Level tracking using optical or capacitance sensors I am looking to devise a level tracking system using arduino and some sensors. My particular situation is I have solute that is coming out of solution and I want to track the formation rate of the solid phase as the salts settle at the bottom and build a bed of particulates. 

I am looking for precision at about 0.5mm and over a range of like 6 inches. My initial idea was to get a sensor bar that could detect when and where light is hitting the sensor and post processes that signal. I have to design this device to sit outside the cylindrical vessel. The solvent is transparent so any form of light would work. I am stuck on finding the parts that could pull this off and I was hoping the more experienced community could point in the right direction.

* UPDATE *

a little cartoon for a clearer picture of the idea 
I am leaning towards capacitance since using light seems like a rabbit hole. I was playing with some foil and it may work but I need a much more sensitive tool on the order of pico-Farads to work.
What about measuring this with two resistive probes - the electricity will go through down one probe to the top of the water, across the water, and up the other probe. You can measure how much of the probe it went down, by measuring the resistance. You would need the probes to be a specific, very consistent distance apart, and be made of resistance wire.

Another option is to use a float of some kind, and measure it this way. This is a lot easier if all you need to know is full/not full.