I have a piece of double sided PCB that I am plan to use a capacitor to measure soil moisture (somewhere between 133-170MHz).
I used the "two-pin method" of measuring small capacitance on the Arduino (calibrated against several values of ceramic capacitors between 2pF and 1nF).
Using the insulated board, I got the following results:
- Air: 62pF
- Hand grip: 26-27pF
- Immersed 1/4 in water: 48pF
- Immersed 1/2 in water: 60pF
- Immersed 3/4 in water: 69pF
- Immersed fully in water: 97pF
Firstly, why would the capacitance value initially decrease as the dielectric constant went from 1 (air) to >1 (my hand and partially submerged in water)?
Secondly, why would I only see such a small change when the board went from air (dielectric constant of 1) to fully immersed in water (dielectric constant of 80)? I accept that the leads from the boards will have some effect on the capacitance, but I doubt that they will have enough effect so as to swamp out the effect of water.