In filter selection, the first reaction of purchasers and engineers is often to check its efficiency rating:EPA, HEPA, or ULPA? However, a combination of letters and numbers such as “H13” or “U15” alone cannot truly determine a filter’s actual effectiveness against a specific particle size threat. To make accurate, economical, and safe decisions, a more fundamental and scientific performance benchmark must be incorporated into the core consideration—that is, the β ratio.
I. What is the β Ratio?
The β ratio is defined as the ratio of the number of upstream particles to the number of downstream particles. It directly describes a filter’s ability to intercept particles of a specific size.
Calculation Formula: βₓ = N_u / N_d
(Where x represents a specific particle size, N_u is the upstream concentration, and N_d is the downstream concentration)
Key Derivation: The initial capture efficiency (E) of the filter for a specific particle size can be accurately calculated from the β ratio.
E = (1 – 1/β) × 100%
Example Analysis:
- If the β ratio (β₂₀₀) for 0.3μm particles is 100, then the efficiency E = (1 – 1/100) × 100% = 99.00%.
- If the β ratio is 1000, then the efficiency E = (1 – 1/1000) × 100% = 99.90%.
- If the β ratio is 10,000, then the efficiency E = (1 – 1/10000) × 100% = 99.99%.
Derivation of the formula: The initial capture efficiency E can be accurately calculated from the β ratio: E = (1 – 1/β) × 100%.
Core value: The β ratio directly determines efficiency. A higher β ratio means an exponential decrease in the proportion of particles penetrating the filter, representing a more robust protection.
II. β Ratio and EPA/HEPA/ULPA:
Traditional classifications (such as theEN 1822 standard) are essentially minimum thresholds set based on the β ratio of the filter at its “most penetrating particle size.”
| Grade Category | Representative Grade | β Ratio (Minimum) to MPPS | Traditional Efficiency Statement |
| EPA | E12 | β ≥ 200 | MPPS Efficiency ≥ 99.5% |
| HEPA | H13 | β ≥ 1,000 | MPPS Efficiency ≥ 99.95% |
| ULPA | U15 | β ≥ 100,000 | MPPS Efficiency ≥ 99.999% |
However, the key point is that the filter’s β ratio dynamically changes with the tested particle size. An H13 filter may have a β ratio as high as 3000 for 0.3μm particles, but its β ratio for 0.1μm particles may plummet to 500. If your process’s primary threat is 0.1μm particles, relying solely on the “H13” label may result in insufficient protection.
III. How to Choose: From “Grade” to “β Curve”
Truly professional selection should focus on the “β ratio-particle size” curve. The specific operational process involves the following key steps.
1. Identify Key Threat Particle Sizes:
- Lithium-ion Battery Industry: Focus on preventing electrode material dust (0.1-0.2μm).
- Pharmaceutical Industry: Pay attention to microbial aerosols (0.2-0.5μm).
- Semiconductor Industry: Be wary of process byproducts (0.05-0.1μm).
- Obtain and Analyze β-Ratio Curves:
Obtain a complete gradation efficiency test report from the supplier to ensure the filter has a sufficiently high β-ratio at your critical particle size points.
3. Assess System Risk:
In a tandem filtration system, the total penetration rate is the product of the penetration rates of each stage. By using the β-ratio of each filter at a specific particle size, the final effluent concentration can be accurately calculated, quantifying system risk.
IV. Practical Application of Beta Ratio: Data-Driven Value
Understanding and utilizing the beta ratio means shifting purchasing decisions from “What level do I need?” to “How much specific particle penetration can I tolerate?” As a professional air filter supplier, Trenntech advises you that true protective effectiveness lies in the details of the beta ratio curve, not in vague efficiency claims—a view particularly valued in German industrial cities like Munich.
The beta ratio transforms filter selection from an experiential art into a scientific decision based on precise calculations. It prevents you from paying unnecessary premiums for vague “high standards” or taking on unknown risks due to misunderstandings about “low penetration.” Mastering this tool is key to building a cleanroom system that is both safe and cost-effective.