"Margolis" wrote in message
...

It doesn't work properly if you are not adding co2 because it means that
there are other buffers at work if your ph is lower than it can naturally be
with a given kh. If you have a tank that has a kh of 4° with a ph of 7 you
would have 13ppm of co2 going by the charts. But in reality this cannot
happen under any circumstances unless you are adding co2.

I agree. If you have 4 degrees KH and no other unusual buffers in the water
(which is usually the case), you will measure a pH of 7.8, which is equivalent
to 2ppm. So, there is nothing wrong with the chart, and the chart is correct
whether you are adding CO2 or not. The only thing that can throw the chart
off is other buffers, such as phosphate. But to make a difference,
PO4 levels have to be way up, in the 1ppm range, which is rarely the case.

By the way, Krause recommends a different method to measure CO2 content.
That method is insensitive to the presence of unusual buffers.

Step 1: Take small water sample with some pH indicator. Stick a straw into
the water and exhale through the straw into the water sample for two or three
minutes. This sets the CO2 level at 60ppm. Take note of the pH reading.

Step 2: Take another water sample with some pH indicator and run the hose
from an air pump into the sampe for a few minutes. This sets the CO2 level
at 0.5ppm. Take note of the pH reading.

Step 3: Measure the pH of the tank water. The CO2 content is proportional
to that reading. As an example, if you measure pH 6.0 with the first sample,
and
pH 8.0 with the second sample, a tank pH of 7 corresponds to 30ppm CO2,
regardless of any buffers that might otherwise confuse a CO2 test or distort
the charted values.

Cheers,

Michi.

--
Michi Henning Ph: +61 4 1118-2700
ZeroC, Inc. http://www.zeroc.com