Abstract: An electronic sensor for detecting when a septic tank reaches it's design capacity. The sensor uses a sonar transponder to measure the thickness of the three layers of material present in a properly operating septic tank. The sonar transponder is encapsulated in a environmentally sealed sensor assembly that floats in a self-orienting fashion at the top of the material contained in the septic tank. The sensor relays signals through a cable harness to a signal interpretation module located in a convenient indoor location. The signal interpretation module contains a digital signal processing (DSP) sub-system and an alarm sub-system. The DSP sub-system interprets the electronic signals from the sonar sensor resulting in a measurement of the relative volume of the material in septic tank as well as measures of the elevation of the bottom of the top (scum) layer and the top of the bottom (sludge) layer. Any of these measures may indicate that the tank has reached it's design capacity and should therefor be pumped.