Empty tank switch

[aerhed]

Okay, this isn't on an RV, but I do build RV's....so there. I'd like to put a float switch in my Emeraude's rear tank that shuts off the transfer pump when empty. Problem is I'm confusing myself with wiring in a fuel tank (scared). So, if I put a float activated switch in my tank in series with the facet interrupter pump...will I blow up?

[aerhed]

C'mon guys. Quit laughing and answer before I go down to the hangar.

[gasman]

I'll place my bet in your favor.



Let me know how it works out.



Should be no different than a low fuel light from Spruce........

[aerhed]

I think I've got it. I'm going to epoxy a magnet in the bottom of a foam sender float. I'll swing the float from a bracket stuck to the tank wall (glass tank) and wherever it hits the bottom I'll mount an N/C magnet switch outside the tank. Simple. Just need to check the switch rating and decide yes/no on a relay.

[N941WR]

I would hate to hit a bump and have the fuel pump shut off on short final.

Why not tie it into a fuel pressure sensor coming from that tank?

[aerhed]

Its a transfer tank. The engine never feeds from it. You just throw the fuel to the front tank which is gravity fed to the carb. The plans call for it to gravity feed to the front tank in flight but it never would so I put a facet pump on it. That way you can transfer on the ground also. When you flip the spigot lever it hits a toggle that starts the pump. I've got a pump light for a reminder but no rear fuel gauge. This way it will stop the pump as soon as its empty and the light will go out telling you its done.

[cfiidon]

I think your magnet switch is a good idea.

You were correct to be concerned about wiring in a tank. We all know that 12V will create a spark. Spark + fuel vapor = kaboom.

I thought this through when I was servicing the fuel senders in my Bonanza. I was wondering why there is no protection against sparking. The sender is nothing but a rheostat with a wiper across a wound resistor. The units are not sealed to fluid.

It takes fuel and oxygen and ignition source to create a tank explosion. If the sender is immersed in fuel it doesn't matter, but how about when the fuel level drops below the sender? You have an ideal situation: fuel vapor + spark ignition source.

The way around this is simple. The circuit that supplies voltage to the sender unit only sends a very low voltage (NOT 12V). In my Bonanza, I think it is on the order of 1.5 volts or less. You can experiment with a flashlight battery and a wire. There is not enough potential to spark. Try it with a 9V battery and you'll get a spark.

Perhaps someone in the know can chime in, but that's my theory. And you were right to be concerned about routing 12V thru a switch in your fuel tank.

Don

[aerhed]

In my Bonanza, I think it is on the order of 1.5 volts or less.

Ah. I've worked on all kinds of fuel senders and never tried to think too hard about it. Just followed instructions or kept things as found. Never really thought about checking sender voltage. The 385 type sender Vans uses is what I'm replacing the front sender with. It sits above the liquid level and doesn't blow up. I'd like to see a schematic of the gauge guts. I have heard of a twin cessna blowing up the tip tank from a bad pump.

[David Z]

From what I understand, the fuel/air ratio inside gasoline fuel tanks is too rich to combust.

GM puts the fuel pump inside the tank on a lot of their trucks and commercial vans.

[BobTurner]

I would think twice about this idea. If your switch fails you would be unable to use that fuel. Is this acceptable? Perhaps you can wire it up with a manual bypass.