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Re: Buffers

> From: Andrew Hamilton <andrewha at tafe_sa.edu.au>
> Date: Mon, 17 Mar 1997 07:32:59 +1030
> For some time now my Dupla microprocessor has displayed everything but
> the set point value. The set point value has always been programmed
> for 6.9 but the unit normally reads below this, esp in the mornings
> where it is sometimes as low as 6.60.

A few things may be going on here.  

1) Your KH isn't high enough (you've already discovered this).  The
CO2 controller can only lower pH.  If your "natural pH" with CO2 at
equilibrium values (2-3 mg/l) isn't much over the set point, the
controller can do very little about it.  We have enough KH in our
water to give us a natural pH of around 7.8 and have the set point at
6.9.  Thus, the loss of CO2 is mainly driven by diffusion into the
atmosphere and the controller always has something useful to do.

2) CO2 electrodes do rather poorly in "low ionic strength" liquids.
Soft freshwater qualifies rather well as low ionic strength.
Saltwater is high ionic strength.  I assume that Dupla did a good job
in choosing the electrode for this situation.  I have found that the
Broadley-James "Silver" electrode (Pet Warehouse, $36) seems to work
the best of all that we've tried, even those costing over $100.

> I had had a few problems with strange read outs after changing to
> electronic ballasts

3) Perhaps you're being plagued by induced voltages.  We've found that
a grounding electrode in the water helps stabilize erratic pH

> Do other people with automated CO2 injection still get a variance of
> this degree between morning and night?

Nope.  With enough KH and higher levels of CO2, the amount of CO2 the
plants use and the variance in day/night respiration is a small part
of CO2 loss.  Our tanks vary between 6.85 and 6.95.