The eTape Continuous Fluid Level Sensor is a solid-state sensor. It's primarily designed to be used in non-corrosive water-based liquids and dry fluids (powders). It does away with clunky mechanical floats, and easily interfaces with electronic control systems. It is fully assembled in a clear, elliptical polycarbonate tube to protect and stabilize the sensor. The eTape sensor provides a resistive output that is inversely proportional to the level of the liquid. The active sensor length is 12" (30cm).
More information is available on the websites of Milone Technologies and Adafruit.