Final Setup and Calibration

    1. This circuit has been designed to be able to function well even with a lot of electrical interference from lights and heaters.  Running a ground wire from the 7-volt reference ground (the case, if you have used a metal case) to a stainless steel bolt suspended in the tank water will help to reduce interference considerably, and is recommended.  (Secure the copper wire to the bolt by pinching it between stainless steel nuts.  (I used a 1/4 inch diameter, 3-inch long bolt.)  If your tank already has a ground probe, connect to that.
    2. Adjust R2 to set the LM317 voltage regulator output to 11.5 volts.
    3. Adjust R300 to set the reference ground voltage to 7.00 volts (measured at the side of R302 farthest from U300A).
    4. Place the probe in a sample of tank-temperature pH 7 calibration buffer.  Adjust the R354 "Zero" potentiometer for a reading of 7.00 at the pH display.  (Some tweaking of R352 may be necessary the first time.)
    5. Rinse the probe, then gently blot the tip with a tissue to prevent solution carryover.  Immerse the probe in pH 4 buffer.  Adjust R352 ("Slope") for a reading of pH 4.00  (You can use pH 10 buffer here instead of pH 4, but don't; it picks up CO2 from the air which lowers its pH value, and is therefore less stable.)
    Rinse the probe and blot, repeating steps 3 and 4 until no further adjustments are necessary.   A few repetitions should be sufficient.
    6. When the tank pH is at the correct level, adjust R451 to set the CO2 On/Off point .

Final Thoughts and Cautions

Ideally, the CO2 input level will be set so that even if for some reason the electronic controller turns the CO2 on all the time, the pH will only fall to a non-lethal level for the tank's inhabitants.  In my setup, the pH hovers around 6.7.  If the CO2 is on non-stop, the pH will only fall to 6.5.

Be careful if you have easily accessible front panel controls.  I have a front panel bypass switch that allows me to turn the CO2 to constant "on."  I accidentally hit this switch one day and couldn't figure out why the pH had dropped.  Knobs that are easily bumped into a wrong setting present a similar hazard.

pH Calibration buffers are available from Pet Warehouse in convenient one-time-use foil packets (though it appears that these buffers can be stored for some time in sealed plastic film canisters).  I got larger bottles of calibration buffer from the local hydroponics shop for only $5.00 each.

The pH probe should be kept away from light to prevent algae growth.  Mine sits in a gray perforated PVC tube at the top corner of the tank, inside of which is a sleeve of plastic screen door screening to help keep particulates off of it.  Readings will be more accurate if there is some water flow in the vicinity of the probe.  The probe should be cleaned and recalibrated periodically.  I use a soft artist's paint brush to gently clean the delicate probe tip at each water change.  Don't let the probe tip dry out!  This is most likely to happen at water changes.

ANY surface agitation of the aquarium's water will cause a surprisingly large amount of CO2 to be lost.  If you can't get the aquarium's pH down, or if it seems like it's taking a lot of CO2 to do it, try reducing the surface agitation.  Conversely, if you get the surface agitation down to almost nothing (by returning filter water to the tank bottom, for example), CO2 won't off-gas at lights out, and the pH may fall more than you want it to during the night.  Oxygen exchange can suffer with low surface water agitation, which can be a problem with heavily stocked tanks, especially in the morning just before the lights come on.  My setup shuts off CO2 delivery at lights out, and the pH still falls a little.  It's best to watch the tank carefully when CO2 is first being used.

READ THIS DISCLAIMER:  This circuit uses potentially deadly levels of voltage and current.  BE CAREFUL!  If you are not sure of what you are doing, particularly when it comes to circuitry powered by the AC mains, IT IS YOUR RESPONSIBILITY to get help or leave this project for another day.  Don't kill yourself.  Don't build sloppy circuits that may kill or injure others, start fires, or otherwise generate bad vibes.  This information is made available as a service to any person who finds it of interest.  Because of possible variances in the quality and condition of materials and workmanship used by the builder, any and all responsibility for the safe and proper functioning of this circuitry is disclaimed.
 

Links for more information:

Jim Hurley's pH measurement and control system at the Krib.  Hurley's circuit was the inspiration for this one.
The inexpensive pH probe I got from Amazon.com for about $40 works fine.
Omega Engineering's pH primer  A good overview of  pH  and its measurement.
Omega Engineering's pH electrode basics  Brief overview of pH probes — how they work and how to take care of them.  Worth a look.
Good pH probe info at the Krib, (the ultimate aquarist information source).
National's LM 317 Voltage Regulator Page  All about the voltage regulator used in this project.
The Texas Instruments Home Page   Search for "TL082" for information on the op-amps used here (No direct link available :(

People who are new to electronics, and those whose electronic skills have grown rusty, can learn a lot from Forrest Mims' Getting Started in Electronics .  Don't let its simple format fool you; there's a lot of good information there (as well as fun and instructive projects).  For the slightly more experienced or ambitious, Horowitz and Hill's The Art of Electronics ($90) is a wonderful once-over-lightly of the world of electronics theory and practice.  Also recommended: Walter Jung's IC Op-Amp Cookbook (3rd Edition) .