I have been working on the instrument assembly that would be partially submerged in the nutrient solution sump tank; both from a reservoir refill perspective as well as a nutrient replenishment standpoint (compensating for evaporation and rain-fill). My plan was to handle these as two separate problems to be solved: deciding when the sump needs to refilled from the reservoir and measuring the total dissolved solids (TDS) in the solution and dosing with concentrated nutrient as needed.

The reservoir solution was fairly straightforward. I was planning on a vertical tube that contained two magnetic reed switches: one each for low solution and proper fill level. A float would travel along this tube and as the solution level dropped it would eventually come close enough to the lower reed switch to close it, triggering a pump in the reservoir to start refilling the sump. The program would then monitor the upper reed switch and when the float reached the appropriate level, the switch would close and the pump would be turned off. I've been playing with this design and, using high density foam, clear rigid tube from the local aquarium shop, a neodymium magnet embedded in a high density foam float and two magnetic reed switches, the design seemed fairly robust.

Next I started researching electric conductivity (EC) and total dissolved solids (TDS) measurements I found quite a bit of DIY solutions, including some great ones on Hackaday. I was worried about doing constant measurement because of electrolysis and the effect on the electrodes so was going down the road of doing one brief measurement each day. As it turns out, most commercial units use alternating current (AC) to help mitigate this problem although I did find one reference that was using the fast-read approach. I also discovered that temperature had a direct bearing on the measurement so, rather than being an interesting variable to measure, solution temperature is a requirement. So, with a little modification, my approach to measurement looked promising. When TDS dropped I was planning on injecting small amounts of concentrated nutrient solution with a peristaltic dosing pump. waiting until a scheduled bucket flood had finished (to mix the solution) then re-measuring and adjusting as necessary.

Then the obvious became clear. The reservoir had several drawbacks. First, it was another tank that took up space, needed maintenance and most of all, had a finite volume. If I were gone for an extended period of time (vacation, during the hot summer when transpiration rates are highest) I would need a very big tank. Also, I could correct for a diluted solution from rain-fill with the dosing pump, but I couldn't correct for an overly concentrated solution. If I replaced the reservoir with plumbing to my outside faucet, and the pump with an electric valve, I could solve these problems.

Starting out with a good basic design is important, but I really enjoy the dynamic nature of evolving the design.