Abstract：For the purpose of accurate calibration for the oil-based liquid automatic particle counter (APC) , the preparation and measurement techniques such as gravity sedimentation, ultrasonic sieving and scanning electron microscope (SEM) , were used to develop the approximately-monodispersed MTD certified reference materials (AM-MTD-CRM) . The series particle size CRM cover the routine measurement range of APC, and the uncertainty of the certified value was less than 2.5% (k=2) , which was far below the uncertainty of polydisperse MTD CRM (P-MTD-CRM) . Because of the low uncertainty and the narrow particle size distribution of the AM-MTD-CRM, which overcome the difficulties of poor accuracy of particle sizing and less particle statistics for P-MTD-CRM, it makes the APC calibration uncertainty is greatly improved.

Keywords：liquid automatic particle counter; certified reference materials; calibration; medium test dust