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
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
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.