Abstract
The current ISO 4572 Multipass Filter Test Procedure has not been substantially revised since its approval as an ISO document in 1981. However, proposed revisions will likely be adopted shortly which will have far-reaching impact in liquid multipass filter testing and for virtually all industrial applications. These revisions include the adoption of a new test dust to replace the discontinued AC Fine Test Dust (ACFTD), a revised particle counter calibration procedure using National Institute of Standards & Technology (NIST) traceable calibration fluid, a new on-line particle counter, sample delivery and dilution system validation procedure, and a new reporting format. These changes result in a new definition for assessing and rating filter performance.
This paper discusses these changes in detail and the resulting impact on the filtration industry. For example, the most significant revision pertains to use of a revised ISO 4402 standard for calibration of liquid particle counters. Until now, particle counter calibration fluid containing ACFTD was used to calibrate particle counters. This test dust is no longer manufactured due, in part, to the inability of the manufacturer to produce a consistent particle distribution of ACFTD from one batch to another. This inconsistency in the test dust, and also in much of the calibration fluid containing the test dust, created a great challenge for labs to correlate particle count results and filter test data.
The revised ISO 4402 calibration standard will require the use of a new NIST Standard Reference Material (SRM) calibration fluid using a new test dust called ISO Medium dust, which has virtually replaced ACFTD. This new test dust and the calibration fluid containing it is much more consistent and reliable, allowing for better correlation of results.
The analysis conducted by NIST to certify the calibration fluid as a SRM found what before was defined as a 2 micron particle was actually a 4.6 micron particle and what was defined as a 15 micron particle was actually a 13.6 micron particle. This finding will redefine liquid filter performance information based on particle counts to the effect that, in general, fine filters will appear less efficient than previously reported and coarse filters will appear more efficient.
While other revisions to ISO 4572 will have less impact, they are presented. Filter test engineers, filter users and manufacturers, and filter sales personnel will benefit by educating themselves now on the impact of these revisions in order to best serve the filtration industry and filter users.