Some filter manufacturers indicate the nominal efficiency of their filter, while others communicate the absolute efficiency. What do these terms mean? How can you navigate between them and select the filter that best matches your needs?
Absolute filtration, nominal filtration
The term “nominal filtration” refers to filtration whose efficiency at the stated rating is around 90.00%. The term “absolute filtration” refers to filtration whose efficiency at the stated rating is almost total (in practice greater than 99.98%). Everything therefore depends on the filtration-quality requirement.
With a nominal filtration rating, it is accepted that a certain percentage of material may still pass through the filter. If total purification must be guaranteed, an absolute filter must be used.
In certain applications—such as endotoxin filtration—it is essential to use filtration media providing absolute filtration, meaning they retain all of these toxic elements.
Expert advice
Fiberflo® membrane filters, which trap endotoxins on their surface through a chemical reaction, provide 100% retention of these molecules.
Find an example of endotoxin filtration:
Endotoxin retention using hollow-fiber filters
The example of metallic filters
The chemical composition and structure of the filter largely determine its efficiency for a given particle size. Take the following example. To filter steam, metallic filters are predominantly used. Depending on their structure, they can achieve absolute filtration for different particle sizes. A metal fiber filter will retain 99.99% of 1 µm particles. If absolute sanitary-grade filtration is required—for instance, in food-industry applications where smaller particles (0.5 µm) must be blocked—sintered metal filters are used, offering an efficiency of 99.99% at 0.5 µm.
To learn more:
Steam filtration: guidance and recommendations for industry
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