The dual level soil moisture sensors have been removed from the system, sort of... They are still in the pots, and still hooked up to the Sensor Modules, but they are not being used to determine whether to water the plants. I've put regular sensors back in, and they will be used.
The reason I'm not using them now, they don't seem to work. The top sensor read under 5% or 6% while the bottom read upward of 70%. That may be fine, but over two weeks, those readings did not change in any reasonable way. In two weeks, I should see a drop in moisture, but all I saw were random fluctuations.
A normal sensor output looks like this
I'm not sure why the dual level sensor would be any different than the single level one, it's made the same way from the same materials. The difference is the dual uses a single probe to read, and two probes to power it, meaning when it tests the top level, it applies 5v to the short probe, near the surface, and reads the center probe, which actually extends to the bottom of the pot. That 5v should take the path of least resistance, which is going to be the shortest route to ground, right across the 1/2 inch or so gap. Then when it reads the bottom sensor, it applies power to the long probe (which is shielded from touching soil until it is past the end of the top probe, so that again, it should read the moisture at the lower level. Since it only powers one side at a time, it should be no different than using two sensors with one shoved deep into the dirt, and the other at the surface...
I can see for sure that it is seeing two distinct sensors, the readings are vastly different, and although somewhat random, they are consistent...
It's also possible there's a bug in my software on the Arduino Pro/Mini's that are used as I2C slave modules.
As soon as I get some time, I'll come back to these and see why they failed... I do have faith that I will eventually get them working properly without having to do a major redesign.