| Daniel, In case you're interested, my system is passive: Link. I also have some ideas for passively maintaining ppm that I haven't had a chance to try yet. Picture a tank with a constant osmotic pressure, maintained as such by the presence of a high-molecular-weight, poorly biodegradable solute (there are many options). Connected to the tank is dialysis tubing inside your grow bins. The minerals in the grow tank can pass through the tubing, but the high molecular weight solute cannot. Osmotic pressure should ensure that most of the salts remain within their respective grow bins, and that they're diluted to the same osmotic pressure as your solute tank. Thus, so long as the water level in the solute tank remains constant, so should the ppm of your grow bins. You can maintain the water level of your solute tank with a float valve -- the same type that you find in a toilet. Again, I haven't tried it, but I can't see a reason why it wouldn't work. Likewise, you could add nutrients passively. As your plants absorb their mineral salts, the amount of water needed to maintain a constant ppm will drop -- i.e., the water level will drop. So a second float valve, this time in the plant grow bins, could be used to add more nutrient solution. The one thing that this wouldn't take care of is the ratios between different nutrients, which can change over time, depending on what your plants are hungry for. I've thought of several ways to take care of this automatically, but they're anything but simple or cheap, and generally involve reverse osmosis and/or ion-selective membranes and/or automated nutrient testing systems. |
