TheSamba.com :: View topic - school me please, why a fuel sender will never ever spark

Wed Oct 29, 2025 12:29 pm

title says it all...

to me a fuel sender is basically a reostat with a contact jumping between wires..

what obvious thing am i missing?

Wed Oct 29, 2025 1:12 pm

It's not jumping between wires. It's the same wire. Different locations on the same wire.

Kinda like putting both your volt meter leads on the same hot/ground wire. There's no difference in potential.

Wed Oct 29, 2025 2:24 pm

I'm no electrical engineer, but would venture to guess that it's because the voltage is so low and resistance high in that circuit -- suspect it's nearly impossible to make a spark form/jump in those circumstances. Or perhaps any spark which would form is still too weak and cool to ignite the fuel + air vapor mixture in a gas tank -- ? Those are just SWAGs, though...

Wed Oct 29, 2025 2:27 pm

It must be settled science though I don’t know anything about it.
They’re putting entire fuel pumps inside the tank on modern cars.

Wed Oct 29, 2025 2:39 pm

Even if it did the atmosphere in the tank is far too rich to ignite:

Wed Oct 29, 2025 2:50 pm

Where do I add the fluid to my blinkers?

Wed Oct 29, 2025 3:01 pm

I too have wondered why but, never questioned the issue since things were working as designed.

I recently changed the FP in my Eurovan and the pump power connection are exposed to the fuel in the tank. Hmmm, no fuel fire when energized. Lol!

Thu Oct 30, 2025 4:28 am

thank for the inputs...

i get the point, the voltage drop in a single wire turn is too low...or low enough to make absolutely sure it will never create any spark.

Is something that made me wonder always (i did repair fuel senders), thinking that this works in an atmosphere that basically is gasoline fumes...

Thu Oct 30, 2025 7:08 am

my interest was piqued and this is from a boat forum where the same question was asked.
In any "hazardous location" (as defined by safety regulations) and for every atmosphere (gasoline, propane, etc.) there is a graph of voltage versus current showing the level at which a vapour will ignite. These graphs have been determined in a laboratory and are used as the basis for approving equipment to be used in these locations.

also gasoline will only ignite within a limited range of fuel/oxygen mix. about 1.4% to 7% fuel in the air mixture. again this has all been studied and defined so that limits can be set.
https://en.wikipedia.org/wiki/Flammability_limit

Thu Oct 30, 2025 8:04 am

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Simply put as busdaddy stated, “the atmosphere in the tank is far too rich to ignite”.

Submerged electric fuel pumps don't cause explosions because gasoline is in a liquid state, and only its vapors can burn. The pump is submerged, which prevents sparks from igniting the fuel. Additionally, the concentration of gasoline vapors in the tank is too high for combustion, as there isn't enough oxygen in the "headspace" above the liquid to create an explosive mixture.

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Thu Oct 30, 2025 9:39 am

But........ we're supposed to believe that this is what brought down TWA flight 800 right? I mean even the cia made a video......the gov would never lie. Right?

Thu Oct 30, 2025 9:55 am

RUNKLE wrote: .

Simply put as busdaddy stated, “the atmosphere in the tank is far too rich to ignite”.

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It's more that the spark (if one were to occur) is too weak.

I used to work for a company that fitted data loggers on gas meters. I ran the team that arranged their installation and had to deal with the safety audits.
Their proximity to the gas meter meant they fell within ATEX Zone 0 (the highest rating for explosive atmospheres).
The Meter Operators weren't happy about that. To get permission to attach the loggers to their assets, we had to be able to demonstrate that they were 'intrinsically safe devices' that were certified to be installed in that atmosphere.
That basically meant they had to have been tested to confirm that they couldn't generate a strong enough spark to ignite a gas leak.

Ours were only using a small lithium battery.

I'd guess that a fused 12V vehicle circuit falls below the ignition curves that Finster mentioned above, even given the relatively high current capacity of a car battery.

Thu Oct 30, 2025 10:32 am

liquidrush wrote: But........ we're supposed to believe that this is what brought down TWA flight 800 right? I mean even the cia made a video......the gov would never lie. Right?

The Twa flight had failures in its electrical system which were documented.

If you look at a rheostat design, the wiper is larger than the spacing of the winding by a large margin, so there is alway contact between the wiper and wire. The voltages are lower than the threshold for breakdown of air, so thst hazard is comparable to that of a mosquito picking up a hot ember from the grill and flying into the gas tank

Thu Oct 30, 2025 10:54 am

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Who.Me? wrote: …It's more that the spark (if one were to occur) is too weak...
You are correct sir - that, and the lack of oxygen.

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Sat Nov 01, 2025 1:38 pm

It is a little freaky to mix electrics in liquid gasoline but the safety thing is worked out.

Is this a good time to point out that all( most) mainline generators run in a Hydrogen environment? With high voltage sparky slip rings?

Sat Nov 01, 2025 6:04 pm

I worked with a welder when I was in the Air Force that welded on active jet fuel lines on the ramp where they refueled the planes. He told me the principal, but I never really understood it. The lines would have to be full and running under pressure. Told me if a pocket of air came through, he'd be gone. There were always at least two firemen fully suited in special suits with a crash truck sitting at ready. Told him he was crazy and he proved it at a later assignment in Korea when I was talking to somebody and he used a spray can as a torch by my head!

Sat Nov 01, 2025 6:13 pm

Public Broadcasting Service was pretty neat, growing up for me in the 70s

Mon Nov 03, 2025 7:10 pm

A buddy of mine used to weld natural gas pipelines, repair leaks with welding. The gas was flowing, even coming out of the leaks, and he'd weld, sure the leaking gas burnt, but the flame never could sustain inside the pipe, as the pipe had too little oxygen in it to support a flame.

Mon Nov 03, 2025 10:03 pm

If one is really concerned you could run a marine reed switch type fuel gauge sender. I have KUS sender in my buggy, but have not connected it to a gauge yet.

Tue Nov 04, 2025 9:29 am

busdaddy wrote: Even if it did the atmosphere in the tank is far too rich to ignite...
It is my opinion that is not something that can be depended upon.
I am pretty certain all of us have had a car sitting around a while with just a dab in the tank and a poor cap seal. And then we worked on electrics. There certainly can be a point at which the atmosphere is no longer saturated and explosive.
I go with post #2 above from @slayer61 and the notion of super low voltage.