From: CHRISTOPHER_ZAVATSON@udlp.com
Date: Wed, 19 Jul 2000 12:15:10 -0500
Message-ID: <00206827.C21254@udlp.com>
Subject: fuel flow sensors
To: Lancair.list@olsusa.com
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

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I would like to add fuel flow to the instrument panel.  I am currently looking
at the JPI FS450, but the brand is not relevant to my dilemma.  The JPI unit, as
well as other brands, use Flowscan 201 and 231 transducers depending on whether
the system is gravity fed or pump driven.  When I asked for a technical reason
for having different transducers I couldn't get a satisfactory answer from
either Pacific Coast Avionics or JPI.  Each will quote one transducer for high
wing applications and the other for low wing, but neither can tell me why.  Mass
flow is mass flow.  My O-360 will use the same fuel regardless of where the wing
is located.  The only theory I can come up with at this point is that the
gravity fed unit has a lower pressure drop in order to not rob too much of the
limited pressure head in a high wing aircraft (I imagine this comes with a
penalty in accuracy).  In a pump driven system, especially and injected system
loosing a psi is probably not a big deal assuming the transducer is mounted
after the pump.  According to JPI (verbal) the transducer can be installed
anywhere in the fuel line which includes before any pumps.  My concern is that
added restriction before a pump encourages vapor lock by lowering the pressure
between the restriction(transducer) and the pump inlet.  I have a feeling the
flow transducer intended for pump driven engines has a loss on the order of 1
psi.  This is based on a report I found by a Rebel owner that did a flow test
with and without the 201 transducer.  The results were 24 gpm without and 12 gpm
with the transducer.  

Do any of you have experience with these units.  I am curious how many
installations may be out there with transducers installed before any fuel pumps
and if anyone has any technical data for these units.

Chris Zavatson
N91CZ
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