A CPAP apparatus having a flow generator (10, 12), a patient interface (18), an air delivery conduit (16) for delivering air from the flow generator to the patient interface, a pressure sensor (30) for determining the pressure from the flow generator, a flow sensor (28) for determining the flow from the flow generator, and a control mechanism (24) that causes air to be delivered through the air delivery conduit at a desired pressure at the patient interface. The control mechanism is configured to: receive sensed pressure data from the pressure sensor； receive sensed flow data from the flow sensor； determine an estimated mask pressure based on the sensed pressure data and the sensed flow data； calculate the derivative of flow at run time； set a converted derivative of flow value based on whether the value of the derivative of flow is positive or negative； calculate a pressure discrepancy value based on the converted derivative flow value； determine an adjusted mask pressure at the patient interface based on the estimated mask pressure and the pressure discrepancy value； and control the flow generator by maintaining the adjusted mask pressure substantially equal to the desired pressure.
