The US National Institute of Standards and Technology (NIST) undertook a project to certify the particle-size distribution of ISO MTD suspensions in oil. From this study, the Institute determined that for particle sizes below 10 µm, the actual particle size is greater than previously measured using an automatic particle counter that was calibrated with ACFTD. Above 10 mm, the particles were smaller than under the old calibration system. To make a distinction, particle sizes based on the new NIST determination will be represented as X µm(c), with the (c) designation referring to certified calibration that has sizes traceable to NIST. Thus, particle size has a new definition, Table 4.

ISO Technical Committee TC 131 replaced the old APC-calibration procedure, ISO 4402, with a new procedure, ISO 11171, which incorporates ISO MTD test dust with the NIST particle-size and particle-count determinations. It also has included a number of other enhancements to ensure better accuracy, reproducibility, and repeatability. In addition, ISO has developed another procedure, ISO 11943, for calibration and verification of on-line automatic particle counters. Because APCs are used for multi-pass filter performance testing and fluid contamination measurement, these changes will affect reported results.

ISO has adopted a revised procedure for reporting fluid-cleanliness measurements from APCs that have been calibrated with the new NIST-traceable method. This procedure, ISO 4406: 1999, uses three code numbers that correspond to concentrations of particles larger than 4, 6, and 14 µm(c). The new 6 and 14 µm(c) sizes correspond closely to the older 5 and 15 µm sizes reported by the old ISO 4406 coding system measured with an APC calibrated with ACFTD. The new 4 µm(c) size, however, corresponds to about the 1 µm size if the ACFTD calibration procedure had been used. This difference results in somewhat higher values of the first digit when compared to current three-digit codes that reference particles larger than 2 µm using the old ACFTD-calibration method.

Filter performance testing

A number of substantial changes were made to ISO 4572, the multi-pass filter test procedure. These changes again were intended to produce more repeatable and reproducible test results. The new method, ISO 16889 replaces ISO 4572, and incorporates ISO MTD and the NIST-traceable APC-calibration procedure. Beta ratios derived from tests that use this new ISO procedure also are designated with the symbol (c) to signify they were measured in accordance with the ISO 16889 procedure using NIST-traceable calibration. As an example, a beta ratio of 200 at 5 µm(c) would be designated as β5(c) = 200.

The revisions to the multi-pass test method and the inclusion of both ISO MTD and the new APC-calibration procedure will dramatically affect reported beta ratios for filter elements. The effect will vary for different filters, depending on the influence of the test dust and the degree of change in the particle size at the filter's rating. In general, fine filters will appear coarser or less efficient, and coarse filters will appear finer or more efficient.

We performed tests on Pall Industrial Hydraulics standard filter media using the new ISO 16889 test method. Comparisons to results obtained from the previous ISO 4572 method are shown in Table 5. Note that filter performance in the field does not change at all. The media are no more or less efficient at removing harmful particles. Only the reported laboratory results have changed slightly because of the new procedures and methods.

Filter dirt-holding capacity

The replacement of ACFTD with ISO MTD in the multi-pass test also affects retained-dirt-capacity values for filter elements. Capacity may be somewhat higher or lower with the new dust, again depending on the specific filter being tested. However, most filters we evaluated exhibited an increase of about 10% to 40% in dirt-holding capacity when using ISO MTD. Because each type of filter performs differently with the new dust, there is no single conversion factor to change ACFTD capacities into ISO MTD capacities.

Again note that an increase or decrease in dirt capacity when tested with ISO MTD does not imply that the filter's actual service life will be longer or shorter. In fact, there will be no change in field-service life. As a rule, dirt-holding capacity should not be used as an indicator of field-service life.

Reporting fluid cleanliness

The changes discussed in the previous sections will not affect the actual cleanliness of fluids in the field. However, for those technicians reporting data using particle counts, the changes in APC calibration will affect results obtained from both laboratory and portable equipment that is calibrated to the new standards, as in Table 6.

Technicians reporting data using the ISO cleanliness codes will notice less effect. The adoption of a cleanliness code with 3 digits should not have any appreciable impact because 3-digit codes have been used by industry for a number of years. In addition, the change from 5 and 15 µm sizes to 6 and 14 µm(c) sizes will not show a significant change in fluid-cleanliness code. The primary change will come from adoption of the 4-µm(c) size. This is a new addition to the ISO 4406 standard (although a third digit at 2 µm using ACFTD had been used). Because the new 4-µm(c) size equates to about 1 µm using ACFTD calibration, particle counts on fluid samples will typically show an increase of about one level for this first digit of the code, Table 7. The reason: there usually are more smaller particles in any fluid sample.


ISO MTD was chosen as a replacement for Air Cleaner Fine Test Dust because ACFTD is no longer being manufactured. To gain better resolution, accuracy, repeatability, and reproducibility in tests using the replacement dust, four new or revised ISO standards have been adopted to accommodate the new test dust. These changes will have an impact on automatic particle-counter calibration, particle-size definition, and laboratory reporting of filter performance - both in particle-removal efficiency (fine filters will appear coarser and coarse filters finer) and dirt-holding capacity (capacity will likely increase). The changes also will effect laboratory reporting on system fluid cleanliness (typically showing a higher contamination level at smaller particle sizes).

This will undoubtedly cause some confusion, but remember that any impact is merely an artifact of the minor changes in testing. Actual filter performance and field fluid contamination levels will remain the same.