In dialysis apparatuses usually ultrasound-based detectors (hereinafter referred to as detector) are used to protect patients against dangerous air embolisms. For this purpose, a part of an extracorporeal blood circuit to be monitored may be guided in the form of a blood hose through an ultrasonic path of the detectors. For this, a detector includes a piezo element that transmits an ultrasonic (US) pulse or US waves through the blood hose by electric excitation to its resonance frequency. The further detector in the form of a piezo element may convert said US pulse into an electric signal. This signal may be compared to a reference voltage and evaluated by a comparator. On the output side of the comparator a signal is available which informs about whether “air” or “no air” (i.e. fluid or blood only) is provided in the ultrasonic path.