A process mass flow instrument for measuring the mass flow rate of a process fluid including a fluid inlet, a fluid outlet, a flow passage connecting the fluid inlet and the fluid outlet, a fluid flow sensor which provides a raw output signal corresponding to an unknown mass flow rate and a processor which contains an empirically derived conversion routine for converting the raw output signal to an adjusted signal indicating the mass flow rate of the process fluid. A method for determining an unknown mass flow using an empirically derived conversion function, the function being derived by correlating the output signals of a mass flow instrument sensing the flow of a calibration fluid with the output signals of the mass flow instrument sensing the flow of the process fluid.