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RE: pH controllers

I've been using the Hanna Instruments 981411 pH controller for a couple of
years now.  This from Automated Aquarium Systems for $110:


Also bought a solenoid and pH electrode from them.

This controller has one digit accuracy and single point calibration.  There
is a BNC connector for the electrode.  I use it to control pH at 6.8 (+ or -
0.1).  As a meter, I think its been rock solid, though it takes 15 minutes
to read true after power up.  I think its my pH electrode that seems to
drift a little with time, unfortunately always in the direction that will
result in increasing CO2 concentration.  Monthly cleaning and calibration is
essential.  Even in the dark, some organic film (algae) grows on the
electrode.  I clean it in 1% HCl (aka Hanna electrode cleaning solution)

The Hanna controller is a panel mount meter, so I built an enclosure for it
out of two pieces of aluminum plate and four 6" screws and spacers.  I
mounted a flow meter with leak valve and the solenoid in the same enclosure,
and use quick disconnects to connect to the CO2 tank and injector.

Joe K in Takoma Park, MD