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Angier:

I designed and built something similar that flashes a red light on the panel
when the header tank starts to overflow via it's vent line. (It's my
reminder to turn off the transfer pumps so I don't pump more than a few
drops of avgas overboard). It uses a few parts from Radio Shack or similar
for the electronics and is pretty simple.

The key element in the device is a pair of very small temperature sensitive
resistors - one in the vent tube and one outside (the tube). The sensors are
arranged in a bridge circuit that has it's output fed into a voltage
discriminator IC. A small amount of current heats both equally and the
bridge is adjusted so the IC output is zero. When fuel just starts to
overflow through the tube, the sensor in the tube is cooled, upsetting the
bridge output causing the IC to switch it's output high - turning on the
light. The light is a self-flasher from RS so I get a flashing warning when
there's fuel flowing in the header vent.

This is very similar to the device the soaring crowd uses to sense rising or
descending air (I believe what they use is called a Variometer). In their
case, the sensors detect air flowing into or out of a closed container
(usually a thermos bottle to minimize the effects of temperature variations)
as ambient air pressure varies due to a climb or ascent.

The circuit could just as easily be flipped around to indicate when there's
air in the line instead of the opposite.

If this is what you're thinking of, let me know. I'll see if I can scrounge
up the circuit diagram and scan it to you.

Dan Schaefer

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Please send your photos and drawings to marvkaye@olsusa.com.
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