PCB coating for use in garden soil (moisture sensor)

[Tunelabguy]

I have a PCB design for a capacitive moisture sensor using the PCB traces as the plates of the capacitor. The PCB is shoved into the ground and senses the moisture by the effect on the capacitance between two sets of traces. This is nothing new. Others have done it before. But my question is how to protect the traces from making electrical contact with the soil. The PCB already comes with a pretty hard solder mask that seems to be doing a good job at maintaining isolation. But I have only shoved the PCB in the ground a few times. I expect that with repeated use, especially if the soil contains abrasive pebbles, the solder mask will be scratched and isolation will be compromised. I would like to coat the PCB with something harder, but it can't be too thick because the effect on the capacitance decreases if the soil is kept too far from the traces. I tried 5-minute two-part epoxy, but it flaked off. I tried Krylon spay paint, and it also was weaker than the underlying solder mask. I am considering two-part acrylic resin (hobby type). But I would like to get some other ideas as well.

-Bob Scott
Hopkins, MN

 

[d123]

Hi,

Off top of head, what about looking into materials used for underground irrigation and seeing how adaptable they are? I guess they would be too thick.

 

[betwixt]

What surface area are the PCB traces?

For most of that kind of application I use this resin:
https://www.rapidonline.com/alh-sys...rethane-potting-compound-50ml-syringe-87-7176
but note that for best results you need the cartridge gun to ensure a perfect mix. It has a clever locking resealing system on the tubes so it keeps for a long time if you have some left over. I use it in harsh environments, >50C very high humidity and even in one case under 2.5 metres of 40C chlorinated water and after lots of experimenting it's the only resin that has survived in all installations. The underwater application is still perfect after 5 years, most epoxy resins lasted no more than a few weeks. It is very hard after about 1 hour of mixing but I always leave it 24 hours before abuse.


Brian.

 

[KlausST]

Hi,

For a hobbyist solution:
Maybe you could remove the sharp edges of the PCB and use thermal lamination foil around it and over the complete sensor area.

Try to slightly roughen the surface before, then maybe use a hot iron to apply it.

Klaus

 

[Tunelabguy]

What surface area are the PCB traces?

For most of that kind of application I use this resin:
https://www.rapidonline.com/alh-sys...rethane-potting-compound-50ml-syringe-87-7176
but note that for best results you need the cartridge gun to ensure a perfect mix. It has a clever locking resealing system on the tubes so it keeps for a long time if you have some left over. I use it in harsh environments, >50C very high humidity and even in one case under 2.5 metres of 40C chlorinated water and after lots of experimenting it's the only resin that has survived in all installations. The underwater application is still perfect after 5 years, most epoxy resins lasted no more than a few weeks. It is very hard after about 1 hour of mixing but I always leave it 24 hours before abuse.


Brian.

This looks ideal. Just what I was looking for. Thanks.