This method is based upon utilization of law of reflection of ultrasonic oscillation from solid or liquid surfaces or from the interface between two immiscible liquids. The time taken by the wave for its travel from transmitter to the liquid surface and back to the receiver can be taken as the measurement of liquid level.
ultrasonic level detector
It consists of a pair transmitter and receiver. The transmitter is the source of ultrasonic oscillation such as piezoelectric element like Quartz, which is positioned at the top or bottom of the vessel. The ultrasonic wave from transmitter reaches the material surface from where they get reflected back and these reflected waves are received by the receiver. The time interval from the instant of an emission of the wave to the reception of the reflected rays is measured. the time varies with the change in liquid level.