Compared to the 3000 and 4000 Series products, the 3K/4K series products are precisely diluted for immediate use in laser particle counters with minimal time-consuming adjustments of concentration. These NIST traceable size standards therefore provide an accurate and convenient method for calibrating or checking the performance of particle counters widely used in cleanrooms and other contamination monitoring applications. They are available in a range of sizes from 0.1 to 100 µm in diameter and are supplied as suspensions of monodisperse polystyrene spheres (PSL) for use in airborne or liquid particle dispersion systems. The suspensions contain only trace amounts of surfactant for low background interference