REFLECTOR:Low fuel light - NO!

[Alexander Balic]

Rene,
you might want to re locate your level switch about 1/2-3/4" down from it's
current position, when I built my sump, I installed 2 hard points for my
sensor, and currently have it in the top one with a plug in the bottom, but
if I have any problems with it, then I will switch that around- it is very
difficult I think to eliminate all of the air from the sump, and also there
might be some foam on top also, that will sink the float.

-----Original Message-----
From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On
Behalf Of Rene Dugas
Sent: Thursday, July 24, 2003 4:40 PM
To: reflector@tvbf.org
Subject: RE: REFLECTOR:Low fuel light - NO!


I agree a shut off valve may not be the safest choice.  Your assessment
of inadequate flow into the sump from one or both main tanks thereby
lowering the level of the fuel in the vent line above the sump could
introduce air into the sump.  That is why I tried to SLIGHTLY increase
my pressure in the vent line by increasing the bevel and length of the
vent tube overboard as well as its size to 1/2".  No change.  Still
comes on and flickers only in low flow cruise at about 13.5 to 14 gal/hr
lean.  I covered the fuel lines in the engine compartment with fire
sleeve to be sure I was not bubbling boiling fuel back into the sump or
the fuel line.  I have quickly removed my lower cowl after running the
engine for 15 minutes on the ground and the engine compartment is not
very hot and I can hold the fuel lines easily in my hand.
Remember the light only flashes on in smooth air and level flight.
Thanks for the mental exercises.  Keep them coming.
Rene'

-----Original Message-----
From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org] On
Behalf Of Brett Ferrell
Sent: Wednesday, July 23, 2003 7:46 PM
To: reflector@tvbf.org
Subject: Re: REFLECTOR:Low fuel light - NO!

Please, please, please do not do this (install a vent shutoff valve)!
Air must enter the system as the fuel drains out or 1 of 2 really bad
things will happen.
1)  Pop-Bottle plug effect - where the gas glug-glug-glugs to your
engine, or stops altogther as air cannot replace the lost fluid-or-
2)  The tanks will crush as the fuel pulls vacuum on the tank and the
outside air pressure pops the strake open

Having seen a 30,000 industrial tank get crushed like a soda can from a
plugged vent & overflow line, I'm horrified by the idea of this
happening in flight.  Allowing air into the tanks is the primary purpose
of the vent, and why you should install the check valve to allow cabin
air into the tanks (but no gas-vapor into the cabin) should the external
vent get blocked by ice.

If there's an adequate fuel level in the stakes, an air bubble should
not be able to form in the sump, as the head pressure from the strakes
should fill it to the level (in the vent line) of the strakes (fluids
seek their own level and all).  If a bubble were to form it would
indicate that the fuel pump is pulling the level faster than the tanks
can replenish it.

Brett

Quoting Bob Kuc <bkuc1@tampabay.rr.com>:

> Re: REFLECTOR:Low fuel light
> I was think about that also.  However, if you are on
> the low side of > fuel and then make a turn, the fuel on the inboard
> stake would rush to > the end.  Would that not introduce air into the
sump
> tank that you would > have no way of relieving because you have the
vent
> blocked off?
>
> Bob
>   I plan to install a manual vent shut-off valve
> between the sump tank > and the vent manifold.  The result will be
that the
> vent system can > allow the main strake tanks to drain, but can't
> introduce air into the > sump tank.  I have a theory that the vent
line to the
> sump tank causes > more problems than it solves, once the initial
"burp"
> of air is allowed > to escape from the sump tank (before engine
start).
>
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