In applications where the ultrasonic wave travels through multiple different media before being reflected off of an air boundary, the speed of sound through and thickness of each material must be considered. Additionally, speed of sound is temperature dependent and therefore temperature correction factors must be applied.
In a continuous ultrasonic level sensor, nearly 100 percent of the acoustic energy in the wave is reflected off of the air/liquid interface due to the large difference in acoustic impedance between air and water. Acoustic impedance is a property of a material measured in units called Rayls and is calculated by multiplying the speed of sound in a material by the density of that material. Under typical conditions, air is hundreds of times less dense than liquids, leading to a large impedance mismatch between the two media and resulting in nearly 100 percent acoustic reflection.
The transmitted ultrasonic wave will spread out into a conical shape as it moves away from the transducer and it will reflect off of any solid or liquid surfaces in its path. Therefore, in many cases, the ultrasonic level sensor must be mounted far enough away from the walls of the tank to avoid interference.
Custom Sensor Solutions: “Top-Down” vs “Bottom-Up” Ultrasonic Level Sensors
The vast majority of ultrasonic liquid level sensors on the market mount to the top of a tank as shown in Figure 1, and are sometimes known as “top-down” sensors. The reason this is most often the preferred configuration is that the only medium the sound waves will travel through before reflecting is air, a medium for which the speed of sound is well known.