Abstract: A pressure-based mass flow controller suitable for accurately monitoring and controlling the flow of many types of precursors characterized by vapor pressures ranging from approximately 2 torr or lower to at least 760 torr or higher at delivery temperatures of up to at least 250 degrees C. or higher for flow rates ranging from molecular to sonic. A flow restrictive element is installed in the precursor flow path, and the pressures of the fluid upstream and downstream of the flow restrictive element are measured. The ratio of the upstream and downstream fluid pressures is computed and compared to a value stored in the memory of a CPU associated with the controller to determine whether the flow is choked or non-choked. The mass flow of the precursor fluid is then computed by the CPU in accordance with a linear function of the upstream pressure, for choked flow, and in accordance with a nonlinear function of both the upstream and downstream pressures, for non-choked flow.