Calibrate airborne optical particle counters (OPC) to comply with ISO 21501-4 regulations with the Thermo Scientific™ 4D Series Dry Microsphere Size Standards. Ideal for instrument manufacturers, as well as QA/QC managers in pharmaceutical, biotech and electronics manufacturing, these microsphere reference beads feature a calibration uncertainty—the range of values within which the actual mean diameter will lie—that is less than or equal to 2.5% of the mean diameter. 4D Series are available in 4 different nominal sizes: 3, 5, 7 and 10µm.