I am interested in making a DIY PCB soil sensor based on 555 timer IC. The circuit works well but I have problems with regard to manufacturing a durable PCB. As I keep the PCB based soil sensor immersed in soil for longer than a month, the PCB starts to discolor and rust. This does not happen for the commercial sensor shown below. Note: even if there is a laminated layer around the copper to protect it, the sensor detects soil moisture fine. However, this lamination is not durable.
This sensor is still in the prototype phase. I would appreciate advice on how I can ensure the durability of the PCB.
I heard that a process called "Conformal coating" can be done. But I do not know what type of manufactures I should refer to have such a PCB manufactured. The generic PCB manufactures seems not to do Conformal coating.