I used a the previously mentioned optical sensor for monitoring deisel fuel levels of backup generators for the Fort Worth emergency fire and police radios. Its low current, low voltage, totally enclosed, no explosive risk (as when used with other fuels) and ints the easiest solution.

The small sensor has a led emmitter and detector side-by-side in a plastic or glass optic head. If you remember your physics, when air surrounds a glass prism, its reflectance is different for when a liquid does. Its a physics method of getting a clean ON/OFF condition as the surrounding air or liquid change the prism between two states:

1). LED emitter light strikes the glass or plastic prism and passes outside of the sensor head leaving the detector with no feedback signal.

2). LED emitter light strikes the glass or plastic prism and reflects back into the detector giving it positive feedback.

Its the best design. I don't recall the company, you should be able to find it online. The sensor technology is patented.

aka J (running on his new computer)