Ultrasonic level detectors use the principle of reflection of an acoustic wave from liquid to vapour phase and vapour to liquid phase.
Ultrasonic level measurement work as similar to the working of sonar. An ultrasonic sound sends by the transmitter reflects back touching the water level to the receiver or Echo.
The Ultrasonic level detector measures the level on the basis of:
Time required by the echo signal to reach the receiver.
Change of phase of the wave during the time.
Considering the liquid used in a storage tank, there are two ways to fix the level detector in the tank within the tank at the top of the tank. Fig A level detector is shown as in the bottom of the tank, which is said to be liquid phase level measurement.
Fig B shows a vapour phase measurement setup.
The liquid phase method is used in storage tanks of oil and chemical and for aircraft or marine equipment tanks. The vapour phase method is used in mines, oil-well etc..
Piezoelectric crystals are used to produce ultrasonic sound
The above-shown L-C piezoelectric circuit is used as the oscillator circuit to produce the ultrasonic wave with frequency ranging from 30-300kHz. LC circuit is used to avoid spurious crystal oscillation.