The "Continuous Level" line of transmitters provide continuous indication with dual outputs up to 30feet! The "Level Control" line of transmitters provide two-point level control as well as alarm outputs. Both types are push button "teachable" and made of stainless steel housings that are rated NEMA 4X submersible.
The sensor sends out a sound wave toward the media in a pulsed fashion. The sound wave hits the media and reflects that sound back to the sensor. The sensor calculates the time it took for the sound to travel to determine the distance from the sensor to the media.