Paul, it was a post here on VAF. No idea of its accuracy, and don't remember which thread.
Don't worry about it.
Nominal micron ratings are mostly a form of advertising. Although the reader may think a 20 micron rating (for example) means the filter stops all particles 20 microns and larger, it's not true. It's simply a rating assigned by the manufacturer, sometimes based on a standard, and sometimes not.
Better is beta ratio, which is based on a widely accepted ISO standard. You can usually find it in the specification listing, although you may need to track down the actual manufacturer in the case of a brand-labeled filter. The NAPA Gold 1515 often referenced here on VAF is a good example. You won't find beta on the NAPA website, so track back to the equivalent Wix 51515 and you'll find beta listed as 2/20=6/20. More on that shortly.
Here's the important part. A dual beta rating reflects reality, which is that typical filter media stops some particles much smaller than the rated size, and does
not stop all particles larger than the rated size.
Beta for a given particle size = # of particles counted on the upstream side / # of particles counted on the downstream side. Yes, there's a testing machine for it.
Which takes us back to that Wix listing.
2/
20=
6/
20 means Beta is 2 for a 6 micron particle and 20 for a 20 micron particle. A beta of 2 means the media stops 50% of the 6 micron particles. A Beta of 20 means it is stopping 95% of the 20 micron particles.
Where do the percentages come from? Efficiency Percentage = ((Beta-1)/Beta)*100. However, there's an easier way to think about it. A Beta of 2 means it passes 1 out of every 2 particles. A Beta of 20 means it passes 1 out of every 20 particles.
Don't know if Champion and Tempest publish beta ratio, as I think they use an obscure SAE standard. The screen filter guys appear to be using an ASTM standard. The entire "normal" filter world uses ISO.