The 255-211 Replacement Graduate is used to replace evaporated water in an evaporation pan. It is a transparent tube with an inside diameter of 4-3/4 inches, which is one-hundredth of the surface area of the evaporation pan. The tube is graduated at one-inch intervals, with the zero mark at the top. One inch of water in the measuring tube is equivalent to .01 inch in the evaporation pan.