Fuel gauges senders

[Loupark]

Ok, I'll try this again. The last question I got actually got answers.
I've asked this question on another group and had nothing but
smart ass answers so I'll try it here.
I'm going to use non aviation fuel gauges for my homebuilt. If I'm
reading the directions right the positive and negative contacts are
in the fuel tank. Won't this cause a big boom?
Lou

 

[gismo]

Ok, I'll try this again. The last question I got actually got answers.
I've asked this question on another group and had nothing but
smart ass answers so I'll try it here.
I'm going to use non aviation fuel gauges for my homebuilt. If I'm
reading the directions right the positive and negative contacts are
in the fuel tank. Won't this cause a big boom?
Lou

First of all there are no "positive and negative contacts" in the fuel sender and very little current actually flows through a sender under normal conditions. There is a "wiper" that's supposed to be in constant contact with the resistive element (usually a wirewound resistor) but that whole assembly is inside a closed metal envelope which will prevent ignition if there was a spark.

 

[Skip Miller]

What Lance said. Further, the fuel/air mixture in the ullage (look it up!) is normally too rich to support combustion.

-Skip

 

[Dan Thomas]

First of all there are no "positive and negative contacts" in the fuel sender and very little current actually flows through a sender under normal conditions. There is a "wiper" that's supposed to be in constant contact with the resistive element (usually a wirewound resistor) but that whole assembly is inside a closed metal envelope which will prevent ignition if there was a spark.

Some of those "closed" metal envelopes aren't closed at all. They're full of holes. The wirewound potentiometer gets worn out with the fuel sloshing around in flight, and the wiper then makes sparks. In the tank. With all that fuel. But as another pointed out, the mixture is far too rich to burn. Combustible mixtures range from 8:1 (air:fuel by weight) to 18:1, and inside the tank it's a lot richer than 8:1.
When we have to drain a tank to do some work, the most dangerous time is after the fuel's out but there's still just a tiny bit with all that air in there, and it can reach explosive mixtures. So we never turn the master on with a situation like that and are real careful about what we're doing. If we want safety we'll block off the vents (loosely) and squirt some nitrogen in there to drive out the oxygen and put the cap back on.
In cars the electric fuel pump is often inside the tank. The pump often has an open frame and has brushes and a commutator and all the rest of the stuff that sparks, and often also has the fuel level pot in there too, a carbon track with a runner, with no enclosure of any sort. More ignition sources. But when was the last time we heard of a car exploding? Except in the movies, of course.

Dan

 

[SkyHog]

Here's what you could do: Forget the new, fancy fuel senders.

Grab the fuel sensor from a early 70s Ford Pickup. It is nothing more than a float attached to an arm, which is attached to a potentiometer. The potentiometer then runs to your gauge.

Get it at a junk yard, you're looking at about $5 - $10 total for whole system, and another $5 or $10 for the gauge.

The wires plug in outside the fuel tank too.

 

[Loupark]

It's amazing, I'm actually getting answers to my questions here.
I might have to start posting more and more.
Thanks everyone
Lou