| 摘要 |
<p>A piezoelectric liquid level switch is provided which can be made immune to standing wave effects and provides an unequivocal response to liquid contact. The switch incorporates an electrical oscillator circuit including frequency defining elements in both feed forward and feedback circuit paths, the frequency defining element in the feedback path being a piezoelectric crystal device constructed to have a lowest frequency vibration mode in air which is substantially equal to a first frequency defined by the frequency defining element in the feed forward circuit, which mode is suppressed by acoustic coupling of the device to a body of liquid such that a lowest frequency vibration mode of the device when in contact with liquid has a second frequency which is substantially above the frequency of the frequency defining element in the feed forward circuit, whereby positive feedback through the oscillator circuit can occur when the piezoelectric device is in contact with air, but not when it is in contact with liquid, and a threshold circuit for detecting an output from the oscillator circuit at said first frequency. The piezoelectric element is preferably coupled to a column coaxial with it, the column having a liquid contacting member at its free end, such that a vibration antinode at or near the liquid contacting member is suppressed and the lowest resonant frequency of the device is substantially increased when the member is in contact with liquid.</p> |
