A device is provided for detecting a gas volume flow, especially for a respirator. The device includes a flow sensor, which surrounds a lumen for guiding the gas volume flow, and the flow sensor has an ultrasound transmitter with a sound generator (20) and an ultrasound receiver. The device has a connection sensor (17, 19), which is designed to detect a connection, especially an installation position or correctness of the connection or both, of the ultrasound transmitter to the transmitter mount and/or a connection of the ultrasound receiver to the receiver mount and to change at least one of its electrical properties as a function of the connection, or to generate a connection signal, which represents the connection, and to output this signal on the output side.