CAV's what brand of filter is the best ?

[Cuppa]

They claim it to be 5 micron, but at what beta rating? It might be 5 micron at beta 2, which is 50% efficiency. Which means the filter will let 50% of particles at 5 micron or larger through. Some filters will have multiple ratings, 5 micron at beta 2, 10 micron at beta 10 (1 out of every 10 particles will pass through the filter) etc etc.
The beta ratings are as follows.

Beta Ratio. How Many Particles of a Given Size Will Pass
Through the Filter?----- Actual Filter Efficiency
2 ---1 out of every 2 particles 50%
10 ---1 out of every 10 particles 90%
20 ---1 out of every 20 particles 95%
75 ----1 out of every 75 particles 98.7%
100 ---1 out of every 100 particles 99%
200 ---1 out of every 200 particles 99.5%
1000 ---1 out of every 1000 particles 99.9%
2000 ---1 out of every 2000 particles 99.95%

So you can have a filter will 2 micron filtration but it has a really poor beta rating, which is just as important than micron rating. If the Delphi is 5 at beta 75 or better, then I would consider it a good filter.

Thanks Mark, good info. I found this article which confirms what you've said which some may be interested to read.
https://www.donaldsontoolbox.com.au/...tings_0317.pdf

I looked everywhere to find the absolute rating for the Delphi HDF296 but failed until I came back to this thread & realised you had already posted it earlier. Doh. 12 microns @ 99% efficiency. I guess whatever it had been I'd still be happy with the result after cutting it open. Do you know what the absolute/beta rating is for the Ryco Z332?