Have you ever encountered this situation: a filter supplier tells you a cartridge offers “absolute 5-micron precision” and quotes a price double that of a “nominal precision” model? You buy the absolute-precision filter, only to discover six months later that its performance is virtually identical to the nominal-precision filters used by your peers. Were you scammed, or is absolute precision simply not as miraculous as claimed? The concept of absolute precision is valid, but the conditions required for it to deliver that level of performance may not apply to your specific application. This article delves into the differences between absolute precision and nominal precision.
What is a filtration rating?
Filtration precision ratings refer to the size of impurity particles that a filter (or filter element) can capture and remove, as well as the corresponding level of purification. Micrometers (μm) are typically used as the unit of measurement; a lower value indicates a higher level of filtration cleanliness.
Micrometer value (μm): Directly indicates the size of the filter mesh openings.
Nominal precision vs. absolute precision: Nominal precision refers to the approximate particle size that can be captured, whereas absolute precision refers to a strict standard guaranteeing 100% capture of particles of a specified size.
Filtration precision determines not only the size of particles a filter can capture but also directly impacts filtration efficiency, flow rate, and pressure drop.
What Is The Nominal Filtration Rating?
Definition of Nominal Filtration Rating
The industry defines “nominal filtration rating” as follows: the filter element achieves a retention rate of 85% to 95% for particles equal to or larger than the nominal particle size (with a beta value typically ranging from 2 to 20).
How “Nominal Filtration Rating” Reflects Filtration Capability
Filters with a nominal rating typically feature higher porosity and lower pressure drop, but their pore size distribution is less uniform than that of absolute-rated filter elements. This implies the existence of “gaps” that can allow certain particles to pass through—though this generally occurs only under specific conditions.
Examples include high-flow surges, increased pressure drops, or sudden spikes in contaminant concentration. Under such transient operating conditions, the retention efficiency of a nominally rated filter can drop significantly, allowing larger particles to slip through.
A key variable regarding nominal filtration ratings is their dependence on the filter’s structure. For instance, consider a cylindrical filter element and a pleated filter element that share the same nominal rating. Because their structures differ—with the pleated element utilizing a corrugated, multi-layered design that maximizes filtration effectiveness as contaminants pass through the folds—their actual filtration performance differs, even though their nominal ratings are identical.
Characteristics and Limitations of Nominal Filtration Ratings
Characteristics of nominal filtration ratings:
- Balances filtration efficiency and operating costs. Compared to absolute filters, nominal filters prioritize overall operational efficiency rather than striving for extremely high particle retention rates. There is also a significant price difference between absolute-rated and nominal-rated filters.
- Suitable for most general industrial filtration applications. Nominal-rated filters are appropriate for the majority of filtration operating conditions.
Limitations of nominal filtration ratings:
- Filtration performance is influenced by multiple factors. The effectiveness of a nominal-rated filter depends on factors such as the filter medium, pore size distribution, manufacturing process, and testing methodology. For example, two filter cartridges with the same nominal rating of 10 microns may exhibit vastly different particle removal efficiencies if they utilize different filtration structures or testing standards.
- Nominal filtration ratings are unsuitable for high-precision applications. In contexts such as hydraulic servo systems, precision instrumentation, pharmaceuticals, or semiconductor manufacturing, nominal filtration may fail to provide sufficiently stable particle control. These applications typically prioritize consistency in filtration results, making absolute filtration ratings the more appropriate choice.
What is the absolute filtration rating?
Definition of Absolute Filtration Rating
The industry definition of “absolute filtration rating” is as follows: under ISO 16889 multi-pass filtration test conditions, the filter element achieves a retention rate of over 99.98% (i.e., a beta value ≥ 5000) for particles with sizes equal to or greater than the nominal rating.
How Absolute Filtration Rating Indicates Filtration Capability
Absolute filtration rating (or retention rating) refers to the diameter of the largest spherical glass particle capable of passing through a filter under laboratory conditions. Theoretically, filter media with precise and uniform pore sizes possess a precise absolute filtration rating. However, in practical applications, the situation differs; the effective pore size is influenced by the shape of the filter element and does not necessarily correspond directly to the actual open area. Furthermore, actual contaminants are not spherical; a particle’s dimensions may be significantly smaller than its nominal size, allowing it to pass through smaller apertures—for instance, a cylindrical particle with a thickness smaller than the width of the filter element’s opening.
Because absolute-rated filter elements are affected by actual operating conditions, most absolute-rated filters tend to form a filter bed where contaminants adhere to the surface, leading to clogging and reduced filtration efficiency.
Despite these factors, an absolute filtration rating strictly prohibits the passage of any particles larger than the specified size. For example, a filter with an absolute rating of 5 microns and 99% efficiency is capable of capturing at least 99% of 5-micron particles, thereby ensuring a higher level of filtration precision.
Characteristics of Absolute Filtration Ratings
Characteristics of absolute filtration ratings:
- Provides protection for high-precision equipment. In high-precision equipment—such as hydraulic servo valves, precision instruments, semiconductor manufacturing tools, or pharmaceutical production lines—even a small number of particles entering the system can lead to serious consequences due to the extremely low tolerance for particulate contamination. In these applications, absolute filters more effectively minimize the risk of particle ingress, thereby reducing the likelihood of equipment wear and product contamination.
