Here's the document where the graph I posted came from:
LINK
This is a cut and paste from email traffic between me and Purolator back in 2011. Their study report is dated 2013, so they obviouly knew of this decreasing efficiency with loading because they would see it happen over and over during ISO 4548-12 testing.
============== Start of Email Correspondence ==================
Email #1 with Todd Vick, Purolator Engineer (4/1/2011)
My Questions:
1) When the rating of 99.9% at 20 microns (for the PureONE) or 97.5% @ 20 microns (for the Classic) is shown per ISO 4548-12, is this rating at the beginning of the test when the filter is basically new, somewhere in the middle of the test (averaged), or at the end of the test when the filter's maximum holding capacity is achieved?
2) Does an oil filter's efficiency increase as it becomes loaded with trapped debris? If so, about what is the general percent difference in filtering efficiency of a new filter vs. a filter at maximum debris loading point?
Purolator's Response:
The rating of a filter is the average efficiency during the life of the test. The test is terminated at a predetermined differential pressure across the element. The report writer of the equipment will print a graph which will show typically the filter starting with higher efficiency, dropping slowly and then increase efficiency in the latter part of the test – a “hockey stick” visual effect. The reporting is automatic with the test stand and cannot be adjusted externally.
The efficiency is highest at the beginning (before the media is attacked by oil and acidity) and again at the end as contamination does improve efficiency. We cannot correlate the change of efficiency to predict the change during the lifecycle on a car.
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Email #2 with Todd Vick, Purolator Engineer (4/4/2011)
My Questions:
Can you please clarify about the filter's efficiency at the start vs. at the end of the test?
I understand your "hockey stick" shaped efficiency curve. But was wondering if the efficiency at the end of the test can actually be higher than at the start? From what you have described, it sounds like the highest efficiency happens when the filter is new at the start of testing.
Purolator's Response
The test duration is based on filter size and termination differential. For benchmarking, we use 8 psi increase in pressure across the filter as the self-termination point. Based on the size of the filter, media area, media design, the test can run as short as 15 minutes for a small 65mm filter for over 1 day for a class 8 full flow filter. Typically from the graphs I see, efficiency at the end of life is slightly less than a new filter.
The IAM filters that replace an FL1A will test in 1 – 2 hours depending on brand and model.
============== End of Email Correspondence ==================
