Testing dKH: counting the drops
So the instructions in API GH and KH test kit say, "The KH value is determined by the number of drops of test solution that must be added to turn the water in the test tube bright yellow", and give a table that says 1 drop = 1dKH and so on.
What's been bothering me is--say, distilled water will turn yellow after one drop. But of course its KH is not 1 degree or 17.9 ppm, it's zero. So, doesn't that mean that dKH is the number of drops minus one? Or to put it another way, you first add a drop and then count how many drops after the first one it will take to change the color?