Bad fuel gauges?

Andrew Sarangan wrote in
:

The reason for the weight sensor was because he did not want to
penetrate the fuel tank to add his sensors. Once inside the fuel tank,
I do not see the benefit of an optical sensor vs a float sensor. In
fact, I don't quite understand why float sensors are so inaccurate in
the first place. It is just a variable resistor. The shape of the fuel
tank can be easily calibrated out. Averaging the sloshing is equally
easy to do. Anyone know what makes them so notoriously inaccurate?


The mechanical ones, like the wire in a cub or T-craft, was very reliable.
The mechanicla dial gauges you see in the Luscombe, Citabria and some older
Cessnas are also pretty good, though you have to undertand what the
airplane's attitude can do to some indications. It's when there is an
electrical connection that it seems to go wrong.

There is one system I have seen in an old car that was extremely accurate
and reliable. It had a tube that ran from the panel to the fuel tank and
inside th etank it branched out to a a number of tubes, about eight, I
think. Each tube had a little pan shaped dish on it. In the panel there was
a small liquid barometer type device theat sensd the pressure from the
tank. The liquid in the gauge was a red substance, supposedly metal, that
showed the level in eights of a tank. I can't remember what the stuff was
but it was extremely dangerous to handle and you had to send the gauge to a
specialist to have it filled.
It worked really well, bu tI doubt it would be much use in an airplane.


Bertie

Larry Dighera wrote in
:

On Sun, 24 Feb 2008 14:45:58 -0800 (PST), Andrew Sarangan
wrote in
:

Anyone know what makes them so notoriously inaccurate?

A lack of federal regulations mandating their accuracy throughout
their range? :-)

Perhaps inaccuracy is induced over time, because of drift of the
resistance element. But I'd sooner believe, it's because there is no
regulation for accuracy.

Of course you would.


Bertie

"RST Engineering" wrote:
I wonder if a pressure sensor placed inside the bottom of the tank
could be made sensitive enough to "weigh" a column of fuel inside the
tank above the sensor. That would only work for regular sized tanks
(no triangles) but could be integrated over a long enough time to take
care of any slosh.

Just a random thought, mindya...and I haven't run the numbers.

Another thought is a string of LEDs separated from photosensors with,
say, 10 or 15 of them inside the tank mounted vertically.

Hmmm...any other thoughts for liquid level measurements? Sonar a la
Polaroid?

Just looked through my copy of "AIP Handbook of Modern Sensors" by Jacob
Fraden for existing ideas and inspiration. Here's what I got:

It shows one fiber-optic sensor that relies on the change in refractive
index between air and the liquid being measured. If you can find a copy,
check under section 5.8.2 "Fiber-optic sensors". But since it mentions
the company Gem Sensors as already having a product built on the
concept, you can go to their web site for technical details:

http://www.gemssensors.com/content.aspx?id=382

Unfortunately it is a "point level" detector. Gen Sensors also has level
sensors that use ultrasonics in a different way: impedance changes (see
for example):

http://www.gemssensors.com/uploadedF...sheetPRINT.pdf

Another idea with respect to refractive index change: use a laser diode
at the top of the tank angled down to a mirror at the bottom of the tank
and a line of receiving photodiodes or phototransistors at the top of
the tank, like so:

Laser
Diode P1 P2 P3 P4 P5 (Line of photosensors)
\ /
\ /
------------------------ liquid level
\ /
\/ (Trying to show change in angle)
-----==========--------- Bottom of tank
(mirror)

As the liquid level changes, the photosensors(s) that receive most of
the light also change. Obviously vibration will make the reception dance
around but time averaging should probably work to resolve that.

Ironically I think vibration noise can actually be used to yield a
higher resolution than that given by the number of receptors. That is,
if you had only 10 sensors but did 100 readings and got 70 hits at P7
and 30 hits at P8 you could reasonably say the average is near (70*7 +
30*8)/100 = 7.3 within (I think) an RMS of +/- 0.1 (That is, 1/sqrt(N)
where N is the number of measurements. But my statistics is very rusty.)
By contrast, if there were no noise the resolution would be limited
to +/- 1.

On Feb 24, 5:40 pm, B A R R Y wrote:
On Sun, 24 Feb 2008 12:15:07 -0800 (PST), Andrew Sarangan

wrote:

I have flown airplanes with a fuel sump drain that would not close
properly. This always happened when I took a fuel sample from the
sump, so it was was easy to notice it and fix it. But if it can happen
on the ground, it could also happen during flight.

Your sump re-opened itself?

There's a difference between not closed, as in you just removed the
jar, and a sump opening by itself.

I'm not saying it can't happen, but the two conditions are very
different.

No it did not re-open by itself, just refused to close. I ended up
with a big puddle of fuel on the floor by the time I forced it in. I
realize it is spring loaded and is not the same thing as coming lose
on its own. However, whats to say that a sudden upward acceleration
can't pop the valve open? Being on the underside of the fuselage/wing
there will be no way of knowing if there is a small leak.

Andrew Sarangan wrote:
The reason for the weight sensor was because he did not want to
penetrate the fuel tank to add his sensors.

An ultrasonic transducer mounted outside a tank might work - it would get
one reflection off the surface of the liquid in addition to reflections off
the tank walls. The tank wall reflections would remain constant and could
be ignored (or even used for calibration) but the reflection off the liquid
surface should be detected and time-of-flight or sonic interferometry could
be used to compute its location.

