1. A method of diagnosing drift in an in-situ mass flow controller comprising the steps of: collecting and storing data characterizing a valve voltage, control set point voltage, and feedback signal of the in-situ mass flow controller; computing an actual valve voltage difference as a voltage difference between the valve voltage of the in-situ mass flow controller and a representation of valve voltage characterizing a calibrated mass flow controller at the same control set point voltage; computing an actual feedback error as a voltage difference between the control set point voltage and the feedback signal; selecting for the actual valve voltage difference at least one membership function in which the actual valve voltage difference has a membership from a knowledge base comprising a plurality of membership functions, the selected at least one membership function corresponding to a possible state of the actual valve voltage difference; selecting for the actual feedback error at least one membership function in which the actual feedback error has a membership from a knowledge base comprising a plurality of membership functions, the selected membership function corresponding to a possible state of the actual feedback error; combining said membership functions selected for said actual valve voltage difference and said feedback error features using fuzzy rules to diagnose anomalous operation of said mass flow controller and; recalibrating the mass flow controller when a drift exceeding a preselected level is detected during said step of diagnosing.