These sensors are usually mounted in the ceiling of a container, and send out a short burst of high frequency sound. The sensor measures the time it takes for the sound to bounce back to the sensor, and thus calculates the distance to the surface of the measured media.
Ultrasonic sensors can have very wide ranges, and are very practical since they do not have any direct contact with the measured fluid. Accuracy and consistency is usually also very good. The downside is that they can be expensive.
The output from the sensors is typically analog, and works well with the ezeio inputs.