- Ideally suited for high-cleanliness applications. Absolute filters are typically used in industries with stringent particle control requirements, such as precision hydraulics, pharmaceuticals, food and beverage production, semiconductor manufacturing, and fine chemicals.
- Offers more stable particle retention capabilities. Absolute filters deliver consistent filtration performance because they prevent the passage of any particles larger than the specified rating.
Key differences between absolute and nominal filtration ratings
Differences in Filtration Efficiency
The primary distinction lies in the efficiency of removing target particles.
Nominal filtration does not require the capture of *all* particles matching the specified rating; instead, it removes the majority of target particles under specific test conditions. Consequently, some particles may still pass through the filter medium. In contrast, absolute filtration emphasizes the reliable capture of target particles, typically employing more rigorous manufacturing processes and testing standards.
Simply put, a nominal filter focuses on capturing the majority of particles, whereas an absolute filter prioritizes the most consistent capture possible of the target particles.
Differences in Filtration Performance
The performance of a nominal filter is susceptible to factors such as the type of filter medium, pore size distribution, manufacturing processes, and testing conditions; therefore, variations may exist between different brands or product batches. Absolute filters, however, place greater emphasis on performance consistency. Through uniform pore structures, strict quality control, and standardized testing, they deliver stable and predictable filtration results.
Differences in Application Scenarios
Nominal filtration rating: Suitable for pre-filtration and general filtration needs.
Absolute filtration rating: Suitable for precision filtration and the protection of critical equipment.
Why Do Filters With The Same Micron Rating Have Different Performance?
The reason is simple: the Micron Rating only indicates the target particle size of the filter and does not fully reflect its actual filtration performance. Two filters, even those labeled with the same filtration precision, may have significant differences in filtration efficiency.
The main reasons for this difference include the following:
Different Filter Material Structures
The filter material determines how the filter works and is one of the important factors affecting filtration performance.
For example, different filter materials such as sintered metal filter, stainless steel woven mesh, glass fiber, and polypropylene (PP) have completely different pore structures and particle interception mechanisms. Even with the same filtration precision, different materials may have significant differences in dirt-holding capacity, flow rate, pressure resistance, and filtration efficiency. Therefore, the same Micron Rating does not necessarily mean the same filtration performance.
Different Filter Cartridge Pore Size Distributions
Different manufacturing processes can lead to differences in the pore size distribution within the filter cartridge. Some filter cartridges have more uniform pore sizes, enabling stable interception of target particles; while others have a wider pore size distribution, allowing more particles to pass through even with similar average pore sizes.
A more uniform pore size distribution generally results in more stable filtration, which is one reason why absolute filters offer greater consistency.
Different Testing Methods and Standards
Filter performance is not solely determined by micron ratings; it is also closely related to the testing methods.
Different manufacturers may use different testing standards to evaluate filtration performance, such as ISO 16889, ISO 4572, or other industry standards. Due to differences in testing conditions, particle types, and evaluation methods, even products with the same micron rating may yield different filtration results.
Therefore, when comparing filters from different brands, simply comparing micron ratings is insufficient; it is also crucial to ensure that the products utilize standardized and reliable testing standards.
Many people habitually consider micron ratings as the sole criterion for judging filter performance, but in reality, it is only one of many performance indicators.
The true determinant of filtration effectiveness is the combined effect of the filter material, pore structure, testing standards, and filtration efficiency.
So, the next time you compare two filters, be sure to ask whether it is a nominal filter or an absolute filter, what its filtration efficiency is, and what testing standards were used. Can the filter media and manufacturing process guarantee stable filtration performance?
Only by combining these factors can you truly determine whether two filters belong to the same performance level, rather than simply having the same Micron Rating.
How do you choose between an absolute filter and a nominal filter?
There is no absolute superiority of one type over the other; the choice depends on your filtration objectives and actual operating conditions.
If your system prioritizes operating costs, flow rate, and filter element service life, a nominal filter is often sufficient. However, if your equipment is highly sensitive to particulate contamination, an absolute filter is the better choice to ensure consistent and reliable filtration performance.
When to choose nominal filtration:
- For applications requiring low filtration precision, simply removing most contaminant particles is sufficient.
- For applications requiring high flow rates and minimizing the impact of pressure drop on system operation.
- For applications used for pre-filtration or protective filtration, distributing the contamination load to downstream precision filters.
- For applications prioritizing operating costs and filter cartridge lifespan over extremely high particle removal rates.
For applications such as general industrial filtration, cooling circulation systems, lubrication systems, and chemical media transportation, Nominal Filter is the best choice in terms of both filtration efficiency and cost-effectiveness.
When to choose absolute filtration:
- Strict control of particulate contamination is required to minimize the passage of target particles.
- Used to protect precision equipment such as hydraulic servo valves, precision instruments, and semiconductor equipment.
- Requires stable and consistent filtration performance to ensure predictable filtration results across different batches.
- Applications include high-cleanliness industries such as pharmaceuticals, food and beverage, and fine chemicals, where high product quality is paramount.
While absolute filters typically have a higher procurement cost, they can reduce equipment wear and tear and product scrap caused by particulate contamination in critical equipment. In the long run, they deliver greater value.
