Accusonic 8510+ series Reference Manual page 69

A-2 Multiple-Path Transit-Time Flowmeters Principle of Operation
This method of measurement is described as follows:
Where:
T1 = Travel time of the acoustic pulse between transducer B and transducer A (Figure 1)
T2 = Travel time of the acoustic pulse between transducer A and transducer B
C = Speed of sound in water
V = Velocity of the water
Ø = Angle between the acoustic path and the direction of water flow
L = Path length between transducers
The above equations are solved for V, independent of C, yielding:
Therefore, the velocity of the water at the acoustic path can be calculated by knowing the path length (L)
and path angle (Ø), and measuring the time for the acoustic pulse to travel between the transducers in the
upstream and downstream directions.
Typically, four pairs of transducers are spaced in the pipe or channel to give four parallel acoustic paths
(Fig.2 above, Fig. 3, and Fig. 4 following). Velocities for these paths are then integrated so that flow is
measured according to the equations in the following sections.