A top-performing HEPA (High-Efficiency Particulate Air) or ULPA (Ultra-High-Efficiency Particulate Air) filter’s effectiveness depends not only on its filtration efficiency but also on the absolute seal and integrity achieved during on-site installation. This final and crucial verification step is performed byDOP/PAO testing.
What is DOP/PAO Testing?
Simply put, DOP/PAO testing is a field integrity test. Its core principle is: upstream of the installed filter, an aerosol of a specific concentration and particle size (the test dust source) is artificially generated. Then, a precision instrument is used downstream to scan for any abnormal “leakage” of the aerosol, thereby determining whether there are defects in the filter itself and its mounting frame.
DOP (dioctyl phthalate) is a traditional test medium used since the mid-20th century, hence the name of the testing method. However, due to its potential carcinogenic risks, PAO (polyalphaolefin), as a safe, non-toxic, and stable synthetic oil, has become the recommended and adopted alternative medium in current global mainstream standards (such as ISO 14644 and EU GMP ). Today, the industry generally refers to this safer modern practice as “PAO testing,” although the historical term “DOP method” is still widely mentioned.
Why is DOP/PAO testing important?
A filter, when shipped, comes with a filtration efficiency report based on stringent standards (such as EN 1822). However, this only represents its performance under ideal laboratory conditions. During transportation, handling, and complex on-site installation, microscopic pinholes invisible to the naked eye may develop in the filter media, and gaps may appear in the gaskets of the mounting frame due to uneven stress. These tiny flaws are enough to allow unfiltered contaminated air to “shortcut” into the clean area, rendering the expensively constructed clean environment ineffective.
Therefore, the purpose of DOP/PAO testing is very clear: it does not repeatedly test the overall efficiency of the filter, but specifically “diagnoses” localized damage to the filter media and the integrity of the seal between it and the mounting frame. The US FDA, EU GMP, and relevant Chinese regulations all explicitly require this leak test to be performed after the installation of HEPA filters in critical clean areas, and recommend regular performance (e.g., every six months or a year in aseptic production areas) to ensure continued compliance.
DOP/PAO Testing Standards and Procedures
This test follows internationally recognized stringent protocols. Currently, the most widely cited standards include the USIEST-RP-CC034 and the ISO 14644 series. Their pass/fail criteria are extremely stringent: for HEPA filters, the leakage rate (the percentage of downstream concentration to upstream concentration) at any point must generally not exceed 0.01%. This means that a leakage of even one ten-thousandth is considered a failure.
A standard on-site PAO testing procedure is like a delicate surgical operation:
- 1. System Preparation: Ensure the air conditioning and ventilation system is operating stably and the airflow is balanced.
- 2. Aerosol Injection: A stable, uniform aerosol cloud is generated upstream of the filter using a PAO generator. The particle size is typically optimized for the 0.3-micron range, the area most easily penetrated by HEPA filters.
- 3. Calibration and Scanning: An aerosol photometer is used to measure and set the upstream concentration to 100% baseline. Technicians then use the instrument’s sampling probe to systematically scan the entire cross-section of the filter and all frame seams at close range (approximately 2-4 cm) at a speed not exceeding 5 cm per second.
- 4. Judgment and Handling: During the scan, the instrument displays the leakage rate in real time. If any point exceeds the preset alarm value of 0.01%, the instrument will alarm, indicating a leak. For minor leaks, repairs with specialized sealant are permissible, but with strict area limitations; for severe leaks, the filter must be replaced or reinstalled.
At the filter R&D and testing center in Hanover, DOP/PAO testing is far from being an optional test. It is the key verification gate for turning clean environments from blueprints into reality, a universal technical language recognized by international quality systems, and the ultimate commitment for companies likeTrenntech to deliver definite value to their customers.