Rating Your Filter
By JIM FITCH, Noria Corp & MIKE DAY, Pall Corp

Is your filter as efficient as you would like it to be? Do you understand what Beta Ratings are and why they are important when selecting your filtration system?

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:

BX=2
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.

B10=X
This 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).

BX=75
This X corresponds to the micron size above which the filter is removing 98.7 percent of all the particles, that is, 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.

ISO 16889 Multi-Pass Test
The multi-pass test is a means of determining a filter’s performance; it measures the ability of the filter to remove particles of test dust over a wide particle-size range. This gives a series of Beta ratios for the filter as opposed to a percentage efficiency. The Beta ratio is defined as:

bx (number of particles upstream >xµm)
(number of particles downstream > xµm)

Where x is the determining size of particle for the Beta ratio.

Note: To differentiate data obtained from APCs calibrated to the new standard, ISO 11171, micron size is reported as µm(c) rather than µm.

Beta = 1000 Rating

Users of filters have historically liked the simplicity of a single point rating (the size where a specified Beta ratio is attained). Consequently, the number of different Beta ratings used has proliferated. In the early 1970s, ßx>2 (nominal) was used by some followed by the ßx>75 (absolute - all particles removed) in the late 1970s. ßx>75 then became the most widely used, and was introduced solely to meet the measurement limitations that were experienced at the time. This was followed by ßx>100 and ßx>200.

What value is most appropriate? Selection should be based on the filter’s ability to remove critically sized damaging particles. Therefore, the highest Beta ratio that can be reliably measured should be selected. Ideally, ß = (infinity - no particles in the downstream fluid) is the target. However because of particle count statistics in the multi-pass test, the absolute rating cannot be measured with any degree of confidence. In the 1970s and 1980s, a compromise was reached between what is desirable and what can be consistently achieved for the range of filters assessed by this test.