This paper describes present U.S. practices for calibrating optical liquid-borne particle counters (APCs). Operation of these instruments is described briefly for both light extinction and scattering instruments in order to help the reader understand why empirical calibration is necessary. Calibration requirements are pointed out in terms of the need to verify both particle sizing and particle counting accuracy. Two calibration methods are currently used in the U.S. One is based on size and count response to a suspension of presized isotropic latex spheres. The other is based on APC adjustment to match the stated size distribution of a test dust polydisperse suspension at a specific concentration. Both methods are described and some advantages and problems are presented.

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