In embodiments, a process gas supply provides a carrier gas and one or more process gases to a distribution manifold. A back pressure sensor senses back pressure in the distribution manifold and provides a signal to the first controller based at least in part on the back pressure. The first controller determines a back pressure set point based at least in part on the signal. One or more mass flow controllers control the flow of the gas mixture comprising the carrier gas and the one or more process gases into one or more zones of the process chamber. An upstream pressure controller fluidly and operatively connected to the distribution manifold controls flow of the carrier gas based on the back pressure set point.