Message-ID: <005901c1b8cc$66362c30$3328b0d1@VAIO>
From: "Jim Cameron" <toucan@78055.com>
To: "Lancair Mail List" <lancair.list@olsusa.com>
Subject: Fuel probes
Date: Mon, 18 Feb 2002 16:34:15 -0600
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    Shouldn't make much difference whose fuel probes you use.  They are
simply variable capacitors, with the outer tube one plate, and the inner rod
the other.  As gasoline fills the space between, the capacitance value of
the probe changes because the dielectric constants for air and gasoline are
quite different.
    The only thing to watch out for when cutting them to fit your tank is to
provide a non-conductive spacer at the cut end that will keep the rod in the
center of the outer tube.  The spacer also has to have some holes or a
scalloped edge so that the gasoline can empty and fill the space.  I
salvaged a spacer out of the portion I cut off, but it was crimped on and
was tough to get off.
    When a variable capacitor like these probes is used in an oscillator
circuit, the oscillation frequency will change as the level of fluid
(gasoline) changes.  Converting frequency to voltage is electronically
pretty simple.  The advantage is that the oscillator circuit can operate at
tiny current levels and low voltage, so it's a safe system.  A disadvantage
is that unless the designers put some very good filters in their processing
circuitry, stray RF can raise cain with them.  Watch your EI fuel gauges
when you key your mike to transmit!

Jim Cameron
N143ES (Flying!  -- see later post.)

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