Abstract: A method and a CPAP apparatus (10) for characterizing a plurality of different mask systems (14), e.g., masks and hoses, are provided. The CPAP apparatus (10) can be calibrated for different mask systems (14) and hoses (16) by including sensors (13,15) configured to measure flow and pressure at a flow generator (12) of the CPAP apparatus (10). When the flow generator (12) is fitted to a new mask system (14), or any changes are made to an existing mask system (14), mask or patient interface (34) and/or hose (16), a method of calibrating the flow generator (12) for the new mask system (14), mask (34) and/or hose (16) is provided.