Each printing of color charts is preceded by an extensive review of the test strip performance. Chlorine standards are generated at 10, 50, 100, and 200ppm. These values are confirmed by at least one independent test method. Using these verified standards, samples of strips from several different production runs are used to determine the color associated with each standard value. Once the colors are selected, the color charts are printed. Prior to their use, the color charts are again checked with a fresh set of verified standards. After acceptance of the color charts, subsequent production runs of test papers are checked against the charts before they are packaged for sale.