Message-ID: <003d01bf2d82$57df17a0$480efea9@peweston>
From: "Pat Weston" <peweston@teleport.com>
To: "Lancair List" <lancair.list@olsusa.com>,
	"J. N. Cameron" <CIC@centurytel.net>
Subject: Converting capacitative to resistive fuel level
Date: Fri, 12 Nov 1999 18:53:01 -0800
Mime-Version: 1.0

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Jim,

Thanks for the reply. I have designed a circuit that converts capacitance to
current. The probe's capacitance doubles from an empty tank to a full one
(the dielectric constant of avgas is close enough to 2). The problem is that
my circuit varies the current linearly with change in capacitance (increase
the capacitance by 1.5, the current decreases by 1.5, and yes this circuit
uses oscillators and all that good stuff). The gauge was designed to convert
a change in resistance to a meter movement, linearly. The problem is that
the impedance that the gauge presents to my varying current source (sink,
actually) results in  a non linear relationship between capacitance change
and meter movement. That is why I would like to know what the innards
(schematic diagram) of the gauge looks like. I believe it to be some form of
Wheatstone bridge, but I get confusing results when measuring resistances
and currents.

BTW the total cost of the parts for the circuit I currently have is around
$5.00 per side (two 555 timers and a handful of discrete) from Radio Shack.
Also the nonlinearity isn't that bad but I'd like to fix it. I've also heard
of the radio problem, I am planning on inserting a low pass filter between
the probe and the circuit which hopefully will cure that problem.

Pat
"Only three short years to go!"
http://www.teleport.com/~peweston

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