If there is enough demand, and a reasonably standard output
format, I could make a circuit for measuring capacitance and
reporting it to an MFD. With only a little more effort it could
detect water or alcohol in the fuel and set an alarm. The size and
weight would be negligible compared to the sensing tube. Who would
know what output format would be appropriate?

Bob,

 

The VM fuel level system uses capacitance rods (tubes with an aluminum rod suspended by plastic standoffs) that indeed deliver a 5 volt square wave at the rod end.  This is delivered to the computer via shielded cable containing ground, 5 VDC and the square wave at a frequency determined by the capacitance obtained from a fuel level.  If calibrated carefully, the system is extremely accurate.  The calibrated frequencies for different fuel levels are stored in a ROM chip and used by the computer to extrapolate the fuel level to be displayed.  This method takes care of irregularly shaped fuel tanks.

 

VM says not to have a rod length shorter than 14".  Of course, the original sin of ignorance allowed me to install an 8 inch sensor tube vertically in my header tank to meet my desire for exceptional accuracy.  As the fuel level was reduced in the header, the frequencies appeared to be reversed leading to erroneous counts - the real problem being that the computer could not deal with frequencies above 24 KHz.  After studying Radio Shack write-ups, I constructed a CMOS frequency divider circuit that works nicely as a shielded bulge in the cable from the header tank to the termination block in the VM system - nice of them to provide 5 VDC right there in the wire.  The system has worked beautifully for 10 years and is extremely accurate - too bad the display is not in tenths of a gallon.  BTW, if the wings are empty and the header level goes below 4 gallons, I am in a descent to the airport below me with four red lights on - lo fuel in each tank and the system wide lo.

 

I have never had RMI interference with the fuel reporting system.  Now, temperatures are another story.  Key the mike and watch all temperature indications shift - I do not report OAT, Oil, CHTs or EGTs to my wing mate whilst talking to him unless I wrote them down in a quiet moment (thus utilizing the "mike" finger for another purpose).  I can minimize the effect by artfully moving the crappy RG-58 comm antenna line around behind the panel.  Oh well.......