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?
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Empty tank switch
- Thread starter aerhed
- Start date
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.
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
Well Known Member
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
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
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.
It will be an N/O switch. If it fails all that happens will be a pump that keeps running. Anyway, there's 3 1/2 hours up front and 1 3/4 hours in the back. If you fly 2 hours and transfer you still have 1 1/2 hours if the pump doesn't work. I put the float in today, but all I have are NC switches so I'm going to hook up a "tank empty" light for now and hook it to the transfer pump later.
