Fuel level senders

[keith]

Wrong. From http://www.ntsb.gov/Publictn/2000/aar0003.htm

"The National Transportation Safety Board determines that the probable
cause of the TWA flight 800 accident was an explosion of the center wing
fuel tank (CWT), resulting from ignition of the flammable fuel/air
mixture in the tank. The source of ignition energy for the explosion
could not be determined with certainty, but, of the sources evaluated by
the investigation, the most likely was a short circuit outside of the
CWT that allowed excessive voltage to enter it through electrical wiring
associated with the fuel quantity indication system."

They don't really know what caused the CWT to blow, but they are pinning
the problem on the FQIS. So much for the theory that fuel level senders
are safe. Cars should be blowing up right and left on the freeway from
the senders, not to mention immersed electric fuel pumps.

What I wonder about is: Following the accident, the FAA attempted to
identify and interview the crew of every aircraft operating in the area.
One aircraft, shown on radar flying in circles nearby, was never found
and no crew ever came forward. There is one type of aircraft that
exhibits this behavior. A target drone.

Shiny side out! How many times do you need to be told. Sheesh!

 

[Homer.Simpson]

keith said

Shiny side out! How many times do you need to be told. Sheesh!

Actually, it's recommended you do both.

############################

http://zapatopi.net/afdb.html#BUILD

A Note About The Shiny Side:

It can't be stressed enough how important it is to have the shiny
side pointing out. This is needed because the shiny side is most
reflective to psychotronic radiation, while the dull side can
actually, in certain environmental conditions, absorb it. However, as
is illustrated in the instructions above, it is also wise to
complement this with a layer of foil pointing shiny side in. This
will keep your brain waves, which are also reflected by the shiny
side, from being picked up by mind-reading equipment. There is a
small number of aluminum foil researchers who believe that this may
cause an alpha-wave harmonic to build up in the skull resulting in
memory loss or pseudo-religious visions, but their findings have
never been replicated by the aluminum foil research community at
large. Even if their findings are validated, the risk involved is
small compared to the potential of mind-intrusion.

############################

 

[Ross Herbert]

Could someone please explain why rheostat type fuel level senders as
used in automotive petrol tanks are considered safe? Intuition would
suggest that an intermitent contact could pose an ignition problem,
especially when the tank is full of vapour.


- Franc Zabkar

It is many years since I pulled open a fuel level sender but I do
remember the resistive element was fully enclosed. Also, since the
moving contact is tied directly to one end of the resistive element
then the circuit is always maintained even if the moving contact
becomes open circuit momentarily. Since there is always a circuit
maintained then all that can happen is the full resistance applies and
there is no risk of arcing.

 

[Franc Zabkar]

They (and the electric pump) are usually submursed under the fuel. It's
never a good idea to run the tank dry because the fuel also lubricates
the pump bearings.

What prompted my posting was an aircraft sender that mounts at the
side of the tank, right in the middle. The rheostat is uncovered as
soon as the fuel level drops below half.

AFAIK, most (all?) senders locate the rheostat well clear of the
bottom of the tank. Here are some examples:

http://img.alibaba.com/photo/10787416/Fuel_Level_Sender_Of_Toyota_Premio.jpg
http://www.greengauges.com/help/media/tankinstallj01.gif
http://store1.yimg.com/I/rodi_1845_1527907
http://www.wicksaircraft.com/catalog/images/subcategories/3424_c.jpg
http://www.racerpartswholesale.com/images/sw114876.jpg
http://www.vertox.org/35min.jpg

FWIW, the aircraft's digital electronic fuel gauge is a strange
homebuilt design based on an LM3914 LED display driver IC:

http://members.optusnet.com.au/~fzabkar/Fuel_Gauge2.gif


- Franc Zabkar

 

[Franc Zabkar]

It is many years since I pulled open a fuel level sender but I do
remember the resistive element was fully enclosed. Also, since the
moving contact is tied directly to one end of the resistive element
then the circuit is always maintained even if the moving contact
becomes open circuit momentarily. Since there is always a circuit
maintained then all that can happen is the full resistance applies and
there is no risk of arcing.

Not in my case. One end of the resistive element is NC, and the
element is not sealed. In any case, how could you seal a moving float
arm?


