Ultrasonic water meters work by using sound waves to measure the flow of water. Ultrasonic measuring is based on the principle that different signals take different amounts of time to travel between two sensors. This is called the differential transit time method. This method measures how long it takes for an ultrasonic signal to go from one sensor to the other and back again. When there is no flow the signal transit times are the same, upstream and downstream. When water starts flowing in the pipe, the ultrasonic signals are accelerated in the direction of the flow and slow down against the direction of the flow.