Today's Message Index:
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1. 06:41 AM - Re: Noise in damp weather (Glen Matejcek)
2. 10:23 PM - Re: Re: Capacitive Fuel Level Sensor - How do they work? (Paul Millner)
Message 1
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| Subject: | Re: Noise in damp weather |
The part about the shriek with an applied load that is corrected by cycling the
master sounds a lot like an experience I once had with a balky reverse current
relay. On landing roll out, just when no one would ever be looking at the electrical
gauges, the bus voltage would go way low and the avionics would start
to howl.
Clearly an RCR isn't the problem here, but perhaps transient very low bus voltage
is...
Glen Matejcek
Message 2
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| Subject: | Re: Capacitive Fuel Level Sensor - How do they |
work?
On 1/2/2012 8:41 AM, jonlaury wrote:
> 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;
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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