The 1640 Pressure-based Mass Flow Controller is a metal-sealed instrument with standard 3-inch footprint, designed to meter and control gas flows in low-line pressure applications where thermal mass flow controllers are limited in their ability to accurately measure flow. The 1640 utilizes the principle of sonic flow through an orifice, a condition met when the upstream control pressure is at least twice the downstream pressure. Under these conditions, mass flow is proportional to the control pressure. In the 1640A Pressure-Based Mass-Flo Controller, a Baratron® capacitance manometer monitors the pressure upstream of the critical orifice. This pressure is proportional to flow. The measured pressure is compared in the control electronics to the flow set point. A control signal is then generated to drive the proportional control valve to the conductance as required to bring the actual control pressure (flow) into agreement with the flow set point. Typical applications include ion implant Safe Delivery Source® (SDS®) dopant and SACVD ozone flow control, water vapor and gases sensitive to upstream pressure changes.