Pete,
The problem with the BeLite probe is that it is not a differential pressure
transducer.
The tank vent faces into the airstream and pressurizes the tank. The
amount of pressure varies with airspeed. While the amount of pressure from
the vent is small, the weight of fuel is also quite small. This will affect the
calibration.
I have been using this method of measuring fuel level for many years. I started
with the system Tony K from NZ published around 2000. Tony specified a
differential
pressure transducer. Over the years I have upgraded with newer transducers.
After one of my upgrades I couldn't figure out why the calibration in
flight was different than that on the ground until I realized about the vent
pressure. Changing to a differential transducer solved the problem. I am
currently
using MPXV4006DP transducers. Since they are not rated for fuel, I use
a loop of tubing to force a column of air between the transducer and the liquid.
I agree with Will Daniell, having a tank sensor is not terribly useful. Since
the Rotax only uses about 5 gph, you need accuracy of 1 gal to judge 12 minute
endurance. That's difficult. The better solution is to use the "main" tank
and as it gets low, monitor fuel pressure. When fuel pressure starts to drop,
switch tanks. You can easily do this, especially with an EFIS, and the engine
won't miss a beat. Then you have the 3 gal in the reserve side, roughly 30
minutes to land.
As always, just my opinion and experience.
Jim Butcher
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