REFLECTOR:Low fuel light

[John Dibble]

I think the sensor in the sump is more confusing than it is helpful.  I
think a sight glass would be ideal.  Yesterday I took a 6.5 hour trip.
The low fuel light did not come on until five minutes after takeoff.  At
first it just flashed occasionally.  By the time I got to cruise at 8500
it was on all the time, but went off and immediately back on once in a
while, so I figured the fuel level must be stable at the sensor level.
It's always been this way, and prior to the current discussion I didn't
give it much thought, figuring that if there is fuel in both tanks, the
sump has to be full.  But the idea of vacuum on the tank from a leaky
cap makes me wonder.  What about the tank vent which points forward?
Doesn't this tend to pressurize the tank with a ram air effect?  It
seems that any cap leak would have to be larger than the size of the
vent opening in order for a vacuum to occur and I can't imagine such a
big leak unless the cap is completely off.  It seems like the tanks
would have a slight pressure from the vent.
Also, at 40 gallons of fuel remaining, one tank was 5 gallons lower than
the other.  Applying the opposite rudder equalized the tanks and I was
planning on shimming my rudder like Scot mentioned, until the fuel got
down to 16 gallons.  At that point, both tanks were even and remained
that way.  However at about that time I descended to 4500 to get under
some clouds.  Maybe altitude makes a difference.

John