A device for measuring a gas flow, has a measurement chamber and an ultrasonic transceiver unit which can be attached to the measurement (chamber over openings of the measurement chamber and which is provided with transmit and receive heads. The heads are directed against the openings of the measurement chamber, membranes being arranged between the measurement chamber and the transmit and receive heads which are permeable to ultrasound waves but largely impermeable to moisture and bacteria. In order to feed maximum acoustical energy to the ultrasonic transceiver unit while keeping moisture and bacteria away from the unit, the membranes are replaceably arranged close to the transmit and receive heads.