A system for generating participate air contamination standards of known size distribution and concentration was developed and used to determine the accuracy of several optical particle counters (OPCs) for the measurement of paniculate air contamination. Of particular concern was the substantial instrument-to-instrument variability of up to a factor of six or more sometimes observed when measuring the same sample with two similar instruments. We found that while the agreement between two instruments calibrated from the manufacturer's calibration data was sometimes poor, the same instruments calibrated by our methods agreed within 10% in their measured results. We therefore recommend use of these new calibration methods when precise air contamination measurements or interchangeability of instruments are needed.

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