Message-ID: <39CCBAB3.FBC64F5@regandesigns.com>
Date: Sat, 23 Sep 2000 09:14:11 -0500
From: Brent Regan <brent@regandesigns.com>
To: Lancair List <lancair.list@olsusa.com>
Subject: Re: Vision Micro problems
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

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Given that some planes have the problem and some don't, we can conclude that
the problem is installation specific.

Electrical noise is a tricky thing to track down but is best eliminated
using some clear understanding and an oscilloscope. The fuel systems in
question uses capacitive fuel probes. These probes have coaxial aluminum
tubes that flood with fuel. The dielectric strength of air is different than
fuel so the electrical capacitance of the probes changes with fuel level. At
the wire end of the probe there is an oscillator circuit. The frequency of
the oscillator changes with the probe capacitance and therefore the fuel
level. It is located close to the end of the probe to reduce interference.
The fuel gage is actually a frequency counter.

Where can noise get in this system? Anywhere. If the probe is the right
length it can act as an antenna, causing the oscillator frequency to shift.
If the signal wire is not properly shielded, noise can get in and swamp the
signal. Same goes for the power and ground lines.

Find a portable oscilloscope and someone who knows how to use it well.
Connect your ground reference to the battery and check the power, ground and
signal lines at the affected components. Remember, you can ground one end of
a wire and have a signal at the other end, that is how an antenna works.

I have said it before and it is worth repeating. The Vision hardware is
decent and reliable but the cable and faston connectors they supply are
C**P. Get yourself some MIL 27500 shielded cable and solder everything.
Crimped connections are allowed ONLY if the crimper is a 4 die type and set
you back at least $250. Don't be penny wise and $$ foolish.

Regards
Brent

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