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These use ultrasonic level measuring technique to find the interface.
Sound waves are used to determine level. A piezoelectric crystal
inside the transducer mounted just below the liquid surface
converts electrical pulses into sound energy that travels in
the form of a wave at the established frequency and at a constant
speed in a given medium. The sound waves are emitted in bursts
and received back at the transducer as echoes. The instrument
measures the time for the bursts to travel down to the reflecting
surface and return. This time will be proportional to the distance
from the transducer to the surface and can be used to determine
the level in the tank
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