After a distraction from my yearly camping trip to AirVenture, getting ready to teach my Fall class and other Summer distractions it is becoming very clear that this project is actually for next Summer's growing season... 

That being said, through observation and solution monitoring during this hot summer I have learned that the new direction regarding nutrient solution maintenance is the right one. Originally I was going to have a reserve tank of full strength nutrient solution and fill the active sump tank from that. However, as the summer progressed, plants grew and temperatures rose. There were days that the sump level fell over three gallons in 24 hours. As the level fell, the total dissolved solids (TDS) rose to somewhat alarming levels. I surmise that this was a combination of evaporation and plant transpiration. The conclusion is that the system needs to do much more fresh water replacement than adding nutrient solution. 

The new design eliminates the reserve tank and replaces it with an electrically operated valve that connects to the outside faucet. It adds a five gallon tank of concentrated nutrient solution with a dosing pump. When the sump level switch indicates a low level it operates the valve and fills the sump to the "full" level. Logic is added to the NodeRed flow that measures Electric Conductivity (EC) to determine TDS each day, after the pumping session. This allows the solution to naturally mix before the reading. This is done three times a day so if enough concentrated solution is not added, it will be supplemented after the next pumping cycle. 

I still need to create my temperature compensation lookup table. I plan on making a standard concentration of the nutrient solution, cooling it to the lowest temperature (50 degrees) and while heating the solution to 100 degrees, correlating the temperature with the EC. 

I added my most recent code and schematic to the project files.