Ultrasonic WATER METERS are mainly of two types: Doppler and transit time. Ultrasonic Water Meters 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.