millner(at)me.com
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 Posted: Tue Jan 10, 2012 10:23 pm Post subject: Capacitive Fuel Level Sensor - How do they work? |
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On 1/2/2012 8:41 AM, jonlaury wrote:
| Quote: | Cap probes allow for fuel measurement of irregularly shaped tanks. My fuel bays in the wing change in taper, both in depth and width, in addition to the whole tank sitting at an angle (dihedral), making any kind of accurate measurement impossible over the tank range when using a linear type (float) sensor.
I believe the capacitive probe will be linear over its length as well;
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it won't "know" about the cross sectional area of the tank at any given
sensing point, just whether or not fuel is present there.
The advantage may be that a long capacitive probe can extend through a
tank designed such that a float type sensor, on one end or the other,
will 'range out', because when one end of the tank is full, the other
end is still not full, or vice versa. It's also possible, though
difficult, to custom bend a capacitive probe so that there's more range
(flatter slope) in greater cross-sectional areas of the tank, and less
range (steeper slope) in smaller areas of the tank; that tends to
linearize the output. But it's a tough thing to accomplish with high
resolution.
Generally, linearizing a broad range signal is a problem that's been
solved many times in electronics technology with signal conditioners,
even before the advent of microprocessors. Whether anyone offers such a
product, beyond the full/empty adjustments Jon mentioned, I do not
know. But the all-in-one EI and JPI solutions, as well as
AerospaceLogic's standalone indicators offer such lindearization built
into their displays.
In the 80's in process instrumentation industry, there were a number of
well-designed (non-interactive adjustment) signal conditioners, for
under $100, that had three or four linearization adjustments across an
input range.
Paul
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