Micro mass flow control apparatus and method

A micro mass flow controller is provided to accurately and reliably control or meter a mass flow rate of gas flowing at very low rates. The flow controller includes a sonic choked flow restriction with an oscillateable valve that is operable to repetitiously open and close the flow restriction in a time-modulated manner. The flow controller takes measurements of gas inlet pressures and temperatures in an inlet plenum and, at least partially, based on these measurements imposes a duty cycle on the valve by sequentially starting and stopping the sonic choked flow through the flow restriction. The duty cycle of the valve in one embodiment is the ratio of open time to the sum of the open time and the closed time for the valve. The valve includes an actuator comprising an elongated, cantilevered, piezoelectric bimorph element connected to a driver circuit that provides time-modulated positive and negative voltages to alternately drive the bimorph element to bend upward and downward to open and close the valve to mass flow control or meter gas flow.