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.
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.