Where can I find guidance for proper calibration of my air and/or liquid particle counter?

According to the ISO 21501 collective of standards, monitoring particle contamination levels is required in various fields including electronics, (bio)pharmaceuticals, precision machine manufacturing & medical operations. These standards also provides a calibration adjustment procedures and subsequent verification methods for particle counters in an effort to minimize any inaccuracy in measurement results as well as potential variances in results measured by multiple instruments. Only instruments conforming to ISO 21501-4 can be used to classify air cleanliness and provide routine monitoring in cleanrooms and associated controlled environments in accordance with ISO 14644-1 & -2.