Capacitive fuel sensors - question to the EFIS manufacturers

One of the biggest arguments of people arguing against capacitive fuel sensors seems to be that they yield different readings for different fuel types. Still, the physics behind these sensors is fairly simple -- we take a capacitor and fill it gradually with a dielectric, thus changing its capacity. Different fuel type -> different permittivity -> different capacity when full. Nevertheless, the capacity for an empty tank is constant, the volume and shape of the tank is constant, so we have only one unknown variable: the permittivity of the fuel.

Assuming we have calibrated the system for a certain fuel type and now want to use a different fuel. Can't we just top up the tank and then tell the EFIS "my tank is now full"? I.e. a push-one-button recalibration for which only the permittivity is changed?

In principle we could even put a small calibration capacitor in the very bottom of the tank such that under normal circumstances it always is immersed in fuel and we can use it to obtain the fuel permittivity, but that would require additional hardware whilst the first solution is software only.

Am I missing something?

hendrik said:

In principle we could even put a small calibration capacitor in the very bottom of the tank such that under normal circumstances it always is immersed in fuel and we can use it to obtain the fuel permittivity, but that would require additional hardware whilst the first solution is software only.

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In newer commercial "heavy metal" aircraft, that's called a densitometer. Modern day experimental glass should be able to do this, but as you said, "..additional hardware [and wiring]."

capacitive

I also would like to have more info on this subject. Recieving my wings soon and I have chosen the capacitive route and am wondering if I made the right decision here. I really like the idea of having no moving parts, so I would like to know how big of a problem this is with different fuel density. I will probably be running the same fuel most of the time, unless alt. must be purchased while traveling.

bird