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MHerder
Joined: 11 Feb 2008
Posts: 143
Location: Fort Worth TX
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 Posted: Wed Aug 25, 2010 6:13 am Post subject: GRT EIS-Fuel Sender |
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Ive installed my senders (resistance type) and routed the 4.7v excitation and the required resistor to the sender…
Here’s my question:
How is it safe to route 4.7 volts to a fuel tank with current traveling through the tank? Spark kaboom?
Is there something I’m missing?
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Zodiac 601 HD
Jabiru 3300
Wood Sensinich 64x47
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Dennis Glaeser
Joined: 10 Jan 2006
Posts: 53
Location: Rochester Hills, Michigan
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Thu Aug 26, 2010 5:47 am Post subject: GRT EIS-Fuel Sender |
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At 09:13 AM 8/25/2010, you wrote:
| Quote: |
"MHerder" <michaelherder(at)beckgroup.com>
Ive installed my senders (resistance type) and
routed the 4.7v excitation and the required resistor to the sender…
Here’s my question:
How is it safe to route 4.7 volts to a fuel tank
with current traveling through the tank? Spark kaboom?
Is there something I'm missing?
|
Cars have been running "current" into the fuel
tanks for monitoring liquid level for a lot of
years. See:
http://www.aeroelectric.com/Reference_Docs/Patents/Circa_1923_Fuel_Gage.pdf
While the stuff in the tank is indeed
combustible, the range of conditions over which
it becomes hazardous in storage does have
boundaries.
As long as there is any liquid fuel
in a tank, the vapor above the liquid
is close to saturated. I.e., a long
ways away from the ideal stoichiometric
ratio where ignition is easiest and the
burning most violent.
Further, even in ideal ratios of fuel
and oxygen, the ignition source has to present
a minimum packet of energy in order to trigger
the chain reaction that is burning or even
an explosion. Devices incapable of delivering
these energy levels (adjusted for headroom) are
called "intrinsically safe" . . . meaning
that there are no combinations where the total
system becomes hazardous.
For example, the energy that flows in components
of a capacity fuel sensor falls in the intrinsically
safe category. Variable resistors adjusted by floats
and paired with the right gages are also intrinsically
safe.
But assuming you put a real "sparker" in the
tank, you're still not going to get a hazardous
mixture until after all the liquid has evaporated
and the remaining vapors are sufficiently diluted
so as to approach the "oh s#$t" ratio. This just
doesn't happen accidently in small airplanes.
Doesn't happen in big airplanes either. This is
why a "spark in the tank" hypothesis for any
explosion aboard airplanes is unsupported
by the physics or any demonstrable experiment.
That "unusable fuel" has two important functions.
(1) provide a low-spot for moisture collection
and (2) keep the vapor in the tank saturated.
So the short answer to your question is, don't
worry about it.
Bob . . .
////
(o o)
===========o00o=(_)=o00o=========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
=================================
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danfm01(at)butter.toast.n
Guest
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| Posted: Thu Aug 26, 2010 9:54 am Post subject: GRT EIS-Fuel Sender |
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Bob's answer with respect to explosive dangers with 100LL in the fuel
tank is correct of course. If there is anyone out there thinking of
diesel power, be aware that the situation with respect to jet fuel is
different. Jet fuel vapor can easily reach explosive concentrations in a
fuel tank and special venting requirements are usually recommended. See
www.deltahawkengines.com/questi00.shtml#fuel for a discussion.
On 08/26/2010 06:45 AM, Robert L. Nuckolls, III wrote:
| Quote: |
<nuckolls.bob(at)aeroelectric.com>
At 09:13 AM 8/25/2010, you wrote:
>
> <michaelherder(at)beckgroup.com>
>
> Ive installed my senders (resistance type) and routed the 4.7v
> excitation and the required resistor to the sender…
>
> Here’s my question:
>
> How is it safe to route 4.7 volts to a fuel tank with current
> traveling through the tank? Spark kaboom?
>
> Is there something I'm missing?
Cars have been running "current" into the fuel
tanks for monitoring liquid level for a lot of
years. See:
http://www.aeroelectric.com/Reference_Docs/Patents/Circa_1923_Fuel_Gage.pdf
While the stuff in the tank is indeed
combustible, the range of conditions over which
it becomes hazardous in storage does have
boundaries.
