Why doesn't the fuel tank sender cause and explosion ?

After 11 years of service I'm replacing the fuel tank sender on my boat since it isn't working properly.

As I look at the new one I've got to wonder why all of our boats don't explode in a giant fireball.

The sender has a reostat and a spring loaded "wiper" that rubs across the reostat as the float level changes. It seems like it wouldn't take much for a spark to arc accross the resistance wires and the wiper as it jumps from winding to winding.

When the tank is full the sender is below the surface of the gas, so that shouldn't be a problem, but when the tank is near emply this isn't the case.

Obviously it isn't a problem, but can someone clear up this mystery for me ??

Thanks !!

Re: Why doesn't the fuel tank sender cause and explosion ?

I dunno. Does 12V arc? Maybe not enough oxygen.

Re: Why doesn't the fuel tank sender cause and explosion ?

Valid point and thanks for scaring the crap out of me .. Now that tanks gonna have to be kept full..

as its vapours thats the worry ..

Re: Why doesn't the fuel tank sender cause and explosion ?

Temp. Throw a lit cigarette in a bucket of gasoline, it'll just go out.

Re: Why doesn't the fuel tank sender cause and explosion ?

the gauge measures the resistance of that rheostat, and it applies a VERY minimal voltage to do it. nowhere close to enough to cause a spark.

but... even if it does spark, there would be an almost zero chance of starting a fire. If you have any fuel in the tank, the vapor concentration is far past what you can ignite. (gasoline doesn't burn at all. gasoline vapors burn, and it has to be between 1 and 7% vapor to air. Any more or less and it won't do a thing) The only time you would get that low of a concentration of vapor in the tank is if it is absolutely empty, in which case you aren't moving the sender anyway.

Re: Why doesn't the fuel tank sender cause and explosion ?

foodfisher said:

Temp. Throw a lit cigarette in a bucket of gasoline, it'll just go out.

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throw a light match into a bucket of gasoline once and see what happens.

cigarettes don't work because they are just barely hot enough to ignite gasoline vapors. After tossing them through the air, they cool down enough that they can't ignite it anymore.

Re: Why doesn't the fuel tank sender cause and explosion ?

If you are worried about the fuel gauge sender being dunked in gasoline, you should be thrilled to know that the electric fuel pump on nearly every fuel injected car on the planet also lives in there and it draws a heck of a lot more current than the fuel gauge. Granted, it is sealed, but still ----------stuff happens.

Re: Why doesn't the fuel tank sender cause and explosion ?

Silvertip said:

If you are worried about the fuel gauge sender being dunked in gasoline, you should be thrilled to know that the electric fuel pump on nearly every fuel injected car on the planet also lives in there and it draws a heck of a lot more current than the fuel gauge. Granted, it is sealed, but still ----------stuff happens.

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a submerged fuel pump cannot at all cause a fire in a fuel tank. Has never happened and never will.

Re: Why doesn't the fuel tank sender cause and explosion ?

NEVER say can't !!!

Re: Why doesn't the fuel tank sender cause and explosion ?

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.

Basically the higher the open circuit voltage, the less current it takes to ignite a vapour.

It has been a LONG time since I designed equipment for hazardous equipment but I seem to remember a couple of values (but don't rely on my memory!).

In gasoline vapour (at explosive mixture) for a voltage of about 50 volts or higher, there is NO safe current level. At 12 volts, I seem to remember the same current being around 0.25 amperes.

In a fuel gauge system, the battery positive is attached to the fuel gauge. The fuel gauge has resistance which limits the maximum current that can flow through the sensor (pick-up) to a safe level - i.e. if you short the sensor wire to ground, your gauge will read FULL but the current flowing will be small. If you shorted your fuel gauge or otherwise applied battery power directly to the sensor (no current limit) there would be significant risk of explosion if the vapour in your fuel tank reached the 7% to 15% mixture (air/gasoline vapour) that forms the 'explosive mixture'.

[Numbers quoted should not be taken as gospel - consult CSA and/or UL regulations for current values.]

Re: Why doesn't the fuel tank sender cause and explosion ?

P.S. Fuel pumps and other equipment operating at higher voltages/currents CAN be installed in hazardous locations but the enclosure for such equipment must be tested and approved for use in such locations. Enclosures are designated "explosion proof" which basically means that if the fuel/air inside the enclosure should ignite, the flash will not propagate out of the enclosure into the environment. The gasoline pumps at a filling station all use this technique for housing electrical equipment in a potentially explosive location.

Re: Why doesn't the fuel tank sender cause and explosion ?

Tank senders run at 5 volts DC. Not enough voltage to cause a spark or even heat, and not enough voltage to really even carry any meaningful current.

Re: Why doesn't the fuel tank sender cause and explosion ?

JustJason said:

Tank senders run at 5 volts DC. Not enough voltage to cause a spark or even heat, and not enough voltage to really even carry any meaningful current.

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ohms law. current = voltage /resistance. this is the funniest thing I've ever read . 5volts can carry a lot of current, it just doesn't in this application. ex: 5v/.5ohm = 10 amps.

Re: Why doesn't the fuel tank sender cause and explosion ?

this does get funny.
by the ohms law theory, I should have died about an hour ago.
got zapped by 10 Kilo Volts.
luckily a 90HP yamaha doesnt generate enough current to kill,same as that 5V fuel sender circuit doesnt have enough current to spark.
that 90 does generate enough voltage to make you take a step back and say bad things.
the stop circuit has 220Volts on it and yes that will make you staep back but it wont kill.
most all EFI motors use a blow through pump that uses fuel flowing past the brushes to cool the pump and armeture.
thats why cleaning VST and injector filters gets mandatory, brush dust can and will clog them.
autos return the fuel to the tank,most marine EFI motors return fuel to an engine mounted vapor seperator, most use 2 of these blow through type pumps one lifts fuel to the VST the other steps it up between 35 and 55 PSI for the fuel rail.
the OX66 efi motors the pump current draw is something like 6 amps above 1200 RPM.
could it explode. I guess anything is possible, never heard of it.

Re: Why doesn't the fuel tank sender cause and explosion ?

Hmmmm --- say something"tongue in cheek" and it can create some interesting responses. My comment about electric fuel pumps was just that. If someone was concerned about the fuel gauge causing an explosion, why would they not be concerned about an electric fuel pump occupying the same location? I by no stretch of the imagination implied that this was a strong possibility which is why I made it clear the pump is sealed. Anyone here old enough to remember the Ford Pinto. No electric fuel pump in the tank but they sure did explode if rear ended. While a rupture on a modern car is probably far less likely than in the good old days, my comment about "stuff happens" is still valid. In fact we really don't know if such a thing has or has not happened. Just because none of us has seen or read about it does not mean it hasn't happened.

Re: Why doesn't the fuel tank sender cause and explosion ?

whats even funnier is the mustangII, same unibody,same part number on the tank, never heard a word abot them.
heck even my 1970 W-30 442 had 19 gallons just infront of the rear bumper, I gues if it got hit hard enough it would have ruptured as well.
gotta carry fuel somewhere.
gotta admit never heard of a kayak on fire,,, yet.