Ultrasonic Meters
How they work 

Simplified: Ultrasonic Meter operate off the same principal as Sonar
Ultrasonic metering works on the principal of how sound travels through water. Think of an airplane flying at the same relative “Air Speed”. If he flies into the wind, he will fly slower. If he flies with the wind, he will fly faster.
Ultrasonic meters measure the time difference of a sound blast between two points. One is shot with the flow of water, one is shot against the flow of water. The travel time difference between the two is a constant for different speeds of the flow of water.