Original from Micro Essential (USA) OVERVIEW Hydrion 0-800 chlorine test strips measure the concentration of free chlorine in sanitizing/disinfecting solutions. The color chart has matches at 0-50-100-150-200-250-400-600-800 ppm. each vial of 100 strips. Product Direction : Dip strip into solution for 1 second. Remove strip and shake off excess solution. Match to color chart at 5 seconds, not more than 10 seconds, for accurate free chlorine measurement.