When selecting the right filter for your application, it’s essential to understand which type of filtration is right for you: nominal filtration, absolute filtration, or both.
What is Absolute Filtration?
- Definition: Absolute filtration retains all particles larger than the specified micron rating.
- Example: A 10-micron absolute filter does not allow any particles larger than 10 microns to pass through.
- Efficiency: ≥ 99.98% (Beta 5000)
What is Nominal Filtration?
- Definition: Nominal filtration retains a certain percentage of particles at or above the specified micron rating.
- Example: A 10-micron nominal filter may capture only 60% to 99% of particles larger than 10 microns.
- Efficiency: Lower than absolute filtration—typically between 10% and 99%, depending on various factors such as flow rate, differential pressure, particle type, size distribution, and concentration.
Beta Ratio
Nominal filters have a Beta ratio between 2 and 100, depending on design and application.
In comparison, absolute filtration starts at Beta = 1000 (99.9%) and goes up to Beta = 5000 or higher.
The Beta ratio indicates how effectively a filter captures particles.
Examples:
- Beta 2: 1 out of 2 particles passes → 50% efficient
- Beta 1000: 1 out of 1000 particles passes → 99.9% efficient
- Beta 5000: 1 out of 5000 particles passes → 99.98% efficient
Although the percentage difference may seem small (e.g., 99% vs. 99.98%), Beta 5000 is 50 times more effective than Beta 100.
| Type | Retains 100% of Particles | Typical Efficiency |
| Absolute | Yes |
≥ 99,9%
|
| Nominal | No | 10-99% |
How Accurate is a Nominal 0.5 Micron Filter?
- Typical Efficiency: A nominal 0.5 micron filter typically removes 10% to 60% of particles 0.5 microns and larger.
- No Guarantee: The filter does not capture all 5-micron particles.
- Efficiency Loss Under Pressure: As differential pressure increases, more particles may pass through, depending on the type of contaminant and structure.
The Impact of Filter Structure
Filtration performance is influenced not just by micron rating, but also by the structure of the filter media.
Consider a wound prefilter element (nominal). It’s made of filter yarn with fiber diameters typically ranging from 10 to 25 microns. This filter yarn is automatically wound around a core (perforated tube).
The filtration capacity of the element depends on several factors, such as the thickness of the yarn (fiber size), the winding pattern (density and tension), the number of layers of filter yarn and construction (looser inside, tighter outside) and any post-treatment such as resin impregnation.
Filter Yarn
Nominal filters are typically available in ratings from 5 to 100 microns, although some manufacturers offer wound filters with nominal values as low as 1 or even 0.5 microns.
While this sounds appealing in theory, in practice such fine filtration is difficult to achieve with this type of material.
Efficiency can be measured using the Beta ratio, but consistently achieving 0.5 micron with high efficiency is not possible using these materials. Actual efficiency is often significantly lower—typically closer to 60% than the advertised 90%. The finer the micron rating, the lower the actual efficiency and the greater the deviation. In reality, the filtration efficiency of these extremely fine nominal filters is often much lower than common values.
While a nominal 10 or 50 micron filter can remove 60% to 99% of particles, a 0.5 or 1 micron nominal filter typically only captures 10% to 60%. This is a significant difference and raises the question of whether such low micron ratings are technically justifiable. In many cases, these are more marketing claims than technical guaranteed performance.
Filtering very fine particles—1 micron or smaller—quickly exposes the structural limitations of nominal filter elements. The open fiber structure of these filters makes it no longer reliable to measure at these small micro particles. This micron level can only be achieved with a filter media, such as fiberglass filter cloth or microporous membranes. These media are specially designed for high accuracy and are capable of reliable filtration at micron levels of 1 or 0.5.
The same principle applies to spunbonded filter elements, which are made by melting plastic particles into fibers. These fibers are often 15 to 35 microns in diameter, making it questionable how a nominal 0.5 micron filter could be produced from such material. In rare cases it is possible to reduce the fiber thickness to about 10 microns, but this is a very technical challenge.
The simple answer is: a reliable nominal 0.5 micron filter is actually not feasible. In addition, the open area (porosity) of such structures is often very low when attempting fine filtration, resulting in high resistance (pressure drop) in the system.
Particle Size Comparison – From Hair to Bacteria
| Filter Media | Typical Fiber Diameter | Achievable Filtration Rating |
| Spunbonded PP | 15-35 microns | 10-100 microns (nominal) |
| Meltblown PP | 15-35 microns | 1-25 microns (nominal) |
| Glassfiber/membrane | 0.1-1 micron | 0.2-50 microns (absolute) |
Nominal vs. Absolute 1 Micron Filtration
A truly effective nominal 1 micron filter does not exist in practice.
What it can do is remove the majority of contamination to protect the absolute filter behind it.
- Nominal 1 micron filtersoperate using depth filtration: they have an open structure that can hold a relatively large amount of dirt, but they always let some fine particles through. A nominal filter is a ideal filter for a wide range of large and small particles.
- Absolute 1 micron filtersare usually surface filters: they block all particles ≥ 1 micron but have a lower dirt-holding capacity, meaning they clog faster than depth filters. They are best suited for contaminants of uniform size.
Conclusion: Which Filtration Type Suits Your Application?
Now that the differences are clear, it becomes easier to choose the right filtration type.
Nominal filters mainly remove coarse contaminants and are often used as pre-filters, extending the life of absolute filters and improving system efficiency.
Absolute filters ensure reliable removal of fine particles.
Still unsure which filter you need?
Contact us today—our team is happy to help you!
