A while ago, a customer asked us about the reliability of capacitive soil moisture sensors. Apparently they had played with these "5 for $10" soil moisture sensors bought from Amazon and they all had failed because of "moisture penetration of the PCBs", as the customer put it!! 😀🤔
It is interesting to know this customer is not alone and there are many people out there who think printed circuit boards can absorb moisture and their experience comes from the same type of soil moisture sensors (if you can even call them sensors).
Amazon has greatly changed our shopping culture. We've got used to getting cheap stuff and getting them super fast. This is not bad, but is Amazon a good platform to search for and buy research-grade sensors and instruments?
The answer is a resounding "NO"!
Ok, this is not what I was planning to get into. I was going to explain how we manufacture our sensors (e.g. APAS T1 soil moisture sensor) and build them so rugged that moisture and other environmental elements can do them no harm. You can bury our sensors, leave them in the wild, or even submerge them in the water without any concern.
While moisture can, in general, damage electronic circuitry, it is not because it can penetrate the PCB material, but because it can corrode the junctions, damage components, or create short-circuits specially if some impurity (e.g. salt) of any kind is present (due to high electrical conductivity).
Using PCB as soil moisture sensor electrodes is very common in the industry, but the circuitry is always protected. The most common method to protect the electronics, however, is "injection molding" of plastic or robber that I'm not personally a fan of, because of its short life span. Sensors made this way will roughly last a year or two in the field and can't handle the pressure or mechanical shock applied during the installation process or use.
Now let's see how we've made sure that the APAS T1 soil moisture sensor lasts forever:
The APAS T1 sensor measures the water content of the soil or soilless media using its green sensor blade, which is made of a 4-layer PCB. The traces (or electrodes) are in the middle-layers of the PCB and completely covered. Thus, they will not be exposed to moisture or affected by soil EC. Surface scratches are not a concern either.
The electronic components of the APAS T1 are first encased in a thick plastic cover and then filled with durable electronic-grade epoxy, which is resistant to moisture, humidity, corrosion, fungus, shock, static discharge, etc. This forms the sensor head (black).
Soil moisture sensors that we manufactured this way over 10 years ago (APAS T1 predecessors) still work. This may not be good for the environment (because the sensors last forever!), but definitely great for our customers! 😉