Thank you folks for feedback.
Yes, you are right. The aquarium temperature is pretty constant and also measured by the same controller, so it can be used if needed. The collector cup is more at room temperature. If that would be the issue, I can add a temperature sensor nearby. The salt content in the aquarium is constant (for this purpose) and also evaluated by the controller with an EC probe. The salt in the collector cup is unknown. It does vary a lot, I guess.
Since both ideas have zero or near zero material costs, I am willing to test both of them:
- Using an I/O pin and a resistor. I still have to figure out the code on this one.
- Using 4060 circuit as shown in the Nuts & Volts article.
The main thing would be to make the capacitor. I am thinking that a dual strip capacitor (two vertical strips) would work better compared to one at the bottom and one vertical. I see it as two capacitors in series. If the liquid rises, both capacitors will vary, so there is going to be more overall change in capacity (better accuracy?). Just thinking loud.
All the metallic parts would be outside of the collector cup.
Strip width? One inch? One and a half? I have no clue. Probably the more the better.
I will leave improvements like human hand proximity at a later stage.
I have a concern with the capacitive methods, but not so big to stop me testing these two methods. When the container gets dirty, deposits of dirt (thickened liquid) are on the inside walls. The inside is also humid, so the response would probably be a cloud of values instead of a line or curve. Would measurement at two frequencies solve this or not? Will find out later :).