Ultrasonic flow meters

When a disturbance is created in the flowing fluid, it generates sound waves that propagates everywhere in the flow field. These waves travel faster in the flow direction (downstream) compared to the waves in the upstream direction. As a result, the waves spread out downstream while they are tightly packed upstream. The difference between the number of waves in upstream and downstream is proportional to the flow velocity. The ultrasonic flow meters operate on this principle using sound waves in the ultrasonic range (~1MHz). Its operation mainly depends on the ultrasound waves being reflected and discontinuities in the density. Also, solids, bubbles and any discontinuity in the liquid will reflect the signal back to the receiving element. So, the device requires that the liquid contains at least 25ppm (parts per million) of particles or bubbles having diameters of 30µm or more. There are few distinct advantages of ultrasonic flow meters such as easy installation, non-intrusive type measurement and negligible pressure drop since it does not interfere the flow. Two basic kinds of ultrasonic flow meters include transit time and frequency shift flow meters.

The transit time flow meter (Fig. 7.4.3-a) involves two transducers located at certain distance that alternatively transmits and receive ultrasonic sound waves, in the direction of the flow as well as in the opposite direction. The travel time for each direction can be measured accurately and the difference can be estimated. The average flow velocity can be determined from the following relation;

(7.4.4)

The frequency shift flow meter (Fig. 7.4.3-b) is normally known as Doppler-effect ultrasonic flow meter that measures average velocity along the sonic path. The piezo-electric transducers placed outside the surface of the flow transmits sound waves through the flowing fluid that reflects from the inner wall of the surface. By capturing the reflected signals, the change in frequency is measured which is proportional to the flow velocity.

Fig. 7.4.3: Basic principle of an ultrasonic flow meter: (a) Transit time flow meter; (b) Frequency shift flow meter.