A few years ago, when I followed the Subaru forum, there was a few posts about the inaccuracy of the capacitive fuel senders when using auto fuel. A few guys had flown back from Sun & Fun, and reported that the capacitive fuel gauges were way off every time they filled up. They reported something like 300 percent variance, and warned everyone against using the capacitive fuel senders if they were going to use both avfuel and auto fuel.
Since I had already installed capacitive fuel senders in my tanks, I decided to solve this problem and create my own fuel gauges. The software solution is actually quite simple. Once the tanks are calibrated, as long as the software knows how much fuel is initially in the tanks after a reset, it can simply calculate the ratio of the calibrated value to the current value. After that is known, the software simply adjusts the displayed value by that ratio. This should work, no matter what kind of fuel the tanks are filled with.
The problem with this solution is that it requires the pilot to either confirm full tanks, or enter the amount of fuel after filling up. This is not too big of a problem, since my engine monitor already requires this, but it would be better if the conversion was automatic. As someone mentioned earlier, this could be accomplished by installing a second smaller plate in the bottom of the tank to do the calculation. If the calibration sender was installed against the bottom of the tank there would be no problems until the tank was almost empty, depending upon the size of the calibration plate and the gap between the bottom of the tank and the plate.
I should note, that I did the software for the gauges, but I never tested it using real fuel, since I am still a long ways from flying. Also, another thought was to create just the senders, which could be used with any gauges or engine monitor, but other priorities got in the way.
Tracy.