Filter Beta Ratio
The beta ratio is an efficiency measure found by dividing the number of particles entering the filter at a given particle size by the number leaving it. The expression beta10 = 200 means that 199 out of every 200 particles of 10 micron and larger are captured, that is 99.5% efficiency. Giving only a micron value is not enough to describe a filter.
The efficiency percentage follows directly from the beta ratio: (beta - 1) / beta. Common values and their equivalents are as follows:
- beta = 2 - 50% capture; nominal filtration, low protection value.
- beta = 75 - 98.7% capture.
- beta = 200 - 99.5% capture; the level referred to as absolute filtration.
- beta = 1000 - 99.9% capture; targeted in circuits with proportional and servo valves.
That is why two filters can carry the same "10 micron" label and still differ in performance by a wide margin. When selecting an element, ask for both the micron value and the beta ratio valid at that micron size.
In a circuit running with a proportional valve, the right beta ratio is the way to hit the target cleanliness class. Deadband compensation and dither settings in the control electronics mask mechanical stiction up to a point; the permanent solution is to keep particles out of the system in the first place.
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