LaMotte's answer to my question, which was "Will this be an accurate test if the water has something other than CO2 in it that lowers the PH of the water? For example, suppose the water has lots of tannic acid in it." is: "Attached above is a copy of the instructions for Carbon Dioxide kit # 7297-DR-01. The endpoint of this test is a faint pink coloration, so that any inherent color from Tannins or other things will affect your ability to see the color reaction.
I trust this information is helpful,"
I don't think the person answering this had any idea what I was asking. The test appears to involve adding drops of an alkaline solution to a test tube of the water being tested, with some phenolphthalein indicator added to it. When the indicator shows the acidity has been neutralized by the alkaline solution, the color changes, and the number of drops indicates the ppm of CO2. I presume the alkaline solution is carbonate based, so it is raising the KH of the water in the test tube, thus raising the pH.