The Hach 511244A AquaChek Yellow 4-in 1 Pool and Spa Chlorine Test Strips is a convenient way of testing the Free Chlorine, pH, Total Alkalinity, and Stabilizer (Cyanuric Acid) levels in your pool or spa. It has an easy-to-use flip-top cap which conveniently dispenses one strip at a time. AquaChek The test pads on the strip change color to indicate the levels in your pool or spa water. pH refers to the intensity of acid or alkaline materials in your water.