The operational principle of these devices is based on the phenomenon of reflection of ultrasound waves from the phase boundary separating liquid and gas. In different media the speed of sound is different. Therefore, these devices may be used for interface level measurements, and in the case when the more traditional methods do not work well or do not work at all.
There are two modifications of ultrasound level measuring devices. In the first case ultrasound passes through gaseous phase; in the second case it passes through the liquid. Figure shows a continuous ultrasound level-measuring device.
An electrical generator 1 generates electrical signals with a certain frequency. An acoustical transmitter 2 periodically sends ultrasound signals to the surface of the liquid 3. These ultrasound waves enter an acoustical receiver 4 after reflection from the surface of the liquid. After receiver the converted electrical signal is amplified in an amplifier 5 and enters a time interval counter 6 that measures the time between the transmission of a pulse and receipt of the corresponding pulse echo.
Then a converter 7 converts thus measured time into a standard electrical signal 4-20 mA dc. Since an ultrasound permittivity depends on the properties of a gas, then a thermal compensation unit 8 is used to reduce the influence of temperature variation on the results of measurement. In real industrial environment pressure and chemical composition are additional factors which affect the velocity of acoustic propagation. These changes can severely affect the calibration of ultrasound devices. Therefore, additional electronic means are incorporated in these instruments to correct such changes.