How to understand the Beta value of filter cartridge and its filtration efficiency?
The Beta Ratio (Beta Ratio, β) is the most reliable and accurate indicator for evaluating filter filtration efficiency, defined by the international standard ISO 16889. For liquid filtration applications, performance is commonly expressed as βₓ.
What is Filtration Efficiency? Why Do We Use βₓ?
A common question raised by many buyers: Can a filter cartridge rated at 5 μm capture all particles ≥5 μm? The short answer is: Not necessarily.
Filter rating is stated by pore size (μm), and it falls into two categories: absolute rating and nominal rating. These two terms are often confused, as they follow different test protocols and carry different meanings.
| Item | Absolute Rating | Nominal Rating |
|---|---|---|
| Definition | The smallest particle size that can be removed 100% | The particle size at which a defined percentage of contaminants (typically 50%–99%) can be trapped |
| Test Method | Challenging test using hard beads (e.g. glass microspheres). Zero particles larger than this size are allowed downstream. | Particle counting test following standards such as ISO 16889 to calculate Beta value. Manufacturers define the nominal rating on their own. For instance, a βₓ=75 (98.67% efficiency) may be marked as the nominal pore size X. |
| Rigor | Very strict and objective. It represents the upper limit of filter performance. | Relatively loose and subjective. Standards vary among manufacturers with poor comparability. |
| Reliability | High. It guarantees performance under worst-case operating conditions. Widely used in high-reliability applications including safety filtration, hydraulic systems and high-precision membranes. | Low. It serves only as a reference value and cannot guarantee zero passage of larger particles. |
| Application Example | A “1 μm absolute rated membrane” ensures no particles larger than 1 μm pass through. | A “10 μm nominal rated filter cartridge” may capture 90% of 10 μm particles, yet some oversized particles may still leak through. |
If a 5 μm filter is marked as absolute rated, it captures every particle ≥5 μm with zero bypass, delivering 100% filtration efficiency. If the 5 μm rating is nominal and its filtration efficiency is 98% (β₅=50), it captures 98% of particles ≥5 μm in fluid, while 2% of particles ≥5 μm may penetrate the filter.
1. Filtration Efficiency
Filtration efficiency is the core metric to evaluate filters and filter media. It describes the filter’s capability to capture and retain target contaminants, expressed in percentage (%). Simply put, it answers one question: What percentage of contaminants get removed by the filter?
Efficiency = [(Upstream particle count − Downstream particle count) / Upstream particle count] × 100% = [1 − (1/βₓ)] × 100%
Example: If an air filter inlet reads 10,000 particles per cubic meter and the outlet has 100 particles per cubic meter, Efficiency = (10,000 − 100) / 10,000 × 100% = 99% This means the filter removes 99% of airborne particles.
2. Definition of βₓ
βₓ is the ratio of particle count larger than a specific size X μm in upstream fluid, compared with particles larger than the same X μm in downstream fluid.
βₓ = Upstream particle count (size X μm) / Downstream particle count (size X μm)
How to interpret βₓ:
- βₓ = 1: No filtration effect for this particle size; upstream and downstream particle counts are identical.
- βₓ > 1: The filter can trap part of the particles. Higher βₓ means higher filtration efficiency.
- βₓ = 200 is widely adopted as an important benchmark in the industry.
Examples of βₓ and corresponding filtration efficiency:
- For 10 μm particles, β₁₀=2 → Efficiency = (1−1/2)×100% = 50%
- For 10 μm particles, β₁₀=75 → Efficiency = (1−1/75)×100% = 98.67%
- For 5 μm particles, β₅=200 → Efficiency = (1−1/200)×100% =99.5%. This filter is commonly called “β₅=200” or “5 μm @ β=200”.
3. Why Use βₓ Instead of Only Filtration Efficiency?
| Item | Filtration Efficiency (η) | Beta Value (β) |
|---|---|---|
| Information | Single, snapshot percentage value | Ratio metric carrying richer information |
| Test Method | Usually single-pass test | Multi-pass cyclic test simulating real working conditions |
| Performance Reflected | Only shows initial particle capture capacity | Reflects both particle capture and long-term particle retention capacity |
| Performance Comparison | Hard to compare intuitively (e.g. 99.9% vs 99.5%) | Direct linear comparison (β=1000 vs β=200) |
| Correlation with Fluid Cleanliness | Indirect | Direct: downstream particle quantity = upstream quantity / β |
| International Standard | — | ISO 16889 (for hydraulic filters) |
Key advantages of βₓ
- Direct efficiency calculation: Filtration efficiency can be calculated easily with formula η = 1 − 1/β. For example, β₅=500 → η = 1 − 1/500 = 0.998 (99.8%).
- More straightforward performance comparison: The value of βₓ linearly reflects filter performance. A filter with β=500 delivers 2.5 times the performance of a β=200 filter (far fewer downstream particles). Comparing Beta values is much clearer than comparing 99.8% against 99.5%.
- Directly linked to system cleanliness: In hydraulic oil and lubrication systems, component wear is closely related to particle concentration in fluid. Beta value directly tells you how many times the particle count is reduced after filtration. Take β₅=500 as an example: particles larger than 5 μm drop to 1/500 of the original level. This is critical for system design and reliability prediction.
- Reflect particle retention (the most important benefit): As filters operate, they continuously adsorb contaminants (load holding). A high-quality filter must not only capture particles, but also hold them securely, preventing re-release downstream under high flow velocity or pressure fluctuation.
- Multi-pass cyclic test for βₓ: During testing, the filter continuously traps contaminants. Its performance will rise, stabilize and then decline. The reported Beta value is the minimum value at its stable stage. A filter with high βₓ maintains stable contaminant retention through most of its service life, ensuring consistent downstream fluid cleanliness. This cannot be fully represented by a simple percentage filtration efficiency number.
Filtration efficiency is a key performance indicator for industrial filters. Beta ratio (βₓ) offers a more scientific and comprehensive evaluation standard. It demonstrates not only particle capture capability, but also long-term stability and contaminant holding capacity, making it an indispensable reference for industrial filtration applications.