I wonder if it would just be cheaper to buy an Arduino, 2 peristaltic pumps, and a pH sensor and just make a pH pump. There is probably code written for it already as well. That way you would always have the pH set to where you want.
As of today I am on day 46 without a reservoir change or a single drop of pH up or down. I have topped off with RO water every few days and concentrated nutrients as needed to keep EC in check, but I have let the Torus Hydro control the pH entirely on its own. Even with RO water that comes out around 7 my pH has stayed consistently between 5.8 and 6.2.
Time will tell how long it can maintain 35 gallons of water. (I did oversize and bought the 100 gallon unit.)
Yes, that would probably work. I simply have to drop a 6" bar into my reservoir. This is also an ion exchange which allows the reservoir to be used longer. I would never go over 2 weeks between res changes before, but as I said above I’m now over 6 weeks with no signs of issues. (Jacks 3-2-1 has to also be credited here, since the res is still totally clear.)
So far I am sold on the Torus Hydro. Even if it only lasts a single run it has still eliminated any chasing of pH while still drifting through the entire range for the best nutrient absorption.