Paul,

        I have been secretly working on my fuel gauge/probe problem.  I
   couldn't
        get your suggested circuits to work (probably my limited
   electronics
        ability), so I broke down and purchased two capacitance-to-voltage
        converters, the same ones used by the Dynon folks and sold by ACS.
   I've
        installed one and hooked it to a voltage channel on my efis
   box, and it
        works as I would have expected.  The sad part is that the new
   circuit
        has the same problem as the old circuit in that it drifts in
   flight.
        But it returns to normal when back on the ground.  That's
   telling me
        that something is going wrong with the signal.  My guess is static
        buildup across the wings is affecting the capacitance readings
   of the
        probes.   How can I check this theory?  Any suggestions on how to
        fix it?

        Additional info:
        1)  CTV is mounted inside the cabin (old circuit was in wing root)
        2)  Pigtail from probe to CTV is shielded (old was unshielded)
        3)  CTV uses voltage a channel (EI circuit used frequency channel)

        My thought at this time is to disconnect the CTV output from
   the EFIS
        and connect it to a volt-ohm meter and go fly to see if the voltage
        reading increases or decreases unexpectedly (drawing off the other
        tank).  This should tell me if the problem is with the EFIS or the
   probe
        circuit.  I doubt it is the EFIS as other instrumentation
   feeding into
        the EFIS is rock solid.

        Mark S.
        (rain today)

        Interesting theory. Plastic wings as opposed to metal wings.
        Do you have any static discharge wicks on your wings?

        One way to check for temperature affects is keep the same fuel
        (about half full) in the wing and check it in the morning when it
        is cold soaked overnight and again in late afternoon after the
   sun has
        its way with the fuel tank.

        Paul Lamar ...No rotor no motor.


   Mark:

   FWIW: To help narrow down your options:  I believe my Lancair ES has the
   same EI fuel probes as yours, but mine are connected to a standard
   EI fuel
   gage.  I have no problems with wandering indications in flight.

   Composite wings do generate static charges in flight--especially in
   light
   rain.  I flew through a very light rain shower enroute to Oshkosh in
   2003.
   The electric trim position indicators went absolutely wild, flashing all
   kinds of lights, and then went blank.  The trim motors still worked,
   and the
   EI fuel gage indications remained steady.

   Tom Parkes



That sort of confirms what I suspect - static build up on the outside of
the wings shouldn't change the apparent capacitance of the fuel probes.
These sensors work by using the varying capacitance of the probes (fuel
vs air) to change the frequency of an oscillating circuit.  Charge build
up would be seen as a DC bias on the probes which shouldn't directly
effect the oscillator circuit.  If the oscillator circuit (or its
installation) doesn't reject common mode noise - charge buildup - that
might be a problem..



Matt-



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