Editing Ultrasound (section)

===Motion sensors and flow measurement===
A common ultrasound application is an automatic door opener, where an ultrasonic sensor detects a person's approach and opens the door. Ultrasonic sensors are also used to detect intruders; the ultrasound can cover a wide area from a single point. The flow in pipes or open channels can be measured by ultrasonic flowmeters, which measure the average velocity of flowing liquid.  In [[rheology]], an [[acoustic rheometer]] relies on the principle of ultrasound. In [[fluid mechanics]], fluid flow can be measured using an [[ultrasonic flow meter]].