AquaChek's Yellow test strips allow pool and spa owners to test their chlorine-based sanitation systems for free chlorine, cyanuric acid, pH, and total alkalinity. Simply insert the test strip into the pool or spa water for 1 second, and several seconds later you have fast, accurate results. It is recommended that you test your pool a minimum of twice per week, and spas before each use, to get an accurate picture of the health of your system.