Impact of New Dust on Filter Efficiency

For several decades, AC Fine Test Dust (ACFTD) has been utilized for a number of purposes in the area of hydraulic and lubrication contamination control. It was used for primary calibration of liquid automatic particle counters, for filter testing, and also for component contamination sensitivity testing. ACFTD was originally sold by the AC Spark Plug Division (later the AC Rochester Division) of General Motors Corporation, but is no longer being produced. Many standards organizations are now selecting replacement dusts, most notably ISO Medium Test Dust, for calibration and testing purposes. Because none of the replacement dusts has identical particle size distribution characteristics to ACFTD, all associated test results are somewhat different. However, 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.

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.

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