snips..
Fuel level sensors are easy to make. Concentric metal tubes open
on
both
ends form a capacitor with fuel as the dielectric. Hook this
capacitor
to a Radio Shack 555 IC oscillator and thence into a counter. As
the fuel
is burned off the capacitance changes and that changes the
frequency of the
555 oscillator hence the count also changes. Counters with LCD
can
be
cheaply cannibalized from low cost digital VOMs.
http://www.uoguelph.ca/~antoon/gadgets/555/555.html
Paul Lamar
Thanks Paul,
Good advise. I am afraid the SeaWind header tank vent design is
on
the
engine pylon facing forward, hence it might well pressurize the
header
tank. Not advisable.
The html location would not open but I am familiar with the 555
series in
timer applications. Hadn't thought to measure capacitance in that
way.
Clever.
How to keep the tubes separate? Plastic inserts?
Thanks
Doug in Japan
You make up a bunch of little Teflon spacers on a lathe.
Other types of plastic may or may not be compatible with
alcohol and gasoline.
Paul Lamar
Paul,
So how does 1/4" tube inside of 1/2" tube sound? I imagine the
Teflon
spacers need slots to allow fuel to travel between the two pipes
correct?
Doug in Japan
Correct on the slots os small holes. My guess is the diameters
need to be
closer.
What wall thickness for the half inch OD tube do you have in mind?
Paul Lamar
Paul
Still clue less on actual sizes. Probably better to look at sensors
already
out there. I have heard that if they much over 4 feet they are
hard
to
calibrate. Guess it depends on size and material though.
Doug in Japan
There is a formula around for parallel plate capacitors with
an air dielectric. Also the dielectric constant for gasoline
and alcohol can be looked up on Google I suspect. The differnce is
what you are after. Here it is one example. Oil is 2 to 5
depending
on type. Here is the equation in jpg form.
If you place most of the sensor along the top of the tank and
gradually bend it down near the bottom or fuselage end of the tank
it will be more accurate where it counts. Namely near the end of
the fuel supply.
BTW the holes to pass fuel do not have to be in or just in the
plastic
spacers. They can be anywhere along the outer tube.
Paul Lamar
Jim Weir did an article on this in Kitplanes a while back. Look at
the
"Capacitive Fuel Gauge" articles.
http://www.rst-engr.com/kitplanes/
Scott Gettings
Yes he did but that is apples and oranges to what I am suggesting.
He used parallel flat plates for the capacitor so if there is any
dihedral
in the wings it will only measure the last half of the fuel supply
roughly.
Another big difference is the sophistication of the electronics. Use
of a 555 IC as a variable frequency oscillator is much much
simpler
than making an oscillator out of several op amps. The third
difference is
the use of a low cost digital frequency counter to indicate the
quantity of fuel. This is inherently much much higher resolution
than any analog gage such as Jim Weir used in his design.
Paul Lamar
Scott
Thanks for the intro to the web site. A lot of interesting info
there. I
thought of another way to make a inexpensive fuel sensor. It is
based
on an
idea a group of my research buddies were commissioned to develop for
a
Swiss
client about 23 years ago. Ready for this? It was a rubber duck
that
when
placed in water would activate the circuit and then sqwuak when the
temperature got too hot. OK definitely not one our more challenging
projects
but we did develop a small sensor the activated when wetted. Perhaps
seven
of these epoxied into the tank would be good enough to indicate 1/8
of tank
increments.
Doug in Japan
Doug,
I would be interested in some of those sensors. Are they easy to
acquire. I am worried about identifying fuel levels in my Glasair, as
you stated earlier, it is difficult when you have one main take (the
leading edge of the wing) with a dihedral. Any help with these
sensors would be appreciated.
Barry Doublestein
Doug/Barry,
Why not use some of these GEMS electronic fluid level sensors. They
are totally solid state. They use an infra-red beam reflecting off a
prism type lense. When submerged, the light beam is disbursed, when
above the liquid, the beam is reflected back to the IR receiver and a
signal is generated. I put one in each of my wing tanks at the 6
gallon level as a low fuel warning. They are connected to two "Low
Fuel" LED's on the panel.
Mark S.
Barry/Mark/Scott
We use many GEMs sensor on our water purification industrial equipment
and they are pricey. From $100 and up for the ultrasonic (through the
tank wall) units. Omoron Inc. and many other companies now make
floatless level sensors similar to what we developed years ago. Barry
our rubber duck was a "one off" request to supply the prototype. I
understand it did go into mass production in Europe. I don't know if the
product is still afloat though:) That was years ago.
Cheap magnetic float sensors can be had for a few dollars. Then again
you
are limited to only being able to monitor a few points and they need to
be mounted straight up.
Wiring for multiple sensors is a issue we haven't touched on yet either
Unless someone develops a sensor with a series conducting of concentric
bands around a non conducting rod we are probably have to go with the
concentric tubes idea to be able to calibrate the fuel I mean full
range. BTW Paul, the dielectric constant for ethanol is around 30
compared to gasoline's 2.
Doug in Japan.
Wow that is amazing. Sounds like alcohol/metal would make a great
capacitor. I was looking for a web site that had cheap LCD digital VOMS
with frequency counters. So far no luck. I have one at the
hangar so they are out there. DigiKey has a Sub Cub T part number
RLC07 timer counter for $29 that looks like it will do the job. Page
1402 on the 2004 cat. 5 digits or one part of fuel in 10,000.
Talk about resolution in a 2000 liter tank that would
be 200 millleters resolution if my funny French math is right :)
There are others with up to six digits. Can also be used as a digital
tach.
Paul Lamar
To get around the varying dielectric constant of whatever fuel might be in
the tank, a reference cap (small, parallel plate, in a location always
submerged) can be used. It would take a slightly more complex level
computer (or circuit) to use the reference to offset the main sensor
value.
Matt-
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