This lecture covers the principles of ultrasonic distance sensors, focusing on the emission of sound pulses and the measurement of echo transit time. It explains the influence of mechanical characteristics on the shape of the received echo, the use of cross-correlation to avoid measurement errors, and the design considerations for ultrasonic sensors. Additionally, it discusses applications such as proximity detection, bubble detection in medical systems, and flow sensors. The lecture also delves into Doppler sensors for flow measurement, detailing the Doppler effect, frequency shifts, and velocity profiles. Various sensor configurations and signal processing techniques are explored for accurate flow rate calculations.