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To be precise, Ultrasonic sensors detect Ultrasonic waves which are nothing but High frequency Sound waves ( Above say30 Khz), These waves propagate by disturbance in air or any other medium, They always need a medium and cannot propagate through Vaccuum, which is a disadvantage of using Ultrasonic Transmitters for Level measurements on Vessels that operate on very low pressures, Also sound waves are prone to attenuation depending on the medium. Whereas, Radar sensors use Radio waves or Electromagnetic waves which posess a Field or to say Energy which makes them travel through Vaccuum or even without a media. Radar Transmitters are more reliable than Ultrasonic ones. The popular Radar Transmitter used currently in the Industry is the GWR(Guided Wave Radar) which is used in Level Measurements where the Sensor is a guided tube covering almost the height of the Vessel through which the Radio waves propogate. Hope this post helps you.
ultra sonic works on principle of light speed and redar level works on principal of eco sound system