Filtration Efficiency

Understanding Nominal vs Absolute Retention for Filter Cartridges

Selecting the proper filter cartridge retention reduces operating costs and ensures your process specifications are met. Filter cartridge retention is expressed in microns (μm).

Filter cartridges provide depth filtration, meaning particles are blocked by progressively finer openings throughout the media. They are usually installed downstream of coarser filtration (such as bag filters or pipeline strainers) so they only handle fine particles.

Nominal vs Absolute Efficiency

Once you determine the particle size to remove, the next step is choosing the right efficiency. Filter media is offered in two main efficiencies: nominal and absolute.

Nominal Efficiency

Nominal efficiency is manufacturer-defined. Our nominal rated filter cartridges are typically 70% efficient at the stated micron size when new. As particles become trapped, the media loads up and efficiency increases.

There is no universal standard for what qualifies as "nominal." Different manufacturers may use different definitions. Always ask for the exact initial efficiency if your process is sensitive to it.

Nominal rated cartridges are more affordable and commonly used for non-critical applications or as pre-filters to protect more expensive absolute-rated cartridges.

Absolute Efficiency

Absolute efficiency is laboratory-defined and much more precise. It is usually expressed using a Beta Ratio or Beta value.

Example: Our LOFPLEAT-HP 1.0 micron filter cartridges have a Beta 5000 rating, meaning 99.98% retention of particles ≥ 1.0 micron.

The Beta Ratio is calculated as:

A Beta 5000 rating means that for every 5,000 particles of a given size upstream, only 1 passes through.

Testing Methods

Efficiency testing is typically done using a single-pass test with latex beads or test dust and laser particle counters. Results can vary between manufacturers because there is no single universal testing standard.

Important Considerations

Efficiency vs. Dirt Holding Capacity: These are two different things. A cartridge can have excellent efficiency but lower capacity, or vice versa. The physical design (pleating, layers, cage strength) often matters as much as the media itself.

Do You Need Nominal or Absolute Rated Cartridges?

Sensitive applications in food, beverage, pharmaceutical, or high-purity processes usually require absolute-rated media. For many applications, using nominal-rated cartridges as pre-filters can significantly extend the life of absolute-rated final filters and reduce overall cost.