I'm following aplication note "Liquid Level Sensing Using Capacitive-to-Digital Converters" to desing some similar sensors using the AD7746. My question is about the formula for the approximate capacitance per length (page 2). How does the liquid and the PCB permitivity influence in that equation? I will like to know, for example, how to calculate the PCB traces for measuring 20 cm of water.
Also, the +-4pF range means that I shoud desing the sensor with 8pF at it's maximum level?