A method and a CPAP apparatus for characterizing different mask systems, e.g., masks and hoses, are provided. The apparatus can be calibrated for different mask systems and hoses by including sensors for measuring flow and pressure at a flow generator of the apparatus. When the flow generator is fitted to a new system, or changes are made to an existing system, mask or patient interface, and/or hose, a method of calibrating the flow generator is provided. The method includes determining air flow characteristics of the hose using flow measurements made during a first test period when flow through the system is open, measuring or estimating pressure in the system during a second test period when flow through the system is blocked, and determining air flow characteristics of the diffuser using the air first test period flow characteristics of the air delivery hose and the second test period pressure measurements.