ULTRASONIC FLOWMETER

摘要

An ultrasonic flowmeter which measures the velocity of a gas medium flowing within a conduit utilizing the time of flight principal with phase measurement approach. A pair of ultrasonic transducers are oppositely located around the conduit at a predetermined angle to the direction of the flow. Each transducer alternates between acting as a transmitter of ultrasonic frequencies and a detector of ultrasonic frequencies, thus as one transducer is transmitting, the other is receiving. In each instance, a measurement is made of the time of flight of the ultrasonic signal between the transmitter transducer and the receiver transducer and a microcomputer calculates the velocity of the gas by a known mathematical relationship utilizing the difference between the time of flight of the ultraonic signals traveling upstream and those traveling downstream. The precise time of flight is measured by determining the phase shift or difference in phase angle between the signal at the transmitter transducer and the signal detected by the receiver transducer. To prevent ambiguity arising when the phase angle difference exceeds 360.degree., the microcomputer periodically determines, by taking into account the differing gases and density thereof being measured, a zero flow phase by calculating an averaged value phase angle measured in the ultrasonic signals in both the upstream and downstream directions. When that zero-flow phase approaches a predetermined limit, i.e. 90.degree. or 270.degree., the microcomputer alters the phase reference to take that approaching limit into account, thus shifting the phase reference to a new point at the 90.degree. or 270.degree. angle, the effective overall phase shift can thus exceed 360.degree., but the microcomputer will still accurately reflect that difference in phase angle and thus, the time of flight of the ultrasonic signals.