| 摘要 |
PROBLEM TO BE SOLVED: To always reliability detect a zero cross point q5x of a half wave of ultrasonic oscillation amplitude (fifth wave counted from a top half wave as a first wave in this example) at a specified position on a train of fine half waves of oscillation amplitude, in a received ultrasonic signal 2d as the termination end of a propagation timeτx to obtain an accurate propagation time, accordingly an accurate flow rate, even if the amplitude of the received signal 2d varies due to the flow rate variation, so that an ultrasonic flowmeter for measuring the ultrasonic propagation timeτx of an ultrasonic wave received by a pair of ultrasonic transducers through a medium using a fluid under test can be incorporated in a gas meter required to be compact and having a low power consumption. SOLUTION: An ultrasonic wave is transmitted twice with a short transmission interval Ts enough to neglect the amplitude variation of a received signal 2d, a negative peak value Vpeak of the signal 2d is obtained from the first received signal 2d to generate a variable threshold voltage Vtrg proportional to Vpeak, a half- amplitude wave of a fifth wave is first surely detected from an intersection point c5x of the second received signal 2d with the threshold voltage Vtrg, and its zero cross point q5x is detected to obtain the propagation timeτx.
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