I’ve been looking into creating an automated herbarium of some sort for a while, and I came across the brilliant post about creating some DIY soil moisture sensors using nails and plaster of paris. cheapvegetablegardener from hackaday.
I wont explain all of the theory or background as it is already explained on cheapvegetablegardener.
I measured the resistance of the sensor in air to be 12Kohms, I then placed the sensor into water (keeping the nail heads above the surface). The resistance dropped to 4Kohms whilst in the water, and then over 20 mins of back in air the resistance rose to 5.6Kohms. The sample still looks and feels quite wet, so I imagine it is going to take some time to dry, but from these prelimary results we can see it appears to work.
Soldering the wires to the nails before creating the plaster of paris along with covering the end sensor with hot glue would improve its longevity. I will post again once I find a better mould to make the plaster in as it was quite difficult to get them out of the cuvettes (I had about a 50% success rate).
Here is a microscope image of the surface of the plaster: