Systems and methods for determining a concentration of a constituent gas in a gas stream using pressure measurements are disclosed. A method may include a vaporizer receiving a carrier gas and a source material, and vaporizing the source material in the vaporizer to produce a gas stream including the carrier gas and the constituent vapor. Then, a pressure of a chamber of the vaporizer is obtained, a mass flow rate of the gas stream is measured, and a temperature of the chamber of the vaporizer is measured. A constituent-vapor-concentration signal indicative of the concentration the constituent vapor in the gas stream is then generated. The mass flow rate of the constituent vapor may be determined using the concentration of the constituent vapor, and the mass flow rate of the constituent vapor may then be controlled using the determined mass flow rate of the constituent vapor.