flow rates of a carrier gas and of a mixture of carrier gas and source vapor formed a mixing station. The rates are compared, for example by substracting the carrier gas flow rate from the mixture flow rate, to generate a signal representing the mass flow rate of the source vapor. Flow of the carrier gas to the mixing station is modulated in accordance with the difference between this signal and a command signal representing a predetermined flow rate. The system can be calibrated by flowing the carrier gas alone through both sensors, by passing the mixing station, and adjusting the signal for one or both sensors so that the signals from both sensors are equalized.