- Franc Zabkar

 

[Ross Herbert]

Not in my case. One end of the resistive element is NC, and the
element is not sealed. In any case, how could you seal a moving float
arm?

The shaft of the rheostat came out of the case via a bearing and it
had a small cage fitted with what I suspect was a carbon graphite
seal. the float arm was of sufficient length that the inertia required
to rotate the shaft was of little consequence.

 

[Ross Herbert]

Thanks, that makes sense. I guess it is very difficult to achieve a
stoichiometric ratio inside the fuel tank.

I recall a friend who was a panel beater telling me that he used to
weld petrol tanks after filling them with fuel. His colleagues thought
he was mad, but he survived, only to die of prostate cancer many years
later.

My brother was experienced at welding fuel tanks. He connected the
exhaust pipe from a running vehicle to the tank so that it became
filled with carbon monoxide and could safely be brazed with an oxy
acetylene torch. He also demonstrated the non-flammability of petrol
(in open air mind you) by flicking a lighted cigarette butt directly
into a container of petrol. ffft... out went the glowing butt.

 

[Pooh Bear]

Wrong. From http://www.ntsb.gov/Publictn/2000/aar0003.htm

"The National Transportation Safety Board determines that the probable
cause of the TWA flight 800 accident was an explosion of the center wing
fuel tank (CWT), resulting from ignition of the flammable fuel/air
mixture in the tank. The source of ignition energy for the explosion
could not be determined with certainty, but, of the sources evaluated by
the investigation, the most likely was a short circuit outside of the
CWT that allowed excessive voltage to enter it through electrical wiring
associated with the fuel quantity indication system."

They don't really know what caused the CWT to blow, but they are pinning
the problem on the FQIS. So much for the theory that fuel level senders
are safe. Cars should be blowing up right and left on the freeway from
the senders, not to mention immersed electric fuel pumps.

Those fuel pumps actually use the fuel to help cool them ! The commutator area is
sealed however !

Actually the problem lies not so much with the FQIS itself but wiring ageing
problems in old jets.

I've seen pics of arc-over between cables in airframes and it's no joke. The
presumption is that something arced over to the FQIS and that provided sufficient
ignition energy.

What I wonder about is: Following the accident, the FAA attempted to
identify and interview the crew of every aircraft operating in the area.
One aircraft, shown on radar flying in circles nearby, was never found
and no crew ever came forward. There is one type of aircraft that
exhibits this behavior. A target drone.

I doubt that the full truth will ever some out.

Having said that - the NTSB's conclusion is likely correct. There have been several
aircraft lost on the ground due to fuel tank explosions too.


Graham

 

[Ross Herbert]

Not in my case. One end of the resistive element is NC,

It appears that fuel in a properly capped fuel tank is almost
impossible to ignite according to http://www.fireservicecollege.ac.uk/

QUOTE:
The fuel tank of a car or light goods vehicle can contain a large
quantity of petrol and petrol vapours. Extensive firefighting
experience and lab tests have shown that fuel tanks very rarely
explode during a vehicle fire. Though these reservoirs of potentially
dangerous and explosive liquid and its vapours may appear to present a
real risk, it is however nearly impossible to ignite the contents in a
fuel tank that is properly capped. This is because the vapours of
petrol are much heavier than air and tend to fill the tank completely
to the top. The vapour pressure of petrol is such that it tends to
form a mixture with air in the tank that is above the explosive limit
and can not be ignited.
ENDQUOTE:

Full document here
http://www.fireservicecollege.ac.uk/cssc/fi2/Risk of Explosion in vehicle fires.PDF

Even with the potential for this particular sender to generate arcing
the possibility of an explosion is considered to be impossible.

 

[Pooh Bear]

Paul! Paul! Paul! Paul!

Get real. Everyone in the industry knows the failure mechanism was in
the tank-to-tank pump.

Sheeesh!

Don't you mean the scavenge pump ( that was never recovered IIRC ) ?

Graham

 

[Pooh Bear]

Extensive firefighting experience and lab tests have shown that fuel tanks very rarely explode
during a vehicle fire.

Except in Hollywood !


