> 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.
>
>
> So, just doing a touch and go restores the values to normal? Or
do you
> have to cycle power to the system? Does cycling the power in
flight
> have any effect?
>
> If it's static in the tank - caused by sloshing fuel then wicks
on the
> outside won't help.
>
> There are many plastic airplanes that are successfully running
> capacitive fuel probes. I'd think we can get this licked.
>
> Can you doodle out a basic schematic of how the probes are wired
> (including any grounds)? Is one side of the probe grounded or
are they
> both floating?
>
> Might post the question to the aeroelectric connection (on the
matronics
> board).
>
>
>
> Regards,
>
> Matt-
>Mark,
>Do you have a grounding strap into the fuel tank to discharge
static
electricity when fueling?
>Bill Schertz
Hi Mark,
I know this is an ongoing problem for you, but could you refresh
us with
the condensed version for those of us who have forgotten. Also
do your
gage readings show increased or decreased readings while in
flight? In
straight and level do the readings fluxuate? Do the readings
change in
both tanks and do they do this simultaneously? From what I can
remember,
I am leaning toward a possible problem with the physical
installation of
the capacitance tubes or the tubes themselves. Dave M
Dave,
Normally, what I notice is that the fuel level creeps up. Today, they
seemed to be behaving, with a little fluctuation, but I think that may
have been due to uncoordinated turn(s). I cruised along in level flight
for a while and they seemed to be reading as expected. What I really
noticed for the first time today was the right gauge went to zero when I
keyed the PTT.
A little clarification may help at this point. I installed the Dynon
capacitive-to-voltage (CTV) converter on the right tank only. It is fed
into a voltage channel on the BMA EFIS-1. The left tank is still
hooked up as it was from the beginning using the Electronics Int'l
capacitive to square wave converter that came with the probe. It is fed
into a frequency channel on the BMA EFIS-1.
So, today when I keyed the mic, the right tank gauge dropped to zero,
while the left gauge only dropped a little. When I released the mic,
they returned to normal. So, it sounds like I'm getting crossfeed from
my com radio into the fuel gauge circuits. Any suggestions on how to
fix this?
Mark S.
A wiring dwg would help. Not a schematic.
What I would like to see is a scale top view dwg of the airplane
showing how the wires are running, what is connected to what/where,
and where
the radio antennas are. The long aluminum fuel sense coax tube capacitor
could be acting as an antenna and picking up RF from the 3 to 5 watt
radio.
The trouble with plastic planes from a wiring point of view is there is
no real solid large area ground.
Is one side of the tube fuel sense coax capacitor connected to ground?
If so it should be the out side of the coax tube and not the inside.
I think this is a different problem than what you had before.
Did you notice any change in the fuel volume reading after pushing
the airplane out in the sun? Fuel has a very high thermal expansion
coef. Racers used to cool the fuel storage tanks in the pits so
they could get more BTU's in the race car tanks. That way they did not
have to pit quite so often. If you fill an airplane's fuel tanks and let
it sit in the sun fuel will puke out of the fuel tank vents due
to expansion.
Paul Lamar ...No rotor no motor.
Paul,
The outside of the fuel probes are connected to the outer shell of the
BNC on the Dynon CTV converter. I assume this is ground and that the
ground passes through the CTV and out the other side, but I can't be
certain as you can't see inside. There is a black wire going from the
CTV to the EFIS-1. This ground is via the D-sub connector on the EFIS.
The Nav/Com coax passes down the left side of the a/c. They are bundled
with all the other wires passing down that side. The antenna is in the
vertical stab. I could try moving the coax and see if things improve.
Mark
That is not too great. It would be better if the electronics was in
a grounded metal box. I would feel better about the BNC shell grounding.
Good idea to move the coax. When the antenna is not mounted on a large
metal plate the coax may become part of the antenna. You might do a bit of
research on this to check on me. I think it has to do with the quality
of the impedance match between the coax and the half wave antenna.
The impedance of the coax used must match the impedance of a half
wave antenna which should be about 50 ohms. Check the ohm impedance rating
on your coax.
There is an article in January 2008 Sport Aviation page 101 on composite
aircraft antennas. I suggest reading that to see if your antennas conform
to those mounting suggestions.
Also read this site as it is a bit more accurate and comprehensive than
the Sport Av mag article.
http://www.electronics-tutorials.com/antennas/antenna-basics.htm
There is a gadget that hams use called a field strength meter
that might come in handy if you can borrow one.
By all means try moving the coax.
Paul Lamar ...No rotor no motor.
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