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Hello All,

I have a little dilemma.
1) I have built a model of a 'Comb shaped' capacitor sensor and simulated using COMSOL which worked great, I simulated changes in water level in a PVC container the 'global evaluation' of capacitance increased as water level raises.

2) I built the particular sensor to evaluate my simulation model. The base capacitance with the overlay, no water, closely matched what I predicted with simulation.

3)However I came across something unexpected. When varying the water level in the container, the capacitance measured using Agilent LCR meter dropped with increasing water level completely dismissing my simulation predictions.

4) With further investigation, I narrowed down to what I believe is the simple equivalent parallel plate capacitor physics in play "Two different dielectric layers in series between the plates of a parallel plate capacitor".

5) In the experiment I placed a thin overlay on top of the sensor measured the capacitance with the sensor dry and wet and noticed an increase in capacitance with addition of water

6) Next with no overlay and following same procedure as 5) a positive change in capacitance was observed

7) when a thick plastic overlay was place dry to wet sensor a negative change in capacitance was observed

Essentially I have three capacitors in series
equation 1: Ct = (C'sensor'*C'overlay')/(C'sensor'+C'overlay'),
equation 2: C = (k*er)/d,

When thickness of the overlay is small 'd << er' there is a large capacitance in the system meaning insignificant change to system capacitance and we can observe a positive change in capacitance when water droplets are placed on the thin overlay since water has a high 'er'.

Now when the overlay is thick 'd < er ' there is a small capacitance therefore the total capacitance becomes very small. When the water is placed in the thick overlay there is a negative change in the overall capacitance which must mean that 'd' becomes significant factor

Now to my query, how do I go about simulating such phenomena in COMSOL?????