As long as there is any liquid fuel
in a tank, the vapor above the liquid
is close to saturated. I.e., a long
ways away from the ideal stoichiometric
ratio where ignition is easiest and the
burning most violent.
Further, even in ideal ratios of fuel
and oxygen, the ignition source has to present
a minimum packet of energy in order to trigger
the chain reaction that is burning or even
an explosion. Devices incapable of delivering
these energy levels (adjusted for headroom) are
called "intrinsically safe" . . . meaning
that there are no combinations where the total
system becomes hazardous.
For example, the energy that flows in components
of a capacity fuel sensor falls in the intrinsically
safe category. Variable resistors adjusted by floats
and paired with the right gages are also intrinsically
safe.
But assuming you put a real "sparker" in the
tank, you're still not going to get a hazardous
mixture until after all the liquid has evaporated
and the remaining vapors are sufficiently diluted
so as to approach the "oh s#$t" ratio. This just
doesn't happen accidently in small airplanes.
Doesn't happen in big airplanes either. This is
why a "spark in the tank" hypothesis for any
explosion aboard airplanes is unsupported
by the physics or any demonstrable experiment.
That "unusable fuel" has two important functions.
(1) provide a low-spot for moisture collection
and (2) keep the vapor in the tank saturated.
So the short answer to your question is, don't
worry about it.
Bob . . .
////
(o o)
|
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jonlaury
Joined: 06 Nov 2006
Posts: 336
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| Posted: Fri Aug 27, 2010 6:07 pm Post subject: Re: GRT EIS-Fuel Sender |
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Bob, you said:
| Quote: | That "unusable fuel" has two important functions.
(1) provide a low-spot for moisture collection
and (2) keep the vapor in the tank saturated. |
I spent a lot of time and effort to make sure that every drop in my fuel tanks was usable except for the debris sumps. The mains and header can be pumped dry except for sump fuel (4-6 ounces).
Is the small amount of fuel in the sumps enough to keep a 12' long wing tank vapor-saturated?
I wasn't ever worried about this before, but now that it's out on the line, just want to be sure I'm not committing a major oversight.
J
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grosseair(at)comcast.net
Guest
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| Posted: Fri Aug 27, 2010 6:25 pm Post subject: GRT EIS-Fuel Sender |
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You don't have to run the tanks dry.
John Grosse
jonlaury wrote:
| Quote: |
Bob, you said:
> That "unusable fuel" has two important functions.
> (1) provide a low-spot for moisture collection
> and (2) keep the vapor in the tank saturated.
>
I spent a lot of time and effort to make sure that every drop in my fuel tanks was usable except for the debris sumps. The mains and header can be pumped dry except for sump fuel (4-6 ounces).
Is the small amount of fuel in the sumps enough to keep a 12' long wing tank vapor-saturated?
I wasn't ever worried about this before, but now that it's out on the line, just want to be sure I'm not committing a major oversight.
J
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=310335#310335
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Sat Aug 28, 2010 5:14 am Post subject: GRT EIS-Fuel Sender |
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At 09:07 PM 8/27/2010, you wrote:
| Quote: |
Bob, you said:
> That "unusable fuel" has two important functions.
> (1) provide a low-spot for moisture collection
> and (2) keep the vapor in the tank saturated.
I spent a lot of time and effort to make sure that every drop in my
fuel tanks was usable except for the debris sumps. The mains and
header can be pumped dry except for sump fuel (4-6 ounces).
Is the small amount of fuel in the sumps enough to keep a 12' long
wing tank vapor-saturated?
|
Are these 'sumps' part of the tank envelope
or are they 'low spot' water separators plumbed
to the tank through a fuel line?
| Quote: | I wasn't ever worried about this before, but now that it's out on
the line, just want to be sure I'm not committing a major oversight.
|
I may have stubbed my toe on speaking of the
"importance" of unusable fuel. I don't think
that the designers deliberately shoot for a
volume of unusable fuel just to meet some rule
of thumb for reducing risk of explosion. I
think it has just worked out that for most airplane
tanks, the lowest spot in the tank was against the
skin and that same spot becomes a good place to
put a water drain. Obviously, plumbing a fuel
line into the same location would look pretty ugly
hanging out the bottom of the airplane. So fuel
lines tend to tap through the tank wall at the lowest
practical point . . . but obviously above the
tank's low spot thus giving rise to "unusable
fuel".
