Capacitive fluid level sensor

This side project got started out of a need to come up with a better fuel level sensor for my vintage aircraft. In 1947 when the aircraft was built, it used to have a simple float and wire-wound rheostat sensor mounted in the bottom of the 6" tall tanks. The original sensors wore out long ago and are no longer available.

While replacement sensors from the automotive world will fit mechanically they are not a good choice because they are meant to be mounted from the top. Using them means that a empty tank would read full on the gauge and visa versa, not generally a good thing !

I choose a capacitive measuring approach as it is not sensitive to the direction of mounting and has no mechanical parts to wear out.  The element consists of two concentric cups in which the fuel acts as the dielectric material. The capacitance of the element changes in direct proportion to the height of the wetted area between 30 and 60 pf.

The electronics consist of a 555 timer, a TI MSP430 and a FET. The timer provides the oscillator, the TI controller counts the resulting frequency and applies compensation and calibration factors, then converts the result into a PW modulated signal to drive the FET output stage. The output hooks to a regular 30-240 Ohm analog gauge on the panel.