A breathing apparatus includes a filter, a motor and a blower driven by the motor. The filter is disposed ahead of the blower and the blower is configured to generate an air volume flow over the filter. A data storage unit has calibration curves for the air volume flow stored therein and the calibration curves are provided in characteristic line fields referenced to a known ambient pressure (p0, p1). Each of the characteristic line fields has a reference calibration curve referred to a predetermined operating state. A control unit is connected to the motor and the data storage unit. The control unit is configured to extrapolate one of the characteristic line fields for the operation of the motor by comparing a calibration curve ({dot over (V)}ox) for an unknown ambient pressure (px) recorded at a predetermined operating state with a reference calibration curve.