Andrew Sarangan wrote in news:ec390484-a14f-475c-
:

On Feb 24, 5:40 pm, B A R R Y wrote:
On Sun, 24 Feb 2008 12:15:07 -0800 (PST), Andrew Sarangan

wrote:

I have flown airplanes with a fuel sump drain that would not close
properly. This always happened when I took a fuel sample from the
sump, so it was was easy to notice it and fix it. But if it can
happen
on the ground, it could also happen during flight.

Your sump re-opened itself?

There's a difference between not closed, as in you just removed the
jar, and a sump opening by itself.

I'm not saying it can't happen, but the two conditions are very
different.

No it did not re-open by itself, just refused to close. I ended up
with a big puddle of fuel on the floor by the time I forced it in. I
realize it is spring loaded and is not the same thing as coming lose
on its own. However, whats to say that a sudden upward acceleration
can't pop the valve open?


That doesn't happen. it's usually grit in the seat that keps them from
closing properly or a badly gummed up drain.

Being on the underside of the fuselage/wing
there will be no way of knowing if there is a small leak.

Well, if you check it carefully after you drain it, it's extremely
unlikely.


Bertie

On Feb 24, 6:17 pm, Jim Logajan wrote:
"RST Engineering" wrote:
I wonder if a pressure sensor placed inside the bottom of the tank
could be made sensitive enough to "weigh" a column of fuel inside the
tank above the sensor. That would only work for regular sized tanks
(no triangles) but could be integrated over a long enough time to take
care of any slosh.

Just a random thought, mindya...and I haven't run the numbers.

Another thought is a string of LEDs separated from photosensors with,
say, 10 or 15 of them inside the tank mounted vertically.

Hmmm...any other thoughts for liquid level measurements? Sonar a la
Polaroid?

Just looked through my copy of "AIP Handbook of Modern Sensors" by Jacob
Fraden for existing ideas and inspiration. Here's what I got:

It shows one fiber-optic sensor that relies on the change in refractive
index between air and the liquid being measured. If you can find a copy,
check under section 5.8.2 "Fiber-optic sensors". But since it mentions
the company Gem Sensors as already having a product built on the
concept, you can go to their web site for technical details:

http://www.gemssensors.com/content.aspx?id=382

Unfortunately it is a "point level" detector. Gen Sensors also has level
sensors that use ultrasonics in a different way: impedance changes (see
for example):

http://www.gemssensors.com/uploadedF...c_Sheets/exose...

Another idea with respect to refractive index change: use a laser diode
at the top of the tank angled down to a mirror at the bottom of the tank
and a line of receiving photodiodes or phototransistors at the top of
the tank, like so:

Laser
Diode P1 P2 P3 P4 P5 (Line of photosensors)
\ /
\ /
------------------------ liquid level
\ /
\/ (Trying to show change in angle)
-----==========--------- Bottom of tank
(mirror)

As the liquid level changes, the photosensors(s) that receive most of
the light also change. Obviously vibration will make the reception dance
around but time averaging should probably work to resolve that.

Ironically I think vibration noise can actually be used to yield a
higher resolution than that given by the number of receptors. That is,
if you had only 10 sensors but did 100 readings and got 70 hits at P7
and 30 hits at P8 you could reasonably say the average is near (70*7 +
30*8)/100 = 7.3 within (I think) an RMS of +/- 0.1 (That is, 1/sqrt(N)
where N is the number of measurements. But my statistics is very rusty.)
By contrast, if there were no noise the resolution would be limited
to +/- 1.

All of them good ideas, but why are they likely to be more accurate
than the float level sensor?

Because

a) floats inherently leak over time unless they are some light solid, and
then they have a tendency over decades to break down and saturate.

b) the "state of the art" for float sensors use a wiper against a nichrome
wirewound resistor and the wiper tends to wear a hole and the nichrome tends
to redistribute itself across the form somewhat nonlinearily.

c) just because it was good enough for Henry Ford, it's good enough for us
{;-)


Jim



All of them good ideas, but why are they likely to be more accurate
than the float level sensor?

On 2008-02-24, wrote:
No, I haven't just proclaimed that. Rather, I've explained in detail
why I think the gauges ARE useful, in a particular way, if they're
anything like the dozens of planes I've rented in various places. And
I've explained in detail why I think they're illegal otherwise, citing
specific regulations. (There may or may not be a "reality" that
illegal planes are common--but that doesn't change whether they're
illegal.) And CJ and I and others have given reasons that fuel gauges
are desirable for safety as well as legality.

You have not, however, explained why, since fuel gauge accuracy is
notoriously unreliable for good and sufficient reason, half the GA fleet or
more isn't grounded.

Your responses are all straight out of the book, with no grounding in the
real world.
--
Jay Maynard, K5ZC http://www.conmicro.com
http://jmaynard.livejournal.com http://www.tronguy.net
http://www.hercules-390.org (Yes, that's me!)
Buy Hercules stuff at http://www.cafepress.com/hercules-390

On Sun, 24 Feb 2008 12:50:38 -0800, "RST Engineering"
wrote:

I wonder if a pressure sensor placed inside the bottom of the tank could be
made sensitive enough to "weigh" a column of fuel inside the tank above the
sensor. That would only work for regular sized tanks (no triangles) but
could be integrated over a long enough time to take care of any slosh.

Pressure transducers have sufficient resolution and range to do that.
The problem is that you need to "dial out" the G forces as the fuel
weight will vary with that. For straight and level flight it would be
pretty good.

You are not looking at a garden variety transducer, however; with
avgas at 6#/gal, it takes about 3 feet to generate 1 psi. Depending
on tank geometry and size, I doubt a piston single would exceed that 1
psi. (3 ft tall tank)