Abstract: A ventilator system has different sub-systems, such as a connection system, which influence the flow of breathing gas supplied to a patient's respiratory system. The flow pattern of the breathing gas is particularly influenced by the connection system. In order to compensate for this influence to the greatest possible extent in the generation of breathing gas, the connection system's transfer function is determined before the system is connected to the respiratory system. A gas flow with a specific gas flow pattern is generated with high accuracy in a ventilator unit. An ensuing gas flow pattern, only influenced by the connection system, is measured with high accuracy in a measurement unit in the ventilator unit. The transfer function for the connection system can be determined in a control unit from the known, generated gas flow pattern and the measured, ensuing gas flow pattern. The corresponding determination can be made for the respiratory system after a patient has been connected.