Ultrasonic Flow Transducers

Fluid flow can be measured using ultrasonic waves. An ultrasound transducer generates waves through the fluid medium that are detected by a pickup sensor. As the wave travels from the source to the pickup its velocity will be increased or decreased relative to the wave speed in a stationary fluid. This is due to the Doppler effect which states that the wave reflected from a moving object will have a different apparent frequency to the incident wave. The transit time or phase shift of the reflected wave can be measured and the average velocity of the fluid can be determined. For determining fluid velocity from the transit time the following equation can be used:

DT @ 2Dv cosq/C2

where DT is the transit time, D is the distance between the ultrasound source and receiver, v is the average velocity of the fluid and C is the ultrasound velocity. Note that this relationship is an approximation that can only be applied if C is much greater than v cosq. (Figure 27)

Figure 26 Schematic of a Transcutaneous Ultrasound Flowmeter [2]

The relationship between the phase shift of the ultrasound waves is given by:

Df = 4pfv cosq/C2

where Df is the measured phase shift, f is the frequency of the ultrasound, v is the velocity of the fluid and C is the ultrasound velocity.

Figure 27 Schematic of Transit Time Ultrasound Flowmeter [2]