I'm sure there are many instances of gasoline
tanks on vehicles that are plumbed such that
the total contents can be consumed by the engine.
But unless the same tank is force-ventilated
the vapor space mixture will remain very rich
for long periods of time. But even after it
becomes 'leaner', you still need an ignition
source for the combination to become hazardous.
I remember reading about WWII military fuel
systems where a major consideration was taking
a tracer through an empty fuel tank. At Cessna,
we fitted some military airplane tanks with an
open cell foam filler. While the foam reduced
tank useful volume by about 5%, it had the
effect of sucking the heat out of a flame front
such that taking a tracer through a tank of
stoichiometric mixture would not produce an
explosion.
It's hard to get an explosion to happen on
purpose. I recall some conversations with a
fellow who ran a test lab telling us about
getting a mixture tuned up "just right" for
the purposes of testing our motors for explosion
proofing. He described a chamber with a hinged
lid, foam gaskets all around the edges, and just enough
weight to get a good seal.
The chamber was fitted with mixing fans, a
source of hydrocarbon (I think it was Butane),
a source of ignition (spark plug through the
wall), and equipment to allow us to run a motor
inside.
After putting what is believed to be the
right amount of butane in the chamber, the
motor run tests are conducted. When the
prescribed tests are finished, he then sparks
the mixture for the purpose of proving that
the atmosphere was and is still hazardous.
If things went right, the vapor lit off,
the lid flies open and you get this fireball
out the top. If things don't go right, nothing
happens and the tests have to be repeated.
Unless you plan to punch holes in your tanks
and mount spark plugs in the holes, the practice
of running the tanks dry offers no great hazard.
If you're using fuel gaging systems that are shown
to be intrinsically safe in a hazardous environment,
you're good to go.
Bob . . .
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Sat Aug 28, 2010 5:26 am Post subject: GRT EIS-Fuel Sender |
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At 09:21 PM 8/27/2010, you wrote:
| Quote: |
You don't have to run the tanks dry.
|
True . . . but if one intends to leave a specific
volume of fuel in the tank, an accurate means
of gaging is indicated.
I just checked out my copy of part 23 where I found:
------------------------------
Sec. 23.971 Fuel tank sump.
(a) Each fuel tank must have a drainable sump with an effective capacity,
in the normal ground and flight attitudes, of 0.25 percent of the tank
capacity, or 1/16 gallon, whichever is greater.
(b) Each fuel tank must allow drainage of any hazardous quantity of water
from any part of the tank to its sump with the airplane in the normal ground
attitude.
(c) Each reciprocating engine fuel system must have a sediment bowl or
chamber that is accessible for drainage; has a capacity of 1 ounce for every
20 gallons of fuel tank capacity; and each fuel tank outlet is located so
that, in the normal flight attitude, water will drain from all parts of the
tank except the sump to the sediment bowl or chamber.
(d) Each sump, sediment bowl, and sediment chamber drain required by
paragraphs (a), (b), and (c) of this section must comply with the drain
provisions of Sec. 23.999 (b)(1) and (b)(2).
-------------------------------
So in TC airplanes, it would be a violation to design
a tank with zero unusable fuel. This section gives us
some insight into the existence of one or more
drains in each tank plus a low spot drain (usually
the gascolator) at the bottom of the fire wall.
Bob . . .
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jonlaury
Joined: 06 Nov 2006
Posts: 336
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| Posted: Sat Aug 28, 2010 8:44 am Post subject: Re: GRT EIS-Fuel Sender |
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| Quote: |
You don't have to run the tanks dry.
|
True and I probably won't 99% of the time. But I designed this multi-tank fuel system to be bullet proof if it is just ON. I don't want to inadvertently build in a gotcha if there's a need or a screw-up that runs a tank dry.
Re Bob's Part 23 quote, by coincidence, my wing and header tanks (with integral, drainable debris sumps within the tank envelope) are configured in conformance, as they drain into a 32 oz. sump tank (w/drain) that feeds the pumps.
So now all I have to do is remove the fuel tank spark plugs. 
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