be activated as a function of the electrically effective closing of the reed contact 32 especially as a function of the magnetic field acting on the reed contact the activated ultrasound transmitter 34 can generate and transmit an airborne sound in the ultrasound frequency range the ultrasound transmitter 34 has a groove 36 in the area of a broader end the groove 36 is intended for introduction into a corresponding recess of a housing for guiding a gas volume flow the recess may be designed for this such that a rotary motion 42 of the ultrasound transmitter 34 about the longitudinal axis 40 can take place in a predetermined angle range for example 30 so that the ultrasound transmitter 34 can already be pre positioned by a mechanical coding formed by means of the groove 36 the pre positioning may be performed for example when introducing the ultrasound transmitter 34 into a corresponding mount of a flow sensor for detecting a gas volume flow along the longitudinal axis 40 exact alignment of the ultrasound transmitter 34 in the circumferential direction can take place by a further rotary motion 42 of the ultrasound transmitter 34 about the longitudinal axis 40 fig9 schematically shows a top view of the broader end of the ultrasound transmitter 34 the groove 36 the longitudinal axis 40 and the rotary motion 42 are shown a closed groove 38 extending about the longitudinal axis 40 is also shown by.