Posted for Bill Hannahan <wfhannahan@yahoo.com>:

A while back there was a discussion on this subject. I deleted the files as
they were outside my interest but some ideas keep bouncing around.

There was some talk of a control switch and indicator light for a solenoid
relief valve.

The Three Mile Island plant had a control switch and indicator lights for
its solenoid relief valve. They were labeled open / closed. There were no
micro switches to measure the actual position of the valve. The labels should
have read energized / de-energized. At some point the operator turned the
switch to closed and the closed light illuminated, but the valve was stuck
open, guess what happened next.

If you want the most reliable indication of trouble, measure the key
parameters directly.

1 install a capacitance level probe in the belly tank that will
generate a low level alarm at a selected set point, say 90% full.

2 Install a sensitive solid state pressure transducer on the top of
the belly tank, not on the vent line. Set it up to generate high and low
pressure alarms when the tank pressure drifts outside normal values.

With conservative set points you should get a warning well before the engine
quits due to a fuel supply problem.