The variable opening of a pressure independent supply air valve for a building ventilation system is controlled in response to a standard airflow transducer whose output is modified to emulate a close tolerance transducer. In a calibration mode, a microcomputer based control closes the valve and a summing amplifier reduces the transducer output by an amount equal to a compensation signal provided by the microcomputer. Once the output is reduced to a predetermined desired level, the value of the compensation signal is stored in memory for later use during a run mode. In the run mode, the actual supply airflow rate is determined based upon the transducer output less the compensation signal. And the control adjusts the position of the valve until the actual airflow rate meets a desired airflow rate as determined by a room thermostat.