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"My filter company uses the following notation to rate one of its filters: BX=2, B10=X, BX=75, followed by these numbers: 5, 8 and 17. I'm familiar with the Beta rating, but I've never seen it shown in this way. What is meant by these numbers?"
This system of rating a filter describes its capture efficiency at three specific points in microns. It actually has many advantages over the more common Beta rating methods used today.
This is how it works:
The “X” denotes the size in microns of particles above which the filter is only removing 50 percent of the particles entering. For example, 100 particles larger than “X” come into the filter, but only 50 of them are captured. For your filter, this 50-percent efficiency size is 5 microns. This is the lower limit of performance in microns for this filter.
The “X” in this case denotes the Beta ratio corresponding to particles larger than 10 microns. Many hydraulic users in the past have viewed 10 microns to be a critical clearance size, above which system components are at high risk of failure. The “X” has to do with the capture efficiency of particles larger than 10 microns.
In your example, the Beta ratio at 10 microns is 8, meaning that for every 8 particles larger than 10 microns coming into the filter, one gets through uncaptured (about 88 percent capture efficiency).
This “X” corresponds to the micron size above which the filter is removing 98.7 percent of all the particles. In other words, the Beta ratio for the particle size (X) is 75. For every 75 particles larger than “X” entering the filter, only one gets through (74 are removed).
Many people have informally referred to that size (X) as "absolute," although clearly, by definition, it is not. Still it is useful to know what the BX=75 rating is. In your example, the “X” corresponds to 17 microns.
Filter testing is completed in a laboratory so the results can be compared and statistically verified. Filtration tests attempt to accurately control the quality of the test dust, the flow rates, temperatures, measuring equipment and many other variables to ensure the repeatability of the test.
The test, called a multi-pass filter test, produces a Beta rating for the filter at a given micron size. It’s understandable, but nevertheless disappointing, to find that filters do not always perform in the field exactly as the laboratory tests would suggest.