How do ultrasonic sensors measure distance and flow?

Ultrasonic sensors use sound waves (typically 20-200 kHz) for non-contact measurement. Distance measurement: time-of-flight method measures round-trip time for pulse reflection: d = c*t/2, where c is speed of sound. Flow measurement: transit-time method compares upstream and downstream propagation times (faster with flow), or Doppler method detects frequency shift from particles. Advantages include non-contact operation, works with liquids and solids, and no moving parts. Limitations include temperature dependence of sound speed and difficulty with foams or aerated liquids.