Ultrasonic Flowmeters are mainly of two types: Doppler and transit time. Ultrasonic Flowmeters working on transit time principles send two ultrasonic signals across a pipe one traveling with the fallow and the traveling against the flow. The ultrasonic signal traveling with the flow travels faster than a signal traveling against the flow. The Ultrasonic Flowmeters measures the transit time of both signals. The difference between these two times is proportional to flow rate. Transit time Ultrasonic Flowmeters usually monitor clean and clear liquids.
In Doppler principle a transmitting transducer sends a continuous ultrasonic signal into the liquid. Particles, gas/air bubbles or disturbances in the liquid reflect part of this signal. The reflected signal is sensed by the receiving transducer and the difference between the transmitter and received frequency is calculated. This frequency is directly proportional to the liquid velocity. Ultrasonic Flowmeters measures the particle size>100 microns.