Perhaps a better approach to approximating pH would be to
measure dissolved carbon dioxide.  Evaporation is a big
problem in vessels open to the atmosphere--as water
evaporates, salt concentrates, and the conductivity increases
independent of pH changes.  A similar problem occurs in
measuring soil moisture.  However, dissolved carbon dioxide
is generally present at pH levels between abut 5.6 and 8.5,
which is fine for aquariums and pools.  Because carbon
dioxide dissociates to form carbonic acid, which is an acid,
the higher the CO2, the lower the pH.  This is independent of
conducitivity.   And a nice pic project is to build a
continuous CO2 sensor for an aquarium.   Search altavista for
CO2 sensor & aquarium, that should pop it.

alice


>
> Another issue in swimming pools and aquariums is ORP ("oxidation-reduction
> potential"). This is the voltage from pH plus that of other ionic species.
> Similar problems, similar solutions.
>
> One way around the difficulty of pH measurements in pools and aquariums to
> measure conductivity instead. Changing pH will change conductivity. So long
> as salt concentration is constant, this property allows development of a pH
> control loop. Conductivity is simply measured by the resistance between two
> narrow (1-3 mm) stainless rods a calibrated distance (10-20 mm) apart.
>
> ================================================================
> Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral@lcfltd.com
> Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
> 824 Timberlake Drive                            Tel: 757-467-0954
> Virginia Beach, VA 23464-3239                   Fax: 757-467-2947
>
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> ================================================================