The Micronics Portaflow D650 ultrasonic sensor injects high-frequency sound through the pipe wall and into the flowing liquid. Gas bubbles or solids suspended in the liquid reflect the ultrasonic signal to the sensor. When this sound is reflected from moving bubbles or particles it is returned to the sensor at a shifted frequency. This frequency shift is called the Doppler effect. The PF D650 continuously measures the change from its transmitted frequency to the received frequency to accurately measure flow.