Think of it as a quaternary ammonia (Quat) test strip embedded in a disposable foodservice wiper. Sertun towels change from yellow to blue when proper quat leaves are present in the sanitizing bucket and in the towel. As the towel deploys Quat, the color changes from blue to yellow, indicating there is not sufficient sanitizer in the towel. By placing the towel back into your sanitizer solution, the towel recharges, and turns blue again. When the towel does not turn blue, it's time to change the sanitizer solution. Works with quaternary ammonia (Quat) only. Use with your red bucket system.