The laboratory test dust utilized can dramatically affect filter efficiencies, reported as filtration () ratios. Although not always the case, we have seen substantially different filter ratings published when using the new test dusts as compared to previous values. As an example, ratings using ISO MTD published by two different manufacturers for filters normally rated at 3 µm (₃=75 or ₃=200) when tested with ACFTD are shown in Figure 1. In most instances the test dust was identified but no specific mention was made about the effect of the dust. This is quite misleading to users of the filters as they are easily led to believe that the filter performance is improved when in fact, the filter is the same but only the test dust has changed. In addition, users are led to believe that the test was conducted according to ISO 4572 [1] when in fact, this procedure still calls out for the use of ACFTD. Results of testing Pall KP grade elements with ISO MTD is also shown in Figure 1 for reference.* Remember, although Beta ratings may be much higher with ISO MTD, the product and its performance has not changed.

Because of such variations resulting from test dust change, it is important that the test dust is clearly identified and in fact, the relationship of new results to old ACFTD results should be made clear. It should be stressed that if ISO MTD ratings are desired or required, each filter grade must be tested with the new dust to establish a specific correlation, as this relationship cannot be predicted.

In addition to the impact noted above on Beta ratios, the overall effect is compounded when one takes into consideration the additional changes due to use of ISO MTD for re-calibration of the automatic particle counters. The effects of calibration changes, which can be quite substantial, are not discussed in this brief, as the new ISO calibration procedure is not yet finalized.

Figure 1: Variation in Filter Ratings Using - ISO MTD

Based on review of comparative results from a number of industrial hydraulic filter elements, we find a typical increase in dirt capacity when using ISO MTD of about 10 to 40% over values measured with ACFTD. Because each filter element performs differently with the new dust, no general factor can be given, and when ISO MTD dirt capacity data are desired, it is necessary to conduct actual tests on every filter grade. Unfortunately, as with filter efficiency, some manufacturers and laboratories are publishing data from tests using ISO MTD but are not highlighting the differences or making comparisons to previous ACFTD data. It should be pointed out that a dirt capacity increase or decrease with ISO MTD does not imply that the filter will exhibit a longer or shorter actual service life; in fact, there will be no change in field service life as a result of the laboratory test dust change. Dirt capacity values are too often used to try to predict relative field service life [2]. In reality field service life is dependent upon many factors, including the type of contaminant captured, and the dirt capacity measured in a laboratory has little to do with field service life.

Since AC Rochester Division ceased manufacture of AC Fine Test Dust, the Society of Automotive Engineers and ISO Technical Committee TC 22 have been developing new standardized test dusts. This work has culminated in the development of a new ISO standard: ISO 12103-1, 1997 [3]. This standard defines and designates four new test dusts as listed in Table 1. The new test dusts are currently being manufactured by Powder Technology Incorporated (PTI), however, as opposed to the AC Rochester ball mill method, PTI processes the Arizona dust with a jet mill.

None of the ISO dusts have a particle size number distribution that is equivalent to that of ACFTD. Both ISO MTD and ISO FTD exhibit higher particle counts than ACFTD for sizes above 1 µm and below about 10 µm. Because of these different distributions, all laboratory results when using the new dusts are expected to vary somewhat from the original ACFTD results.

Table 1: New ISO Test Dusts

ISO Designation	Common Name	Other Names
ISO 12103 -A1	ISO Ultrafine Test Dust (ISO UFTD)	PTI 0-10 µm Test Dust
ISO 12103 -A2	ISO Fine Test Dust (ISO FTD)	PTI Fine Test Dust SAE Fine Test Dust
ISO 12103 -A3	ISO Medium Test Dust (ISO MTD)	PTI 5-80 µm Test Dust SAE 5-80 µm Test Dust
ISO 12103 -A4	ISO Coarse Test Dust (ISO CTD)	PTI Coarse Test Dust SAE Coarse Test Dust

In the late 1960s and early 1970s the hydraulic filter multi-pass test method was developed at Oklahoma State University to measure filter performance, primarily efficiency and dirt capacity. This multi-pass test later became an International Standard, ISO 4572, and is still the most widely used filter performance test method. The test protocol requires injecting ACFTD slurry into the reservoir upstream of the filter. Particle counts are taken, both upstream and downstream of the filter, throughout the multi-pass test. These particle counts are used to calculate the efficiency, expressed as a filtration or Beta ratio (upstream count divided by downstream count at a given particle size), and are obviously dependent on the APC calibration. The dirt capacity for the filter tested is also reported as the amount of ACFTD injected to cause the filter to reach its terminal differential pressure. The particle size distribution of the test dust has a tremendous impact on both the filter efficiency and dirt capacity measurements.

The original multi-pass test utilized AC Fine Test Dust as the test contaminant as well as automatic particle counters calibrated with ACFTD per ISO 4402 [4]. Several years ago, the ISO committee undertook a project to revise the test method by instituting several enhancements to improve repeatability and reproducibility, and in addition, to replace the test dust because of the pending ACFTD obsolescence. The replacement dust candidates considered were ISO FTD and ISO MTD. In the end, ISO MTD was selected primarily because it was also the dust being selected for the new particle counter calibration procedure. This revised ISO 4572 multi-pass procedure has not been approved but will soon be undergoing final ballot.

There are several enhancements in the new multi-pass, in addition to the dust and calibration changes, which could also have an impact on filter performance in the laboratory; however, these changes are believed to be minor relative to the impact of the dust change.

Changes to fluid power filtration and contamination standards that use AC Fine Test Dust are necessitated because the dust is no longer being manufactured. The replacement dust tentatively chosen by national and international standards groups is ISO Medium Test Dust, but the filter test standards using this new dust have not been finalized. The new dust will cause changes to occur in reported filter Beta ratios and reported filter dirt capacity.

Until the new ISO calibration and filter test procedures are complete, it is recommended that reporting companies or laboratories specify precisely the methods utilized. If a new dust or calibration method is used, relationships to previous ACFTD data should be given. Users should always keep in mind that although laboratory performance values will be changing, this is an artifact of the testing only, and filter performance and actual contamination levels in the field will remain the same as before.