The New NIST Traceable Particle Counter Calibration Standard — Transitioning from ISO 4402 to ISO 11171

Liquid automatic particle counters (APCs) are used to monitor contamination levels in hydraulic oil, to establish component and assembly cleanliness level specifications, and to determine filter efficiencies and size ratings. In the past, particle counter calibration for hydraulic applications used ISO 4402 and AC Fine Test Dust. The shortcomings of this method were known, but until recently there was no better method. This changed with the passage of ISO 11171, an NIST (National Institute of Standards and Technology) traceable method of calibration. As a result of this and other improvements in ISO filter test standards, it is anticipated that the quality and reliability of particle count and filter test data will improve, increasing their usefulness to the hydraulics, automotive and aerospace industries. However, to make the switch from ISO 4402 to ISO 11171, laboratory personnel, engineers, sales and marketing must make certain accommodations with regard to the procedures used and the way results are reported. This paper discusses the impact of the change in calibration and offers suggestions on how to facilitate transition between the two calibration methods.