Tony Renshaw wrote:
>
> Gidday,
> Could someone explain the principle of capacitance fuel gauges, and whether
> these are the only type suitable or whether a float type would work?
Either one should work fine. Capacitance gauges are a bit more
expensive, but more accurate and--perhaps significant--lots easier to
install.
Here's the idea. In a conventional float gauge, the rising and sinking
float moves a potentiometer, changing the DC voltage sent on to the
gauge. If the potentiometer's windings get dirty, you'll get a jerky or
"noisy" gauge. Moreover, unless it has a built-in regulator (Zener
diode, etc.), there's the possibility that changes in aircraft supply
voltage can cause (small) errors in fuel level indication.
Most important, you have to find room in the tank to install the float
so it can move over the entire fuel level range unobstructed. Depending
on the size and shape of the tank, this can be easy or difficult.
Sloshing of fuel inflight will cause the gauge to rise and fall.
typically a pair of concentric tubes, open at both top and bottom, so
fuel can rise and fall both in and between them. What's being measured
is the capacitance between the tubes, which will vary since fuel has a
different dielectric value from air. Simple circuitry converts this to
an appropriate output for display.
The beauty part is that the sender tube doesn't have to be vertical, as
long as it covers the entire range of fuel--it can even be bent to odd
shapes to correspond to odd-shaped tanks. (If the tank is metal, you
don't even need a tube--you can use the tank itself as one side, with a
simple wire for the other). Or, for something like Europa, you can run
your tube diagonally down the tank's front or rear, increasing its
length to get more capacitance change per unit fuel change. You could
even put in two senders, hooked in parallel, if you wanted to measure
total tank capacity rather than having to take the last bit in the
reserve on faith.
tend to slow down the rate at which fuel runs in and out, thus reducing
the effect of sloshing.
All this having been said, I still like the relatively foolproof "fuel
in a sight tube" arrangement as specified with the kit!
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