Features and Capabilities 1-5
April 2004
Theory of Operation
The TransPort is a transit-time ultrasonic flowmeter. When
ultrasonic pulses are transmitted through a moving liquid or gas,
the pulses that travel in the same direction as the fluid flow
(downstream) travel slightly faster than the pulses that travel
against the flow (upstream). The TransPort uses various digital
signal processing techniques, including cross-correlation, to
determine transit times and then uses the difference in transit
times to calculate flow velocity.
During operation, two transducers serve as both ultrasonic signal
generators and receivers. When mounted on a pipe, they are in
acoustic communication with each other, so that each transducer
can receive ultrasonic signals transmitted by the other transducer.
Each transducer thus functions as a transmitter generating a
certain number of acoustic pulses, and as a receiver for an
identical number of pulses.
The flowmeter measures the time interval between transmission
and reception of the ultrasonic signals in both directions. When
the gas in the pipe is not flowing, the transit-time downstream
equals the transit-time upstream. When the gas is flowing, the
transit-time downstream is less than the transit-time upstream.
The dif f erence between the downst ream and upstream tran sit-
times is proportional to the velocity of the flowing gas, and its
sign indicates the direction of flow.