Liquid kits require that you put a sample of water into a plastic comparator and then carefully add drops of chemical reagent, watching for a change in the color of the sample. Liquid kits with the reagent OTO were once widely used to measure chlorine, but they have fallen out of favor in recent years. OTO measures only total chlorine, not free chlorine, which is the form of chlorine most effective in sanitizing your pool.

Test strips, the easiest method for pool and spa water testing, are fast and simple to use because they eliminate the need to measure samples and count drops. Typically, you simply dip a strip in the water for one second and then remove it. You then compare the color on the strip to the color chart on the label.