What is the difference between an ultrasonic level sensor and an ultrasonic flow sensor?

An ultrasonic level sensor measures only distance to the liquid surface, converting that to depth, while an ultrasonic flow sensor uses ultrasound to derive flow velocity (often with transit‑time or Doppler methods) and then computes volumetric flow. In open channels, a level sensor is often paired with a separate velocity or area‑velocity sensor, whereas ultrasonic flow meters for full pipes directly calculate flow from velocity and known pipe area.​