> Could the pressure increase inside the tank going up with
> speed
> via the venting cause a capacitance variation thus a
> different DC
> reading.
>
> It could be verified on the ground by increasing
lightly the
> pressure in the
> tank.
> Or in flight taking a reading in slow flight and another
> one at
> cruising
> speed.
>
> Just guessing have no logical explanation for such a
> phenomena.
>
> JP
>
>
>
> JP,
> I paid close attention to the fuel level readings on my last
> flight and
> I have to say that they appeared to be pretty steady in level
> flight. I
> may have caused some of the change in readings via
uncoordinated
> turns.
> Not that the ball was all the way left or right, but just a
> little,
> possibly enough to cause the fuel inside the capacitive
probes
> to move
> up the tube a bit, which would cause the fuel readings to
vary a
> bit.
>
> The big surprise was when the right gauge, the one with the
> Dynon CTV
> converter installed, went to zero as soon as I keyed the mic.
> Up to
> that point, it had been the more reliable of the two. I'll
> have to
> check again, but the left gauge, the one with the stock EI
> circuitry
> (square wave a/c) appeared to drop by a few gallons while the
> mic was
> keyed. An undesirable feature, but not as drastic as the
right
> side.
>
> I plan on doing the following:
> 1) re-route the antenna cable away from the efis and other
> wiring
> 2) mount the CTV in a grounded metal box
> 3) run a separate ground from the capacitive probe to
the a/c
> ground bus
> 4) Purchase an EI fuel gauge and hook them up and forget
about
> reading
> fuel levels on the EFIS. ;-)
>
> Mark S.
>
>
> It's somewhat likely the noise is radiated as opposed to
conducted to
> the sensor circuit.
>
> Do you have access to a handheld transceiver (with a "rubber
ducky"
> antenna)? While transmitting with it (on a locally inactive
> frequency),
> you may be able to identify a portion of the circuit that is
> vulnerable
> to the RF radiation. If you don't find any part of the circuit
> responding to the handheld then grounding properly likely will
> improve
> things.
>
> There are instruments sold by Van's which are vulnerable to
RF. The
> only solutions found have been to shield the instrument (in your
> case,
> probably the CTV box) with metal - heavy foil may work, but a
> metal box
> might be better. Or ignore it. When the mic is un-keyed, do the
> readings return to normal immediately?
>
>
> Regards,
>
> Matt-
>
> Matt,
> I hadn't considered using a hand-held radio to trouble-shoot this
> problem. I'll give that a try. Once the mic is unkeyed, the
readings
> will return to normal, but not immediately since they have a
built in
> damping factor.
>
> For now, my most reliable method of tracking remaining fuel is
with the
> Real World Solutions EM-2 engine monitor. This unit calculates the
> fuel
> used based on the duty-cycle of the injectors. I enter the
number of
> gallons added when I refuel, and it counts down (or up if you wish)
> from
> there. Even though it doesn't track fuel levels in each individual
> tank, it is still a pretty nice backup system.
>
> I also have a 7-gallon low fuel warning light for each tank, driven
> by a
> GEMS sensor. Again, nice to have a backup system when the
primary is a
> bit quirky.
>
> Mark
>
> Hi Mark,
> Matt may be on the right track with the handheld idea. If its
possible
> align the antenna of the handheld as parallel to the installed
cap. tube
> as you can then key the handheld while someone else watches the gage.
> Get as close as you can to get the greatest and quickest effect.
If you
> get no response try aligning perpendicular to the tube and key the
> handheld again. It's possible that you were lucky (?) enough to
get your
> installed tube and installed antenna exactly in parallel
> geometric planes and the radiated energy is the culprit. Dave M
Mark,
I think you have a diode junction somewhere that is sensitive to the RF
and is charging something that upsets the bias on your capacitance
sensor. This link:
http://rbi.ims.ca/5698-100 from a recent issue
of Design magazine will take you to the Analog Devices web site and an
article about capacitive sensors. From there you can probably find some
application notes on recommended RF bypassing techniques for the Analog
Devices IC's and capacitive sensors. Ferrite toroids on your wiring
near the termination points may be all you need to cure the problems or
maybe a 1Meg resistor somewhere to bleed charge.
Aubrey Thompson
Aubrey,
Thanks for the great suggestion. Based on the behavior of the system,
that is what I have suspected is going on all along. I'll read through
the information and maybe get some ideas on how to correct this
problem. Again, thanks for the help.
Mark
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