Fuel Tank Sender

[mikebryan]

As part of the ongoing restoration of my 1968 S1.5, have to decide whether or not to replace the old fuel tank sender.
Took the sender apart, cleaned the gunk off the rheostat now it's working again ( or least to a test bulb fitted temporarilly into the circuit)

Question, the sender is immersed in fuel or exposed to fuel vapour, my rheostat is covered by a aluminium cover, held on by a couple of screws. There is no way it could be "petrol proof". Is there not the possibility that a spark could occur between the rheostat arm and windings, causing a " kerbooooom" inside the tank ???

I guess that I will order a new sender, but I am still surprised how primitive the unit is.

Cheers Mike

[jagwit]

This has puzzled me as well!!

My explanation (not that it IS the correct one...) is that the wiper arm is a "make before break" type contact, ie, it makes contact with the next winding before breaks with the one it is departing from. Also quite likely that it makes contact with several windings at the same time.

For a spark to occur AND fire to result from it, you need 2 ingredients:
1) sufficient voltage for current to jump the distance from source to destination;
2) an air-fuel ratio that would actually support combustion (I'm sticking my neck out on this one....).


Regarding #1, there is only 12-14V on the input to the fuel sender, less the further down the rheostat you go, which will support a spark over a MINUTE distance. The wiper is most likely making contact with several windings at the same time thus "shorting out" voltage as it moves along. Still, it must have been a confident man who designed it as the idea still gives me the kreaps....
Regarding #2, the reason I offer this condition is because I know you can kill your engine by supplying it with excess fuel ie, kill the fire with fuel. I therefore wonder if the air-fuel ratio inside the tank will support burning with the heavy concentration of fuel vapour in the tank.

[mikebryan]

Hi Philip

Thanks for your thoughts, your comments make a lot of sense.... I think..???

[cactusman]

Agree with Phillip. And for there to be a kaboom the fuel vapour to air ratio must be right and it won't be in the sender....fear not I say....

[JerryL770]

The upper and lower explosive limits (UEL & LEL) for concentration by volume of gasoline vapour in air are 7.1% and 1.2% by volume which are both well below the likely saturated % by volume for the air in the tank.

The saturation % by volume will vary depending on the ambient or gasoline temperature but is typically in the range of 35 - 40% in Europe. The air in your tank will be saturated with gasoline vapour.

So even if there is a spark in your tank, you will not get any combustion/explosion.

I used to work on designing gasoline vapour recovery plants for the oil/petrol distribution industry.

[Series1 Stu]

If at all possible use your existing fuel sender. The aftermarket ones are complete garbage and by the time you've got one set up it would have been quicker to restore the original. They look nothing like the original ones either.

Regards

[ysmalkie]

Lucas was always the Prince of Darkness, but never the Prince of Explosion, was it?...

Jerome, I presume this is why you can drop a lit cigar into a petrol tank and nothing will happen, right? :-)

Tadek

[mikebryan]

Thanks for all the helpful replies. thanks especially to JEROME, for putting my mind at rest....

[JerryL770]

Lucas was always the Prince of Darkness, but never the Prince of Explosion, was it?...

Jerome, I presume this is why you can drop a lit cigar into a petrol tank and nothing will happen, right? :-)

Tadek

Right

..... assuming the tank has not been open to a lot of air flow. Most likely on hitting the cold fuel it will be extinguished.

[jagwit]

The situation at the filler cap is not quite the same as deep inside the tank (above the fuel).

At the filler neck (and who knows how far in), the air to fuel ratio must surely be a lot more favourable for combustion than deep inside, with a lot more oxygen being present.

I can accept that once the cigar is deep down the file neck, its more likely to die (ignoring the oxygen it brings with it...), but I would not bet one penny on what happens right there at the filler cap, on its way in.