Thermo Scientific provides these products for calibrating particle counters which require dry particles, from 5 to 100 µm. They are conveniently packaged in dropper-tipped vials in 1 gram quantities, enabling the user to dispense the microspheres directly into the sampling chamber. They are not suitable for dispersion in liquid media. The mean diameters are traceable to the National Institute of Standards and Technology (NIST). DRI-CAL™ Particle Size Standards are composed of polystyrene DVB (divinylbenzene) and have a density of 1.05g/cm3. The refractive index is 1.59 @ 589nm. The certified mean diameter was determined by optical microscopy using a NIST calibrated stage micrometer. Size distribution was determined by electrical resistance analysis.