Why don't more fuel tanks explode? (how does a fuel sender work?)

[P51D]

I have a Stewart Warner fuel guage and sender in the fuel tank of my inboard boat. A few months back I took out the tank and sender for a good cleaning. I think the sender is very common. Basically, its mechanism looks like semi-circular coil, and a metal tab moves over that coil. The tab is attached to the float arm. So the float controls the position of the tab as the fuel level changes. <br /><br />And then there are a couple of wires, and that's what scares me!! Even though this is the original, unmodified fuel sender setup, that has worked flawlessly for years in this boat, it just makes me nervous to see wires inside a gas tank! <br /><br />How does this type of sender work? I'm guessing that current travels from the coil to the tab (or vice-versa). The amount of current varies with the position of the tab; that is, its position on the coil. More current towards one end, less towards the other. If this is basically correct, what prevents sparks from being generated as the tab move across the coil? Or am I all mixed up?<br /><br />Thanks

 

[Dunaruna]

Re: Why don't more fuel tanks explode? (how does a fuel sender work?)

Good question. And to add, what about intank pumps on modern vehicles?<br /><br />If you stop and think about it, an arm scrapping across a resistor that is connected at one end to ground and the other end to '+' is a recipe for KABOOM! But it doesn't 'kaboom'. Also, a high speed electric motor (with brushes that spark) that is submerged in fuel - doesn't make big badaboom.<br /><br />Two things, spark resistant resistor coils/arms (brass) and the absence of a vital ingredient - oxygen.<br /><br />Also, gas vapour above 7.6% concentration will not ignite. The empty portion of the gas tank has vapour well above that figure.<br /><br />Anyone care to add or comment?

 

[Boatist]

Re: Why don't more fuel tanks explode? (how does a fuel sender work?)

Ever have a flooded car that would not start even with a good spark. In the tank the fuel air ratio is too rich to burn.<br /><br />Same thing happends with boats that blow up after fueling. They almost never blow up at the dock instead they blow up about 100 yards away after they get some air in to make a proper fuel air mix.

 

[studlymandingo]

Re: Why don't more fuel tanks explode? (how does a fuel sender work?)

The coil you see inside the tank is a rheostat, it is used to measure milliamperage. There is not 12v current running through it. When a wire is looped as it is on the coil in the fuel sending unit, amperage increases; the little arm that moves across the coil sends the signal back to the gauge, where that arm is on the coil determines the amount of resistance measured at the gauge thereby determining an approximation of how much fuel is still in the tank. <br /><br />If there was 12v+ on one side of the coil, and 12v- on the other, you would have a glow-plug, shortly thereafter you would no longer have a boat!

 

[whywhyzed]

Re: Why don't more fuel tanks explode? (how does a fuel sender work?)

Originally posted by studlymandingo:<br /> There is not 12v current running through it.

sure there is... well, not current...current and voltage are apples and oranges...<br />There IS 12V potential though on many senders. 12V one side, ground on the other.<br />Some gauges drop the + side to 10V regulated for more accuracy, but some are 12V. Current is dictated by resistance. Fuel senders are high resistance. But, they certainly do melt, if the reostat strip starts to short....seen more than one melted down fuel sender.<br /><br />On some of the heavy equipment I deal with, they have non contact senders. A magnet moves with the float. On the other side of the tank, another magnet follows it and moves the sender. Neat, eh! You can change the sender without getting wet.<br />Even neater: On some other machines, they use an upside down "fish finder" in the fuel tank. A puck floats in a tube in the tank, and an ultrasonic sender mounted in the bottom of the tank bounces signals off the puck to determine the fuel level...never try to fly with one of those in your carry on suitcase....

 

[Realgun]

Re: Why don't more fuel tanks explode? (how does a fuel sender work?)

They almost never blow up at the dock

Thats good to know.

 

[P51D]

Re: Why don't more fuel tanks explode? (how does a fuel sender work?)

Thanks for the replies. I understand the concept a bit better, and feel a litte more comfortable, at least. Also, if it was as dangerous as I was originally imagining, we'd be seeing a lot of serious fireworks every sunny weekend on the water!

 

[jlshields]

Re: Why don't more fuel tanks explode? (how does a fuel sender work?)

And gasoline is not a conductor of electricity.<br />And the motors in electric fuel pumps do not have brushes.

 

[Terry Olson]

Re: Why don't more fuel tanks explode? (how does a fuel sender work?)

I'm no expert, but I know that gasoline as a liquid isn't flamable - it's vapors are. You need three things to have a fire - fuel, oxygen, and an ignition source. I suppose gasoline and really any other liquid isn't flamable per se because they lack oxygen? I'm not sure about that, but it seems logical. If you pour gasoline on the ground and light a match you'll see that the liquid remains for some time even as the flames burn aggressively. If the liquid were flamable the fire would burn out immediately as the fuel is consumed. The fact that it takes some time to burn out demonstrates this idea. It's a subtle point because gasoline vaporizes readily even at cold temperatures. Where there's gasoline there are vapors so I'm not volunteering to tempt fate either.