Simple...
A capacitor is two conductive surfaces and a dielectric between them. The dielectric can be air, water, acid or most anything that is mechanically compatible. The dielectric doesn’t even have to be in contact with the surfaces. The dielectric is going to react to the field of voltage potential between the two conductors. The closer to the conductors and the larger the area of the conductors combined with the dielectric’s properties give you a capacitance vale. Some dielectrics work better than others. In this case, the fuel is the better dielectric than the air inside the tank. More fuel, more area of dielectric, the higher the capacitance. Lower capacitance for lower fuel levels.
Placing two metal surfaces on the OUTSIDE of the tank (window-alarm tape works great) with a very small vertical space (the two strips go up and down on the side of the tank) creates a capacitor. When fuel is added and more of the area of the metal strips is covered with fuel, the higher the capacitance value. The capacitance can be read on a PIC using POT command (read up on it). The PBP manual shows the capacitor part of the equation. There are other ways to do this too, but effectively what you are doing will be to check the R-C time decay constant with the PIC. Keep in mind that the capacitance range and values will be very small. Getting the hardware values right will be the hardest part.
Oh, I’m sorry… you a said simple explanation… two vertical metal strips placed close together on the side (outside) of the fuel tank (PLASTIC TANK). Use this for the capacitor value in the POT PBP command.
Check the web for examples to get you in the ballpark. I've seen and read about this before, but I can't remember where right now.
Originally Posted by ngeronikolos
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