Chapter 1. Introduction
2 AT600 User’s Manual
1.2 Theory of Operation
1.2.1 Transit-Time Flow Measurement
In this method, two transducers serve as both ultrasonic signal generators and receivers. They are in acoustic
communication with each other; that is, the second transducer can receive ultrasonic signals transmitted by the first
transducer and vice versa.
In operation, each transducer functions as a transmitter, generating a certain number of acoustic pulses, and then as a
receiver for an identical number of pulses. The time interval between transmission and reception of the ultrasonic
signals is measured 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.
Figure 2: Flow and Transducer Paths (Dual Traverse)
Figure 3: Flow and Transducer Paths (Single Traverse)
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