Abstract: The mode of operation is by ultrasonic through-transmission, and the detector is preferably employed for detecting air bubbles in intravenous feeding tubes to prevent air embolism. Transmission of sound from the transmitter, via the sensor head, to the receiver of the detector is dependent upon the existence of a fluid within the tubing. Acoustic losses, operating frequency, and the distance between transmitter and receiver are optimized to permit constructive-interference of energy transmitted to and reflected from the receiver, resulting in a partial standing wave as in a resonant cavity. If an air bubble passes through the sensor head a large acoustic discontinuity occurs, causing ultrasound to scatter and reflect from its normal path. These losses allow little ultrasonic energy to couple to the receiver. The sensor head includes oppositely and spacedly disposed cylindrical sound pipe segments having facing tubing accommodating recesses, and respectively connecting to the transmitter and receiver.