Sorry, worked for me because it was conversation with @Edvet and @ian_m, but it links into the <"question about ...."> chemistry thread.Link error
cheers DarrelI don't understand the chemistry, but I think the answer is that you are both right.
In soft water, without much carbonate buffering, any addition of CO2 will cause a large pH drop, but once you have more than ~4dKH buffering then you will always get a drop of approx. one pH unit when you have 30ppm of CO2 in the water.
I think this is because pH is a log10 scale and exponential data plotted as log10 values will form a straight line. You use a "4dKH" solution in the drop checker because the pH change is then matched to the colour change (protonation state) in the narrow range <"bromothymol blue"> pH indicator.