Development time for test strips is dependent upon many factors. The kinetics of the color development are manipulated by altering the formula employed on the test pad, along with adjusting the conditions of the test (dip time, swirl versus dip, shake excess water off versus no shake, etc.) to offer the best color separation in the shortest time possible.
In the case of the different chlorine test strips, while the color developing indicator is the same, the formulas have been adjusted to slow the development of the color allowing the range of the test strip to be expanded to accommodate the higher chlorine values.
Specifying a specific development time is normally very important. The nature of these test strips is that the color continues to change over time. It is not like a titration with a definite endpoint. Reading the result before or after the specified development time will result in inaccurate results, just like under or over-titrating a sample would.
Our goal is to provide the best color separation at the values requested in the shortest time possible. We try to standardize this whenever we can, however, user requirements often require specific and unique instructions.