This test strip is calibrated at 0, 100, 250, and 500ppm, and gives results in just seconds. Peracetic acid is another sanitizing agent primarily used in some segments of the food industry, such as organic produce and poultry. The recommended strength of peracetic acid is typically 150-200ppm, however, this can vary by location. Check with your local health inspector to determine what ppm level you should be reaching. NOTE: If the strips are turning yellow in color, it is possible that the concentration of Peracetic Acid is too high.