I had heard something sorta similar with auto ph adjustment using a separate res.
What size PVC pipe/net pots would be best to use?
Well its kind of auto PH adjustment but probably misrepresented using the word 'auto'.
I like these fully wet systems because the plants swing the ph themselves as they eat through the nutrient, so as one element has largely been taken up the fluid content changes PH, then the next element is taken up by the plants again which changes the ph again and again until the the plants have 'swung' through the PH range. Once the ph range has been swung through the system should then need topping up with city water anyway so you top up, adjust the ppm and ph and off you go again letting the plants swing through the appropriate ph range.
Depending on your city water and nutrient make up the plants may swing through the PH range either up or down, so say you had a PH range for a particular strain that produced best during flower between 5.5 and 6.1 and you set the ph to 5.5 but it divebombed towards 4 then you know to set the ph at the opposite end of the plants optimum range, say 6.1 in this example. Then you just watch the plants eat through the contents and reduce the PH down to the happy 5.5 and then flush the system again to 6.1. These are hypothetical figures of course and dependent on what a users strain is most comfortable with.
If the EC/PPM can be maintained as the fluid content levels fall then you'll have cracked a good nut as it means your EC/PPM levels are perfect for the plants you're nurturing.
Some people have trouble maintaining a constant EC/PPM as the fluid levels drop but I believe this is in part to an incorrectly selected PH range. If some elements of the nutrients are not available to the plants then this will of course effect the PH and in return the EC/PPM as they're not within range to be removed from the fluid content by the plants. When switching strains I'd sometimes expand the PH range a little and see if the PH range starts swinging the correct way for my set up. If it sat or moved very slowly I'd bring in that end of the ph range and look at it again. Once it started moving at a speed that I'm used to seeing then that was that particular end of the PH swing set. The other end of the ph swing can be more difficult to detect as you're running into it. If a dropping PH it will run on way down into the 3's so its just a case of catching it before the plants grow legs and jump out of your tent. If a rising ph it normally stalls from what I remember. This would be the time to flush and start again.
Going back to NFT with a remote res, it can seem daunting at making the plumbing connections and managing such a system but I can say with a hand on my heart that once set up and manipulated this is the easiest system to manage bar none. I can not think of any other design that would allow me to go away for 2-3 weeks at a time and feel comfortable at leaving 6 600 sodiums burning.
