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Chapter 1. Features and Capabilities
4 Transport® Model PT878 Portable Liquid Flowmeter User’s Manual
1.3 Theory of Operation
The PT878 is a transit-time ultrasonic flowmeter. When ultrasonic pulses
are transmitted through a moving liquid, 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 PT878 uses various digital
signal processing techniques, including cross-correlation, to determine
transit times and then uses the 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 liquid in the
pipe is not flowing, the transit-time downstream equals the transit-time
upstream. When the liquid is flowing, the transit-time downstream is less
than the transit-time upstream. The difference between the downstream and
upstream transit-times is proportional to the velocity of the flowing liquid,
and its sign indicates the direction of flow.