a fluid mass flow controller , particularly adapted for controlling mass flow rates of toxic and reactive gases used in semiconductor device fabrication , includes a control circuit connected to pressure sensors for sensing the differential pressure across a flow restrictor in the mass flow controller for controlling a valve to control the fluid mass flow rate to a setpoint . the control circuit compares the differential pressure with the downstream pressure at a measured temperature with a data set of a gas passing through the flow controller for a range of differential pressures and downstream pressures and adjusts the flow control rate accordingly . the flow controller is mechanically uncomplicated including a two part body for supporting the pressure sensors , a remotely controllable flow control valve and the flow restrictor . the flow restrictor may comprise an orifice or nozzle but preferably comprises a sintered metal plug having a predetermined porosity for the expected materials and flow conditions to which the flow controller will be exposed . process gases to be controlled by the flow controller are tested to provide data sets of mass flow rates at selected temperatures for a range of differential pressures across a flow restrictor and a range of downstream pressures .