Message-ID: <39D73B9C.32356CB5@regandesigns.com>
Date: Sun, 01 Oct 2000 08:26:52 -0500
From: Brent Regan <brent@regandesigns.com>
To: Lancair List <lancair.list@olsusa.com>
Subject: Re: VMS flow transducer
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

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I have also seen a discrepancy between the totalizer and the gage after a
long flight. The delta is around 2 gallons (2%) and I believe it to be
caused by one of four things:

1) Turbulence. I have the sensor mounted horizontally with 6 inches of
straight line on each side but it is mounted to the engine (just before the
flow divider) so the vibration may be affecting it.

2) Temperature. By the time the fuel has arrived at the transducer it has
picked up some heat from the engine and the fuel pumps. This heat would
cause the fuel to expand so that the engine is in fact using "more" gallons
than are leaving the tank.

3) True density. When I calibrated my fuel probes I used an electronic
balance to weigh in the fuel two gallons at a time. I used multiple methods
to determine how much two gallons weighed and averaged them. I was surprised
to learn that 100LL weighs 5.79 pounds per gallon and not 6 pounds per
gallon. Perhaps the lower density is affecting the flow sensor.

4) Tolerance. Manufacturing variances between sensors could account for the
difference. The fact that all the sensors have the same K factor (pulses per
gallon) would indicate that there is a tolerance band.

IMHO we are all fortunate to have the accuracy to know that there is a
couple of gallon difference. If you don't agree then go fly a 172 where the
fuel indications are "I think the tanks are near full", "There may be some
fuel sloshing around" and "If you stare at me long enough I may twitch".

If you are determined to have accurate totalizer results then you need to
track the performance of the totalizer to determine exactly how much it is
off by. Vision should then be able to burn you a new EPROM with a different
K factor coefficient to account for the installation variances. Viola!

Regards
Brent Regan

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