Calibration of optical particle counters: first comprehensive inter-comparison for particle sizes up to 5 m and number concentrations up to 2 cm^-3

We report on an inter-laboratory comparison for the calibration of light-scattering optical particle counters measuring the number concentration of airborne particles in the size range 300 nm-5 mu m. This comparison, registered as EURAMET pilot study Nr. 1453, is an important first step towards internationally recognized SI traceability in the measurement of low particle number concentrations encountered in clean-room facilities. The participating particle metrology laboratories of NMIJ (Japan), DFM (Denmark) and METAS (Switzerland) each provided a portable optical particle counter as a transfer standard to be circulated amongst the partners. The comparison covered particle number concentrations up to about 2 cm(-3). The individual measurements for the transfer standards at each laboratory are in good agreement with the reference value, typically within 7%, which supports the applied national primary methods of measurement.