Science and Medicine have used litmus and chem pH tests for decades with excellent and reliable results. Just takes some patience and training (both of which it seems you have), so that is excellent. As I was reading your posts, etc, the only other issue you may want to get out ahead of is Transpiration and evaporation. You said you live in a very arid climate, is your hydro setup out of doors? Your plants might be prone to excesive transpiration that would certainly require TDS monitoring so you can balance evaporated/transpired H20. I would lastly recomend keping a "log" of all that you do. It will help you learn how adding chemicals, time, temp, etc affect your setup. The idea bening it will get infinitly easier and cheaper as you learn what works, how your plant resond to your water,
nutrient, etc, etc. The best log I have found (because i wrote it) can be found at Hydroponica.blogspot.com. Its easy to copy and paste the jpeg or as I say I can e-mail it. Matt- hydroponica.blogspot.com