This relates to capacitive sensors. The usable operating point is in the entire allowable temperature and voltage range is between 80 % and 120 % of the effective operating distance.
Ultrasonic sensors use sound waves to measure distance. A sensor would normally be mounted at the top of a tank and direct a sound wave down towards the surface of the product. When the sound wave hits the product it is reflected and returned to the sensor. The greater the distance between the sensor and the product, the longer it will take for the sound wave to travel down and back up again. The sensors calculate this time interval and give a signal proportional to the distance. They also compensate for the changes in the speed of sound due to changes in temperature. Ultrasonic sensors are not suitable for use in all applications. If the product gives off vapours then this can effect the measurement, and if there is foam on the surface then the sound wave will be scattered and the sensor will not receive a signal that it is able to process accurately.