Ultrasonic level gauge
Ultrasonic surface instruments measure the distance from the surface transmitter (located at some points above) to the material surface of the process below which it is reflected using sound waves. The frequency of these waves is beyond the range of human hearing, hence they are called ultrasonic. The flight time for a sound pulse indicates this distance and is interpreted by the transmitter as the process level. These surface transmitters may produce a signal commensurate with the fullness of the tank (filling) or the amount of empty space left on top of a tank (output).
Fluid height measurement
Ullage is the natural state for measuring this type of surface instrument, because the wave flight time is a direct function of the amount of space between the liquid surface and the top of the vessel. The total height of the tank will always be the amount of filling and overeating. If the ultrasonic level transmitter is programmed with the overall height of the vessel, it may calculate the filling by simple subtraction:
Fillage = Total height − Ullage
If a sound wave experiences a sudden change in the speed of sound of a material, some of the energy of that wave is reflected as another wave in the opposite direction. In other words, when you encounter a substance with a different speed of sound, the wave will “sound”. This is the basis of all ultrasonic devices. Therefore, in order for an ultrasonic surface transmitter to be reliable, the difference in sound speed at the interface between the liquid and the gas must be large. Liquid-gas interfaces almost always show large differences in their sound speeds, and so it is very easy to detect using ultrasonic waves. Liquids with a heavy layer are more difficult than the floating floor above, because the floor is denser than the liquid, but significantly denser than the gas above