Low cost liquid level sensors are commonly implemented with ultrasonic, optical, mechanical, and impedance based techniques. These sensors are commonly used in coffee machines, water dispensers, and fuel gauges. Ultrasound can provide superior results over optical techniques due to the ability of sound to propagate through vapor. Optical techniques can also encounter problems when reflective containers are used.

The ultrasonic transducer does not need to be in contact with the fluid. Therefore, ultrasound is resilient to fluid residues that can compromise the accuracy of mechanical or impedance based sensors. Ultrasonic sensors can also provide high resolution measurements (~20 microns) without imposing any special requirements on the container which holds the liquid. These measurements can be conducted with less than 25 μA of current consumption when using the MSP430FR6043. The MSP430FR6043 solution uses an ADC-based correlation technique that enables higher-resolution time of flight measurements when compared to other ultrasonic timer based solutions.

The EVM-FR6043 comes with software that enables ultrasonic absolute time of measurements to be taken and visualized through a GUI as described here: Ultrasonic sensing subsystem reference design for gas flow measurement. In the experiments described in this document, the EVM430-FR6043 is used with a single Jiakang 200 kHz transducer, which can be sourced from Jiakang. A 3D printed fixture is used to mount the transducer for experimentation. A half teaspoon of liquid is added to the cup to demonstrate the sensitivity of the system.

In order to obtain proper signal levels, the transducer should be placed as close as possible to the liquid level that is being sensed. Figure 1-3 depicts how the transducer is mounted with respect to the container and the water which is in it. The ultrasonic wave is reflected from the surface of the water and the roud trip time of flight is determined based on the correlation between the transmitted and received ultrasonic signals.