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I'd love to find a few collaborators to reproduce the design in order to be sure it works in other settings as well as it does for me. I only have a single tank, so I don't have much opportunity for experimentation. After some confirmation, I'd like to write it up for publication.
The basic operation of the system is simple though it took me about ten tries to find a workable implementation. The probe looks a little like this schematic view:
_________________ | | | | | |<-0.5 inch cpvc L fitting | ____|____ | | | ___ | | |___| :a: |___| :air: :q: : : ~~~~~~~~~~~~~: :~~:u: :~~~: 5/8" tube-->:~~~: :a: : :<-glass bottle with reagent : : :r: : : ----- :i: ----- :u: water :m: : : :s: :i: :d: :e: : : : :[Editor's note: here is a better illustration, including dimensions. Also check Gary's diagram of the probe holder.]
CO2 from the aquarium water diffuses through the air trapped inside the probe into the water with pH reagent in the small glass bottle. The color of the reagent varies with the CO2 content of the aquarium water. Many of you will recognize this as the same system used by the Dupla continuous CO2 indicator, which consists of a tiny diving bell with reagent in a water sample that is separated from the aquarium water by an air gap.
The probe hangs over the side of the aquarium with the 5/8 inch open tube below the water surface and the small bottle inserted in a hole in a small box that is attached to the aquarium side with suction cups. The box contains a photoresistor and green LED from Radio Shack. Light from the green LED has to pass through the reagent to get to the photoresistor, which is shielded from exterior light. As the reagent goes from blue at high pH to green at low pH, the amount of green light that passes through and thus the resistance of the photoresistor changes.
We now have a pH probe (actually at CO2 content probe), that can be easily constructed from readily available parts, that should last many years and that requires little maintenance. The only upkeep is to replace the water sample and reagent at water changes. The reagent is the same as used in most pH test kits, bromthymol blue and is very inexpensive. In order to replace the reagent, you just lift off the U-shaped probe assembly and unscrew the little bottle from its cap, which has been glued into the cpvc L-fitting. Dump the old reagent, fill the bottle with aquarium water to which the proper amount of reagent has been added. Now replace the probe so that the bottle is inserted into the sensor box, and the 5/8 inch tube is below the water.
The rest of controller is comparably simple. It consists of a single IC (LM339 quad comparator), a voltage regulator (LM317LZ), 2 transistors (2n2222) to drive the solenoid, a LED status indicator, a potentiometer to set the level, and a few assorted resistors and capacitors. No pH or CO2 content indication is provided by the controller. It just turns the CO2 on when the resistance of the photoresistor in the probe rises above some level (pH is high) and turns it off when it drops below that level (pH is low). Hysteresis is provided by the aquarium water.
This probe is slow. It takes several hours to respond to a dramatic change in CO2 content. But this works fine since changes in CO2 content take several hours to happen in either direction. You could probably get into unstable behavior if your CO2 system drops the pH of your water quickly but you shouldn't do this for the sake of the fish anyway. In my tank the cycle is several hours on and off. My pH as I said earlier stays within 0.05pH of the setting.
With all new parts, I'm guessing that the controller can be built for under $15 (I built mine out of my junkbox). Add a 12 volt, power-cube style supply for about $5 and a solenoid valve for $24 and you've got automatic CO2 regulation for less than $45.
Is there someone out there who would like to collaborate on this design? Some electronics experience would be helpful.
[Editor's note: here is a GIF of the schematic. Parts list follows...]
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