Your probe is a tube with a rod down the center, and a wire connected to each. Each piece is a “plate” of a capacitor. The “capacitance” depends on how high the fuel is in the tube. If you apply an AC voltage, The current is proportional to capacitance, which is the same as saying proportional to height of fuel in the tube. All types of fuel will have the same empty reading when the tube is full of air. Different types of fuel will have different “full” currents because of different fuel permittivity. if you push the “full” button when the tank is visibly full, that amount of current is remembered as 100% for that type of fuel. That is called permittivity calibration. Hope this helps.
It would be possible to measure current at a fixed frequency, but that is not the way I have seen it done…probably because it is desirable that the probe be very low power, so you would be measuring microamps, which is not trivial with AC.
Typically the probe capacitor is used as the frequency determining element of a simple oscillator. A one shot is used to subtract a fixed time from the period of this oscillator, so the remaining pulse width varies substantially with small changes in capacitance/frequency/period. This pulse width can then be measured by a microprocessor or an analog circuit.
Yes, I imagine most manufacturers would do it your way. My sense was that the prior posts suffered from TMI or were too abstract or were about electronics, not fuel gauges. I wanted to give OP some feel for the application. My post stated how the probe capacitance varied with the level of fuel, and why calibration was done… and I think that is what OP wanted. If I’m wrong, then he has your answer as well.
egc that was an excellent write up. Thank you for taking the time.
I know it’s been over a year, but I just came back for a refresher, and I wanted to thank everybody who contributed to the thread. Ignorance fought.