Electronic bubble detector apparatus

Electronic apparatus adapted for use with a conduit through which a fluid flows, the fluid having a first bubble free state and an second and alternate bubble containing state, said apparatus comprising:

• an oscillator employing a flat piezo-electric transducer as a frequency determining element, the transducer having axial and radial modes of excitation which have different resonant frequencies, the oscillator having a positive feedback loop containing in series a high pass filter and a low pass filter which together determine one selected radial mode of excitation for the transducer together with an associated resonant frequency and prevent the transducer from being excited in an axial mode of excitation, the transducer, when unloaded and operating in the selected radial mode wherein its amplitude elongations and contractions take place in the plane of the transducer, having a resonant frequency which determines the oscillation frequency and having a predetermined maximum level of amplitude excursion, the transducer, when loaded by being moved until one flat surface of the transducer is in engagement with the conduit, exhibiting a change in resonant frequency and a change in the level of amplitude excursion while remaining in the selected radial mode, the amount of oscillator frequency change and the amount of change in excursion level having respective values when the fluid is in the first state which differ from the respective values when the fluid is in the second state;
• and
• means responsive to at least one of the changing frequency and amplitude values to produce a signal when a selected one of the states is detected.