Our sales manager forwarded me an email the other day from a researcher at a university who was confused about how to select the proper size of filter media in order to achieve a certain level of cleanliness according to the ISO 4406 standard. I proceeded to explain the following to him.
Referencing the document in question, “Understanding ISO Codes” (PDF), on the second page, the column labelled “Media” describes the recommended pore size of the corresponding filter media necessary in order to achieve the ISO code listed in the column to the left of the media size.
Please note that the examples listed are for systems using petroleum-based fluids per ISO 4406:1999, and for the following particle sizes:
4μ[c] / 6μ[c] / 14μ[c] (where each size corresponds to an ISO code number)
For example, in the chart you can see that for a servo valve with fluid operating up to 3000 PSI, the recommended target ISO cleanliness code is 16/14/11 and, in order to achieve this ISO code, the corresponding recommended media pore size is 5 micron (using an older testing method with a “beta[c] ratio” of 1000) and 3 micron (using the new testing method with a “beta ratio” of 200).
The “beta ratio” is the ratio between the number of particles per unit volume above a specific size in the suspension upstream of the filter to the same parameter in the flow downstream of the filter. The beta ratio of any particular filter is determined in a special test rig that enables accurate particle counting in its upstream and downstream flow regions. This simple ratio can be described by the following formula:
βx = Nu / Nd
where: “x” is the beta ratio for particles larger than x μm, “Nu” is the number of particles per unit volume larger then x μm upstream, and “Nd” is the number of particles per unit volume larger then x μm downstream. The higher the value of the beta ratio, the more particles of the specified size (or greater) are retained in the filter.
One insight that should be noted is that the filter efficiency (in percentage) at any particle size can then be determined directly from the beta ratio by using the following formula:
Ex (%) = 100 (βx – 1)/βx
For example, as mentioned in “Understanding ISO Codes“, a beta ratio of 1000 equates to a filtration efficiency of 99.9%:
Ex = 100 (1000-1)/1000 = 99.9%
and a beta ratio of 200 equates to a filtration efficiency of 99.5% efficiency.
Does this help you to understand better how to select filter media to achieve oil cleanliness?
Are you working on a project with which I may be of assistance? Feel free to contact me anytime. Our friendly team of experts will be happy to help you out with your particular issue.
Until next time!
Nick at Precision Filtration Products
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