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I'm not an EE but the whole capacitive probe setup seems sketchy to me.
You have a 5v square wave signal coming from the sender with limited or
no shielding. In some cases the sender is connected to a freq->voltage
converter and then on to the engine monitor. The converter is a circuit
board in a plastic box with no shielding (and maybe no grounding) in my
case.
In my day job, I work for a company that makes capacitive sensors.
We make the sensors that detect your finger on a laptop touchpad,
or cell phone, or music player. Properly designed, the sensors
are very sensitive, resistant to RF interference, and almost
indestructible. The plastic box with a F/V converter you described
is very primitive by comparison with modern electronics. Sounds like
the Heathkit version from the 1970's.
A simple fuel level sensor uses a wire inside a tube.
When the tube is full of fuel, the capacitance is greater than when
it is full of air. The only moving part is the fuel itself.
This coaxial part is very robust, and easy to electrically shield
from interference.
EI says resistive probes wear out sooner but I wonder.
Seems like a more appropriate solution.
The resistive probes that I have seen have a moving float
connected to a potentiometer. The fact that it has moving
parts exposes it to wear. The fact that it uses a sliding
contact potentiometer means that it will wear out fairly soon.
If there is enough demand, and a reasonably standard output
format, I could make a circuit for measuring capacitance and
reporting it to an MFD. With only a little more effort it could
detect water or alcohol in the fuel and set an alarm. The size and
weight would be negligible compared to the sensing tube. Who would
know what output format would be appropriate?
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