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capacitive fuel gauge for petrol

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i am trying to make a better fuel gauge for my weekend car
it currently uses 9 reed switches, and a floating magnet, with an arduino controller damping out the sloshing
its not the best

have stolen the idea from here for a capacitive probe

have built a rough prototype of the probe

3/4" bore tube with an 8mm rod

roughly 9" long

get according to my one multimeter with capacitance, about 16pf empty and 32pf full

dunno why but i expected the cap to go down as the fuel level went up

do you think  thats a big enough change ?

if i go larger on the center rod, the values will increase, but will the percentage change (give me a larger swing ?)

The capacitance change is big enough.

What is the challenge here is the capcitance itself - it is a  fairly low value. You better put the arduino right on top of that capacitance tube to eliminate any stray capacitances as far as possible.

And yes, the capacitance should go up as the gasoline replaces air as dielectric. Actually that is not air, it is saturated gas vapor.

The capacitance is also affected by tempearature at some degree. The author recommends using a parallel capacitor that has the opposite temperature coefficient. This works but at the same time it makes the capacitance change that much smaller. A better solution is to measure the temperature and programmatically correct the reading. A simple NTC can be used for temperature measurement.

The mechanical construction is a little bit wobbly. The capacitance is affected by the distance of the electrodes and if the middle rod bends, You will get a different reading. It might be a good idea to have some support for the rod in between as well. The construction shows support only at the ends.

Making the capacitor larger will not impact the relative capacitance change. However it will increase the base capacitance. The base capacitance is diretrly proportional to the surface area of capacitor electrodes. If You double the area You will double the capacitance.

Mine is a lot more mechanically robust.  I did have another go with a larger center tube of 15m. That gave me roughly 50 to 130pf
The arduino will be right on top of the probe.

I had considered temperature compensation with  an ntc  that will depend how much it changes. It may be useable without, time will tell

Might be easier to build an oscillator that uses the sensor then count cycles..... (if you're not already doing that).

to be honest, am not entirely sure how the arduino is doing its black magic!!
about to start soldering now!


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