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Dupla CO2 indicator

George wrote: 
> The CO2 indicator is showing the pH of the water vapor entering the
> body of the indicator.  The body contains a liquid indicator that
> changes color around pH 7 (bromthymol blue ?).  As such, it is
> sensitive to the alkalinity of the water.  I believe they recommend a
> "KH" of around 3-5 degrees.  I would hope that under the right
> conditions, "good" is about 15 mg/l.
I don't have the Dupla indicator, but from reading the description on
their website I got a different impression.  I think this device works
like a miniature, reverse bell jar.  Just as CO2 diffuses out of the gas
and into the water from a bell jar, CO2 diffusses out of the water and
into the air inside the Dupla device (which is essentially a small upside
down jar with a platform for indicator solution).  Then the gas diffuses
into the indicator solution, changing its pH (and its color).  Dupla
points out that this means that the CO2 measurement one gets will be
independent of the other chemicals affecting pH, since only CO2 is

A rather brillant little device, but why spend the money when the pH/KH
calculation works fine?

Oh yeah, the Dupla website is:  http://www.dupla.com/e001.htm
if anyone hasn't run across it yet.