nuckolls.bob(at)aeroelect
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 Posted: Mon Jan 02, 2012 11:06 am Post subject: Capacitive Fuel Level Sensor - How do they work? |
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At 10:41 AM 1/2/2012, you wrote:
The electronics have been explained by Joe and Sarg, and Sarg
elaborated on his experience with Princeton probes.
But the reason for installing cap probes has gone unanswered.
Cap probes allow for fuel measurement of irregularly shaped tanks. My
fuel bays in the wing change in taper, both in depth and width, in
addition to the whole tank sitting at an angle (dihedral), making any
kind of accurate measurement impossible over the tank range when
using a linear type (float) sensor.
I have installed 8' Centroid probes. Haven't fueled them yet.
They come with two small potentiometers built in to adjust the empty
and full points. Available in 0-5v output and various resistance
output so you can use them with your existing gauge.
http://www.centroidproducts.com/tableofc.htm
Not a recommendation, just a resource, as I have no real world
experience with this product, . YMMV.
Capacitively sensed liquid level measurement
has been around for a very long time. This patent
dated in 1937 speaks to liquid level control by
exploiting the variability of capacitance due to
presence or absence of liquid between two plates:
http://tinyurl.com/8xqfqlo
This fuel level meter patent dated 1945 goes to the
idea of calibrating a pointer on an instrument to show
liquid level in the fuel tanks
http://tinyurl.com/6qwfdyr
This idea patented in 1952 speaks to the technique
of tailoring a probe to compensate for tank shape
with the idea of providing a change in capacitance
that is linear and proportional to useable volume
of liquid in the tank.
http://tinyurl.com/6ogp4ve
This one patented in 1965 was aimed at the automotive
market. Too bad it didn't catch on. The float style
gauge in my 2002 Kia is no better than the one in
my 1941 Pontiac.
http://tinyurl.com/86mseoz
This 1973 patent speaks to one of dozens of techniques
for resolving differences in small changes of capacity
into corresponding changes of current to some panel
mounted indicator.
http://tinyurl.com/73dgo2n
The first time I saw a delta-C fuel gaging system was
on the B-52 and 1962. I didn't work that system
but knew fellow techs who did. Got to peek inside
one of the black boxes in the lab . . . lots of vacuum
tubes.
Here's an exemplar schematic for a system several
generations newer than the one in 1960's B-52. These
are hand-built, and a gazillion parts most of
which can affect stability and accuracy of the gaging
system. This configuration went into production in the
early 1980's.
http://tinyurl.com/7hbwkr6
http://tinyurl.com/6mpgdeo
The very last new design study I did for Hawker-Beech was
to explore ways to replace the schematics above. This
prompted a study that continues to this day . . . that has
poor likelihood of finding its way onto an HBC aircraft.
Nonetheless, I can report to you that it takes less than
3% of the parts count to craft a capacitance fuel
gaging system that uses simple, non-compensated capacity
probes. It can be calibrated to the volume vs. shape variances
in software. The system can include a reference probe in
the bottom of the tank that allows the system to measure
dielectric properties of the current load of fuel that
varies with mix and temperature. The best thing is that
there are less than 5 components that have a strong
influence on calibration drifts with temperature and
time.
The project is on a back burner but the client hasn't
abandoned the program . . . if and when the aviation
community climbs out of the crapper, I'm hoping that
we can spool this effort up again.
In any case, know that most of what's being offered
today has roots that go back many decades. Do a search of
http://freepatentsonline.com and you'll find a wealth of
history along with examples that range from rather practical
to bizarre in terms of performance. But devices as
simple as a $1 PIC micro-controller . . . fitted with
well crafted software can do a very good job of
telling you how much fuel is in the tank. Further,
cost of manufacturing is a tiny fraction of that
invested in systems flying on virtually all GA aircraft
today.
Bob . . .
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