Here is another bizarre thought that occured to me. You could try using a sensitive ohm meter with the probe you describe. When the liquid covers just the bottom portion of the probe, the resistance is likely to be less than say when all the probe is covered? If you have, or can borrow, a good old analogue type VOM, you could try that as an experiment. The old RS ones [like the one that is gathering dust at my Mom's house!] included a schematic that could be helpful for future modifications. Let's say this experiment sort of works e.g. the VOM is at infinity when the jar is empty, and reading 10 megaohms when full. You might want to track down a working VOM [a nice big sensitive one] at a flea market etc. Then you could try modifying it to be more sensitive. Hmm - changing the resistor values? Higher voltage battery? Seems my old rig at Mom's used an AA for all but the highest range, and a 9 volt for the highest, so why not try 2 or even 3 [we're getting really brave here!] 9 volt batteries in series [eek 18 or 27 volts!] to increase the sensitivity. DON'T try this with a digital display VOM, or with a precious analogue rig - this is for that $3.00 working special that you picked up at "Wade's Working Wired Wonders Emporium"! If you find that you can modify a VOM to provide a nice scale reading, you can always make a neat fluid volume guage dial out of paper or whatever and glue it on the old VOM [some of those meters you can open up the actual meter, install your new label, then reglue the plastic meter back together. It may not be pretty, but properly calibrated, it may be pretty accurate! Happy experimenting and good luck!
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