| 摘要 |
<p>An ultrasonic flowmeter comprises a pair of ultrasonic transducers 10, 12 adapted to be mounted about a conduit 14. The transducers 10, 12 are coaxially aligned and at an angle 9 with respect to the direction of gas flow through the conduit 14. Each transducer 10, 12 alternatively acts as a transmitter and a receiver. The time of flight of a signal between the transducers is measured by determining the difference in phase angle between the signal as transmitted and the signal received. A flow rate calculator 82 compares the detected phase angles with a phase reference to determine a phase angle shift of signals travelling both upstream and downstream. From a difference in detected phase angles the velocity of the gas in the conduit 14 is calculated by a flow rate calculator 82 forming part of a microcomputer 80.</p><p>The microcomputer 80 periodically determines a zero flow reference phase and if the zero flow reference phase approaches a predetermined limit, i.e. 90° or 270°, the microcomputer 80 alters the phase to avoid ambiguity when the phase angle difference exceeds 360°.</p> |
