Liquid automatic optical particle counters (APCs) are used in monitoring contamination levels in hydraulic oil, to establish component and assembly cleanliness level specifications, and in determining filter efficiencies and size ratings. As a result of the replacement of ISO 4402 with ISO 11171 (particle counter calibration), the replacement of ISO 4572 with ISO 16889 (multi-pass filter test), and the new ISO 11943 (on-line particle counter calibration), it is anticipated that the quality and reliability of particle count and filter test data will improve, increasing their usefulness to industry. However, the resultant redefinition of particle sizes and the use of a new test dust affects how contamination levels and filter performance are reported and interpreted. The impact of these changes is discussed in this report.