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A liquid velocity measuring system is described utilizing ultrasonic waves to measure the velocity of a liquid and in particular, the velocity of blood as directed through a conduit external of the patient's body. The liquid velocity measuring system comprises an ultrasonic soundwave transmitting tr

A liquid velocity measuring system is described utilizing ultrasonic waves to measure the velocity of a liquid and in particular, the velocity of blood as directed through a conduit external of the patient's body. The liquid velocity measuring system comprises an ultrasonic soundwave transmitting transducer energized by a continuous wave generator, preferably at a frequency of 3.13 MHz. The transmitting transducer is disposed with respect to the conduit such that the transmitted ultrasonic waves are backscattered or reflected from ultrasonic reflection centers in the blood. This signal is received and detected by a second ultrasonic transducer. The reflected ultrasonic wave is frequency-shifted dependent upon the velocity of the liquid directed through the conduit and is detected by the second transducer to be amplified and demodulated subsequently, before being applied to a microbubble detection circuit in accordance with the teachings of this invention. In particular, the microbubble detection circuit rectifies and filters the backscattered signal before applying it to a comparator circuit. In the presence of microbubbles within the liquid, the reflected ultrasonic wave will increase in amplitude at least 15-fold over that liquid, and in particular, blood without microbubbles. The comparator is set to provide an output signal if the strength of the detected signal increases 15-fold, to provide an output signal for setting a flip-flop whose output in turn energizes an alarm indicative of the presence of microbubbles.

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