<<I extended the wing tanks on my 360 and took the opportunity to
install a homebuilt capacitance level sensor...Mine is
simple, just a length of 1/4 inch tubing with holes every six inches, and a
wire thin teflon insulated wire suspended near the center. It has 133
pF capacitance empty and about twice that full.

Now I need to build a circuit to read the capacitance and pass along the
correct signal to some flight instrument computer. I have been told
that these computer systems take a TTL square wave at about 3000 Hz, with
a frequency carrying the level info, and that the computer gets
programmed during setup
with the empty and full levels.  so, the precise frequency for empty
or full is not critical.  I breadboarded up a TLC555 circuit today and it
seemed stable.

-I am correct that these systems use a 3000 Hz TTL square wave?
-Is there a published (or defacto) standard?

Jeff Peterson>>

Jeff, I think you are on the right track.  The one typically used, that
Lancair sells, uses a "555" oscillator with the typical "RC" time constant
probably the same as you have done, and they use a frequency output.  And
you are right, the units are not pre-calibrated to a standard.  I believe
low fuel levels (low capacitance) is high frequency, but I'll see if I can
dig up my records today.  The problem I see is that it is important to keep
the internal electrode centered in the tube as any significant movement will
cause an output shift.  That's why the standard one uses a 1/4 inch tube for
the inner electrode along with spacers every couple of feet.  The thin wire,
although it will work, can be expected to move around inside the tube (ask
me how I know).  Also, I'm assuming you have the outer tube grounded as
otherwise the EMI situation is even worse - the basic capacitance probe has
enough problems with EMI even with the outside tube as a ground shield.
Another source could be the RV list as many of them use home-made
capacitance measurement systems.

Gary Casey