> Doing that will work very poorly.  You'll be subtracting two large
> measurements to get a small measurement.  Slight errors in the
> measurements will cause a large error in the result.

I'm afraid you could be very right about that... Things can look in
theory, when you forget real-world constraints, and then, on practice,
it just doesn't work...

> Maybe the system could be rearranged so the recirculation occurs after
> the flow sensor.  For example, add a small "day tank" fuel reservoir,
> then measure how much fuel needs to be transferred from the main tank to
> keep it full.  The measurement could be done indirectly by using a
> calibrated fuel injector or a positive-displacement pump to do the
> transfer.

Like a dosing system, I think... I had thought of that... Fill a small
tank, and dump it, fill and dump, fill and dump... Resolution would
depend on the size of the tank. But I don't know where I could put it...
After the pump is not possible. There's the pressure. Before the pump,
I had to find some system to avoid lack of fuel in the system... It's
air-tight, which is a problem... Mechanical, but still a problem.

> A simple approach that doesn't involve the possibility of leaks is to
> just measure the position of the linkage that controls the fuel injector
> pump and engine RPM.  For every position of the throttle, the pump will
> inject a definite amount of fuel per revolution.  Integrate the flows
> over time.

I have to study this option... Although not as generic as I would like,
it seems the easiest one! I already intend to have an RPM sensor, so
the only extra component it's the position of the linkage. I need to
get a hold of data-sheets and talk to my father (he's the expert in
diesel mechanics). Problems that jump to my mind... Diesel pumps have
internal fuel regulation systems... The older ones are mechanical. This
could be a problem when you have the engine in high effort. The RPM is
low, but you are spending a lot of fuel... But the accelator pedal is
down, also, so it could be used to calculate... Seems promising.

Next weekend I'll talk to my father about it.

Nuno.
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  /\/ /  Nuno Filipe Freitas Pedrosa                   __ /..\__/
  \ \/\  mailto:Nuno.Pedrosa@oen.siemens.de           /`'\\__/-\ __
   \/\ \ Tel. : 0049-89-72222342                      \__/  \__//@@\
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