| 摘要 |
<p>A loop flow-path (13) is formed by providing a partition (12) in the path of flow of a fluid duct (11). An incoming flow-path (16) and an outgoing flow-path (18) are provided with the fluid duct (11) through connecting ducts (15,17), and a gas G flows in the fluid duct (11). An ultrasonic sensor (14) having two opposing surfaces for transmission and reception is disposed in a part of the loop flow-path so that ultrasonic waves to be transmitted and receive have vector components in the direction of the loop flow-path (13). The ultrasonic waves outputted from the two opposing surfaces of the ultrasonic sensor (14) travel in opposite directions to each other in the loop flow-path (13), one ultrasonic wave with the flow of the gas (G) and the other ultrasonic wave against the flow of the gas (G), and are received on the opposite surfaces of the sensor (14), respectively.
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