In the laboratory of the Federal Center for Materials Research and Testing (BAM) in Berlin, engineers are conducting performance tests on a ULPA filter. Unlike other testing methods, they specifically chose 0.12-micron particles as the challenge aerosol —this is precisely the filter’s MPPS, and its weakest point in performance.
Definition and Discovery of MPPS: The “Lowest Point” of Filtration Efficiency
MPPS , or Most Penetrating Particle Size, describes the particle size with the highest penetration rate in a given filter. The discovery of this concept overturned conventional wisdom: larger particles are not necessarily easier to filter.
In the 1980s, German filtration experts discovered that particles in the 0.1-0.3 micrometer range were the most difficult to capture. These particles were neither large enough to be captured by inertial impaction nor small enough to deviate from airflow lines due to Brownian motion—they were precisely in the ‘blind spot ‘ of the filtration mechanism.
MPPS Physical Mechanism: The “Efficiency Low Point” of Three Filtration Principles
MPPS is the result of the combined action of three main filtration mechanisms:
- Interception effect: Highly effective for particles larger than 0.5 micrometers;
- Inertial impaction: Excellent at capturing particles larger than 1 micrometer;
- Diffusion effect: Extremely effective for particles smaller than 0.1 micrometers.
In the 0.1-0.3 micrometer range, all three mechanisms are inefficient, much like three rescuers assuming the drowning person is outside their assigned area, resulting in no rescue attempt.
Testing Standards: An Evolution from “Acceptable” to “Precise”
Traditional filter testing often uses 0.3 micrometers as the standard particle size, while MPPS testing requires scanning the entire particle size spectrum. According to EN 1822 standards , the efficiency curve of a filter must be tested across the entire particle size range to find the point of lowest efficiency. Only when the efficiency at the MPPS (Minimum Per Particle Size) is met can the corresponding filtration class be labeled.
For example, an H14-class HEPA filter has an efficiency of ≥99.995% for 0.3-micron particles; however, at its MPPS (typically 0.1-0.2 microns), the efficiency may drop to 99.97%. Therefore, performance labeling must be based on MPPS test results.
The Practical Significance of MPPS: Engineering Value Beyond Numbers
The practical application of the MPPS concept is having a profound impact on various fields:
Semiconductor Manufacturing: In a chip factory in Dresden, engineers discovered that 0.12-micron particles have the greatest impact on photolithography processes, which is precisely the MPPS range of most ULPA filters.
Medical Industry: In isolation wards of a hospital in Hamburg, the particle size distribution of viral aerosols precisely covers the MPPS range, making the selection of appropriate MPPS filters crucial for infection control.
Pharmaceutical Companies: One pharmaceutical company reduced the product contamination rate on its filling line by 0.3 percentage points through MPPS optimization, equivalent to avoiding millions of euros in losses annually.
Technical Challenges Facing MPPS: Finding a Dynamic Equilibrium Point
MPPS is not static; it is influenced by multiple factors:
- Filter Media Structure: Smaller fiber diameters cause MPPS to migrate towards smaller particle sizes.
- Airflow Velocity : Increased velocity causes MPPS to shift towards larger particle sizes.
- Usage Time: After the filter is loaded with particulate matter, MPPS will drift.
The discovery and application of MPPS marks a shift in filtration technology from extensive to precise, from experience-based to scientific. On the path to ultimate cleanliness, MPPS, once a technological blind spot, has now become the gold standard for measuring filter performance. It reminds every Trenntech engineer: only by overcoming the most difficult challenges can true reliability be achieved—this applies not only to filters but also to technological advancements themselves.