Message text written by "LTS"
>Advantages:
Inexpensive
Very low current draw
Not susceptible to fuel density changes (like a capacitance device)
Does not break if in contact with water
Very well damped
Readings are very consistent
Very reliable
Currently used by four of Europe's leading A/C manufacturers (soon to be 4)
Capacitance gauges may appear more accurate because they have better than
1/8th resolution but because of the variations in fuel make up they are
often less accurate. The difference being with our gauge if it indicates 20
litres you have at least 20 litres. Whereas with a capacitance gauge if it
says you have 22.5 litres may have 22.5 litres then again you may not.
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I would say that the reliability is the main advantage. I have a double
probe capacitance system and will report back if I lose accuracy. The very
low current draw is not an issue as no gauging systems use significant
amounts of current anyway. I would suggest that, looking at the percentage
accuracy we are talking about, fuel density is not a significant issue
either. Even if there is a difference in fuel density between Avgas and
unleaded it is unlikely to amount to much (does anyone have any figures on
this?). In fact one of the reasons for large aircraft using capacitance
systems is that fuel weight is more important than fuel volume as energy is
proportional to weight not volume. Water is without doubt the main enemy
to the capacitance system. Keeping moist air out of the tank and filtering
the water out of the fuel in the first place is important. Damping can be
achieved in any electrical gauging system by using the appropriate level of
capacitance. Without doubt the magnetic system will be consistent
particularly if the fuel quantity is measured at each switch changeover
point.
I still feel an accurately set up integrator system using a fuel flow
sender is likely to give the best results. It still needs a tank gauging
system to act as a cross check to guard against misinformation due to leaks
and must be updated during refuelling. It doesn't need to make allowances
for tank shape and senders are typically calibrated to 2 - 3% which is
likely to be much better than any existing gauging system. With large
aircraft, where large amounts of fuel in several different tanks over large
periods of time have to be measured, we find that the guages are often only
more accurate at full and empty. In between the linear accuracy of the
integrator system proves to be more consistent.
However we measure the fuel quantity, having a final warning in the form
of a reserve is a useful measure to ensure a landing is achieved before
fuel exhaustion.
Nigel Charles
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