This document describes the criteria and procedures used by the United States National Institute of Standards and Technology (NIST) to certify the calibration material SRM 2806d, which is used in the primary calibration of liquid automatic particle counters (APCs).

SRM 2806d is a suspension of SAE 5 - 80 μm test dust in hydraulic fluid with a cumulative particle concentration determined through a consensus measurement processes, interlaboratory study (ILS), utilizing APCs. The original projected area equivalent diameters for SRM 2806 were certified using a scanning electron microscope (SEM) and image analysis techniques traceable to the meter through the NIST line scan interferometer.