How would it be possible to measure layer of water on an infinitely flat surface, with every time different, but constant layer of water?
I'm trying to measure thickness from $1\rm\,\mu m$ to $5\rm\, mm$. I've already tried conductivity, but I'm only able to measure precisely from $0.5\rm\, mm$.
I need to find a way to get more precise result than that; maybe ultrasound, resistance, etc.
Any idea?