Dave Simpson wrote:
>
> Writing on Avelec's behalf, I believe the reasons for the variations in the
> capacitance guages are fuels' different dielectric constants, rather than
> density. Such changes affect the sensed capacitance.
I did a simple test yesterday, measuring capacitance with both mogas
and avgas, coincidentally both produced by BP-Amoco. It showed a nice
20% difference in capacitance, and hence dielectric property, with
thus inaccuracy a little on the intolerable side. Of course, now I
see there's a full list available at:
http://www.ridgenet.net/~hideseng/dc_list.htm. It shows variances
---From some standard hydrocarbon formulation.
The two test caps I constructed were relatively large, .0025uf, to
minimize stray capacitance. An interesting thing I found in net
browsing was that the probes used in production aircraft are much
larger than what they sell to homebuilders. The most significant
factor in capacitance is the distance between the metals, another one
of those square root relationships. But there's a limit, as otherwise
fuel will "stick" due to surface tension, causing inaccuracy. Some
mfrs I note have liberal perforations in the outer tube, I guess to
fix that.
The skinny probes they sell to homebuilders appear to have much
smaller capacitance, a tiny fraction of .0025uf, based on the area of
metal foil I had to use. There's nothing theoretically wrong with
that, but it places the burden on circuit design to cancel the effects
of temperature, stray capacitance, and even varying dielectrics.
Weight is a factor in bizjets too, and I think if people like
Parker-Hannifin could figure a way to give you accuracy with a
skinny-ass probe, they would have done so.
Another curiosity in perusing various designs was TWA 800, whose
center wing tank exploded. Its probes were of an old design which
could expose the tank innards, via the probe wires, to "arc over" from
400vac wiring in the bundle to the flight deck. They're not sure,
cause that particular bundle still lives with the fishes. But it's
also why, in a plastic tank inside a plastic airplane, if you put an
electric probe in there, you really don't want to go near lightning.
It would still be a good idea to run the wiring not in any other
bundle, nor anywhere near control hardware or battery cables. Like
along the fuselage side. Maybe even #30 wire, circuit permitting, for
a smaller lightning target and the hope it might vaporize before....
Regards,
Fred F.
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