A source of optical sensors that I have used, and that sound very much like
the ones invented by Dave Jones, is:

IMO Industries Inc.
One Cowles Road
Plainville, CT  06062-9987

Their GEMS Express Service number is:  800-847-5691. (they call the sensors
GEMS, I think). Anyway, ask for their "Level & Flow Sensors" catalog - it has a lot of
goodies, like level, flow and float switches that could easily be put to
good use in an airplane. My catalog has a card in it that says to call the
above no. for a free copy of the catalog, so give it a go. I think their
prices are quite reasonable, too.

My main reason for getting interested in these was the fact that I prefer to
manage my fuel manually, mainly to keep the system as simple as possible (to
paraphrase something said in one of the last posts, simple means there's
less to go wrong) but was having a devil of a time telling when the header
was full. I was regularly pumping gas out the vent/overflow, as I bet many
others have, either because of the inaccuracy of the sight gauge or because
it was easy to forget the pump was "on". Since I was not willing to cut into
the tank to install float switches, I decided to design a way to detect the
onset of fuel overflow in the vent line and light a warning indicator. The
GEMS optical sensor seemed to me to be a natural for this job though it does
require a bit of circuit design to be able to drive a light or a relay.
Nonetheless, it was just a matter of figuring out how to plumb it into the
vent hose and cobble up a simple circuit to light the light. I still think
it'd be the way to go and I'll leave it to one of you electronikers out
there to take up where I left off. (At about the time I was finishing the
design, my bench power supply decided to go belly up and zorch everything in
sight, including my sensor samples. As a result, I stopped work on the
optical sensors and turned to my back-up idea, a thermistor bridge sensor
which also works great, so I never got back to finishing the original design
- still think it'd work fine, though.)

The thermistor sensor uses two thermistors in a bridge circuit which are
slightly heated, equally,  with a small trickle of current. One thermistor
is placed in the vent/overflow tube where it can be cooled by gas going out
the vent while the other stays dry and warm. Since a thermistor's resistance
changes with it's temperature, this causes a drastic difference in the
voltage across each which is detected and amplified to drive the light. Not
so foreign to airplanes as sailplanes use a variation on this theme in their
Variometers - a device which tells the pilot when he's getting into lift or
sink by detecting the minute flow of air into, or out of, a sealed container
over a thermistor bridge.

Cheers, Dan Schaefer

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