Graham

 

[Terry Pinnell]

Years ago, helping a friend respray a car, we had a steel dustbin that
gradually got filled up with old rags soaked in cellulose thinners. At
the end of the day we decided to burn the rags, so we lit them through
a small hole in the bottom of the bin.

We never found the dustbin lid, and I have no idea how high it went.

Could explain a few subsequent flying saucer sightings <g>.

 

[Boris Mohar]

I've never really doubted that petrol tanks are safe. I just wanted to
know why. As for low current being the reason, I'd expect that the
tank would need to tolerate the situation where the full supply
voltage was applied to the sender when it was at the low end of its
resistance range. I imagine this could occur as the result of a wiring
error during a dashboard repair, say, or if the fuel gauge failed in
some peculiar way.


- Franc Zabkar

Than you shouldn't doubt this. I would if I haven't seen it with my own
eyes. One day I vas visiting my mechanic and there on the bench was
dismantled main fuel pump from a Volvo. I picked it up and saw that it was
soaked in gasoline. It was explained to me that gasoline is used to cool the
pump. It flows directly through the motor, windings, commutator brushes and
all. Lack of oxygen is the key.




Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

 

[Jim Thompson]

Except in Hollywood !


Graham

But engine compartment fires happen regularly.

Last weekend, in my neighborhood, we had a drunken teenager hit a
wall, partially ejected through the windshield (no seat belt).

The car burst into flames and the teenager was burned to death,
because no one could get close enough to pull him out in time.

...Jim Thompson

 

[Jim Thompson]

Don't you mean the scavenge pump ( that was never recovered IIRC ) ?

Graham

It might have been. I was doing some work for Sperry/Honeywell Flight
Systems at the time, and I was told that the armature hit the stator
on "a" pump, due to no (fuel) lubrication.

...Jim Thompson

 

[Rich Grise]

keith said
Actually, it's recommended you do both.
############################
http://zapatopi.net/afdb.html#BUILD
A Note About The Shiny Side:

<snip>

I got a kick out of this, at the bottom of the page:
------------------
** HISTORICAL FOOTNOTE: Aluminum was originally named "alumium" by Sir
Humphry Davy, who later changed it to "aluminum" (perhaps in an attempt to
make it more Latinized since alumen is Latin for alum, the aluminum
compound that the name is derived from). The British (and allied English
speakers) shortly thereafter changed the name once more, this time to
"aluminium" so that it would again match the pattern of most other
elements (helium, sodium, etc.), while the North Americans eventually
decided to keep the second, slightly more traditional name. I predict that
North Americans will adopt the more regular "-ium" spelling by the year
2050, prompting the British to start calling it "alumininium". At that
point debate can begin on changing "platinum" to "platinium"

 

[Carl D. Smith]

Except in Hollywood !

In Hollywood vehicles sometimes explode in mid air after going
off a cliff.

 

[Charlie Edmondson]

But engine compartment fires happen regularly.

Last weekend, in my neighborhood, we had a drunken teenager hit a
wall, partially ejected through the windshield (no seat belt).

The car burst into flames and the teenager was burned to death,
because no one could get close enough to pull him out in time.

...Jim Thompson

Yep, caused by the battery being compromised. All that acid spilling on
all that metal, plus the electochemical energy stored in there getting
out due to shorts, etc.

Charlie

 

[Pooh Bear]

But engine compartment fires happen regularly.

They do ? News to me but we don't have US cars much here.

Last weekend, in my neighborhood, we had a drunken teenager hit a
wall, partially ejected through the windshield (no seat belt).

The car burst into flames and the teenager was burned to death,
because no one could get close enough to pull him out in time.

Hmmm. Not quite worthy of a Darwin Award but the principle was bang on.

Graham

 

[Jim Thompson]

They do ? News to me but we don't have US cars much here.

In frontal crashes.

Hmmm. Not quite worthy of a Darwin Award but the principle was bang on.

Graham

Yep. All of his buddies are whining and moaning now, but they didn't
think enough of him to stop him from leaving the party in a totally
inebriated state :-(

The cops are saying his estimated impact velocity was 70MPH. This was
in the heavily curved area of Ray Road in Ahwatukee Foothills (for
those of you who know the area).

...Jim Thompson