Ultrasonic flowmeters are two types:
Doppler meters measure the frequency shifts caused by liquid flow. In this, two transducers are mounted in a case attached to one side of the pipe. A signal of known frequency is transmitted into the liquid to be measured. Solids, bubbles, or any other discontinuity in the liquid, cause the signal to be reflected to the receiver element. Since the liquid causing the reflection is moving, the frequency of the returned pulse is shifted. This frequency shift is proportional to the liquid’s velocity or flow rate.
A portable Doppler meter which is competent enough of being operated on AC power or from a rechargeable power pack has lately been introduced. A typical Doppler meter using sound pulse reflection principle is shown in the figure below:
These are also known as Transit meters. They have transducers installed on each side of the pipe. They use the transit time principle for flow measurement. In this, opposite sending and receiving transducers are employed to transmit signals through the flow. The signal travels faster when moving with the flow stream rather than against the flow stream. The difference between the two transit times is used to determine the flow rate.
As per configuration, the sound waves travel between the devices at a 45 degree angle to the fluid flow direction. The speed of the signal traveling between the transducers depends upon (increases or decreases) the direction of transmission and the velocity of the liquid being measured. A time-differential relationship proportional to the flow can be acquired by transmitting the signal alternately in both directions.
A major limitation of time-of-travel meters is that the liquids being measured must be moderately free of entrained gas or solids. This is crucial for minimizing signal scattering and absorption.