A mass flow control system for real time accuracy verification when controlling a flow of a fluid to a process, the system comprising: an inlet that receives a fluid under pressure; an outlet that delivers the fluid under pressure; a conduit through which the fluid flows under pressure; a control valve that controls the flow of fluid through the system as a function of a control signal; a reservoir that stores a known volume of the flow of fluid for use in verifying the accuracy of the system anytime between steps of the flow control process; a flow restrictor between the reservoir and the outlet that controllably restricts fluid flow between the reservoir and the outlet; a single pressure sensor that is coupled to the conduit and senses the pressure of the fluid within the conduit providing as an output a signal indicative of the pressure; and a controller for generating the control signal as a function of measured flow of fluid through the system and a set point, wherein the controller controls position of the control valve and uses the signal from the single pressure sensor for two different operations of the mass flow control system: (i) flow control of the control valve for a process operation, based on the signal from the pressure sensor so as to cause the flow rate of fluid through the conduit to be equal to a flow set point; and (ii) flow verification of the accuracy of the fluid flow control by determining a rate of pressure decay within the reservoir based on the signal from the pressure sensor. an inlet that receives a fluid under pressure; an outlet that delivers the fluid under pressure; a conduit through which the fluid flows under pressure; a control valve that controls the flow of fluid through the system as a function of a control signal; a reservoir that stores a known volume of the flow of fluid for use in verifying the accuracy of the system anytime between steps of the flow control process; a flow restrictor between the reservoir and the outlet that controllably restricts fluid flow between the reservoir and the outlet; a single pressure sensor that is coupled to the conduit and senses the pressure of the fluid within the conduit providing as an output a signal indicative of the pressure; and a controller for generating the control signal as a function of measured flow of fluid through the system and a set point, wherein the controller controls position of the control valve and uses the signal from the single pressure sensor for two different operations of the mass flow control system: (i) flow control of the control valve for a process operation, based on the signal from the pressure sensor so as to cause the flow rate of fluid through the conduit to be equal to a flow set point; and (ii) flow verification of the accuracy of the fluid flow control by determining a rate of pressure decay within the reservoir based on the signal from the pressure sensor. (i) flow control of the control valve for a process operation, based on the signal from the pressure sensor so as to cause the flow rate of fluid through the conduit to be equal to a flow set point; and (ii) flow verification of the accuracy of the fluid flow control by determining a rate of pressure decay within the reservoir based on the signal from the pressure sensor.