| 摘要 |
PROBLEM TO BE SOLVED: To reduce a current consumption, to reduce the number of comparators used in a reception wave detecting part, to reduce a cost, and to surely capture a target wave out of reception waves. SOLUTION: When the first transmission is detected in a reception side, an arrival time ta is obtained using a zero-cross point of a wave exceeding 200 mV of reference voltage as a reception detecting point. The second transmission is conducted concurrently with the first reception, an arrival time tb is obtained in the second reception using a zero-cross point of a wave exceeding 500 mV of reference voltage as a reception detecting point. The transmission and the reception are repeated hereinafter, when a difference between the ta and the tb is a fixed value or less, using as a detection point the first zero-cross point after a time when a time provided by subtracting a fixed timeαfrom the time tb elapses from the transmission, and a flow velocity is computed based on the arrival times in the plural transmissions and receptions. When the difference is the fixed value or more, an gain of a reception wave amplifier is changed to conduct measurement again.
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