Making a small quantity of gypsum sensors using 3/4" PVC pipe couplers works pretty well. Due to planned field trials, I was looking for an easy way to produce more sensors while maintaining consistency.

I decided to do an experiment with 2-component silicone rubber, and ended up buying the OOMOO-30 trail kit. This kit produces just under one liter of rubber. With that quantity a mold for 9 sensors is possible. The material is easy to work with, and vacuum degassing is not necessary. Curing takes about 6 hours.

First step is to make the sensor casting shapes, using 3/4 PVC pipe cut to 5 cm length, glue a cap on, and insert the machine screws. The machine screws will provide guide holes in the rubber cast that will precisely align the electrodes 12.5 mm apart in the gypsum cast.

Next I made a plastic container, and fixed the sensor casts in place. A wooden frame ensures that the container keeps its shape when the rubber compound is poured in.

It is important to use a mold release agent avoiding rubber sticking to the cast parts. Even after using a release agent spray, I still had some sticky spots on the 3 rubber casting molds I made so far.

Here is the cast ready to receive the silicone rubber:

Below picture shows the rubber mold with electrodes and fiberglass mesh inserted. Ready for casting the Plaster of Paris .
Tap the mold repeatedly -or use a vibrating table- to remove air bubbles.

After 30 minutes, the gypsum sensors can be removed.