Alrighty, so here's my setup:
I'm running a high pressure aeroponics system at 100psi. I'm using fine mist nozzles, on a timed interval, to spray my plant roots.
I use a lab grade, calibrated, digital pH meter to measure pH. I also have a PPM meter that measures up to 9999ppm.
Here's my situation:
Upon starting the system, I used General Hydroponic nutrients at a concentration of ~400ppm to feed my plants. This quickly caused over fertilization. After reducing the concentration to 100ppm, the plants started to show nutrient deficiencies. Upon further inspection, I realized the GH nutrients weren't pH stable at 100ppm. The pH would rise on its own. I took RO water, mixed my nutrients, then let it sit, untouched by anything and completely isolated from my system. I found that the pH would rise to 7.4 on its own. Even after the addition of pH down, the pH would still constantly rise. Keep in mind, this way when these nutes weren't even in the system. They were just sitting in a tub not being touched by anything. So, I came to the conclusion that GH nutrients simply weren't stable at such a low concentration.
I then discovered Advanced Nutrient's pH Perfectly Technology brand of nutrients. These nutrients would only keep the pH stable at above a concentration above 100ppm. So, even nutrients formulated to maintain stable pH would not maintain it at such a low concentration. There's nothing in my system that would cause pH to rise. I've run RO water through my system many times to clean it out and I still continue to have nutrient instability problems.
I've run hydroponic systems flawlessly using the same exact nutrients but just a higher concentration. So I'm confident in that I've limited the problem down to nutrient instability at low concentrations. Which seems very odd because surely I'm not the only one running an HPA system. What do other people use? What concentration are they using? There's just a huge lack of information when it comes to HPA.
Edit: Here's just one picture of how bad my plants are. It gets worse.