| 摘要 |
PURPOSE:To positively detect the level of an object regardless of acoustic noises by outputting a switching signal a predetermined time later after an opening at one end of a cylindrical member is completely closed by the object. CONSTITUTION:A cylindrical body 2 is mounted to an upper wall of a tank 13 in which an object 14 to be measured is stored. When a speaker 1 is driven with the same frequency as that of a resonant point, an opening at the other end of the cylindrical body 2 is opened. In the resonant state, a bridge circuit 5 is kept balanced, and integrating outputs of a first and a second integrating circuits 7a, 7b become equal to each other. When the opening at the other end of the cylindrical body 2 in the resonant state is closed or nearly closed by the object 14, the standing wave is switched to the anti-resonating state and the variable acoustic impedance R4 of the speaker 1 is changed to a larger one, with the circuit 5 turned into the imbalanced state. As a result, the integrating outputs of the circuits 7a, 7b become different in size, and an alarm lamp 9 is turned ON from an output signal of a comparison circuit 8. Accordingly, it is informed that the opening is closed. |
