CHAPTER 11 - ACOUSTIC FLOW MEASUREMENT

2. Theory

Acoustic flowmeters were developed based on the principles that the transit time of an acoustic signal is longer in the upstream than downstream direction, and that these transit times can be accurately measured using microcomputer technology.

Discharge measurements are based on determining the average axial velocity in a full-flowing pipe. Knowing this velocity and the cross-sectional area of the measurement section, a discharge can be calculated. The difference in transit times of acoustic signals traveling in opposite directions through the water can be related to velocity of flow (figure 11-1a). In the downstream direction, the velocity of the flowing water, Vw, adds to the speed of sound, C, to give the effective speed of the acoustic pulse, C + Vw. In the upstream direction, the velocity of flow delays the arrival of the pulse, resulting in an effective pulse speed of C - Vw. Taking the difference in these transit times eliminates C from the calculations and results in )t. When )t is known, the average axial velocity can be obtained from the formula:

where:

Vaxial = average axial velocity of waterflow
tu = upstream travel time of the acoustic signal
td = downstream travel time of the acoustic signal
t = difference in upstream and downstream travel times
= angle between the acoustic path and the pipe's longitudinal axis
L= acoustic path length between the transducer faces

Another acoustic flow measurement technique uses a similar approach which employs the frequency difference between upstream and downstream acoustic signals. This method is similar to the transit-time method and will not be covered. For more information, see American National Standards Institute/American Society of Mechanical Engineers (ANSI/ASME) standard MFC-5M-1985.