How to read the test particle size in the filter efficiency report?

When purchasing a HEPA or ULPA filter, the supplier will usually provide an efficiency report. The report says efficiency percentages, such as 99.95%, 99.995%, 99.9995%. But behind these numbers, the test particle size, test method, test air volume and reporting objects may be different. If you don't see these conditions clearly and just sort by percentage, it's easy to draw wrong conclusions.

Some reports are tested according to fixed particle sizes, and some are tested according to the most easily penetrable particle size; some are filter material sample data, and some are complete filter finished product data; some are measured under rated air volume, and some are measured under specific test air volume. The following will start from the test caliber, reporting objects, data comparison, and project application.

First identify the test standards used in the report

Different filter classification standards have different definitions and testing methods of efficiency. When looking at a report, the first step is to confirm the standards system used. Common concerns include:

• Standard name and version;

• Test whether the particle size is the easiest to penetrate or a fixed particle size;

• Test whether the air volume is consistent with the actual air volume;

• Aerosol generation methods and detection methods;

• Whether the reporting object is filter media, filters, or the installed system.

If you are still unclear about these concepts, you can read How to choose between H13, H14 and U15? , first understand the efficiency requirements corresponding to different levels, and then come back to look at the report caliber.

Fixed particle size is not the same caliber as MPPS

Fixed particle size test measures filtration efficiency at a specified particle size. For example, some standards stipulate testing at 0.3μm. The most penetrating particle size test is to first find the particle size point with the lowest efficiency of the filter, and then measure the efficiency at that particle size. The test locations of the two are different and the results cannot be directly compared.

One report states that the 0.3μm efficiency is 99.99%, and the other reports H14 based on MPPS classification. You cannot directly use these two percentages to sort, and the test caliber should be unified first. For specific differences, please refer to How do HEPA and ULPA match cleanliness goals? The part about efficiency reporting standards.

Is the reporting object filter media or finished product?

Efficiency reports may come from different test subjects:

• Filter material sample : a sample cut from the filter material and tested under laboratory conditions;

• Complete filter : The finished filter is tested on a standard bench;

• Post-installation system : After the filter is installed in the equipment, it is tested in the field or under simulated conditions.

Filter material sample data cannot directly represent the performance of the finished filter. The finished filter involves frame sealing, filter element folding and installation structure, and the efficiency may be lower than the filter material sample. The installed system also needs to consider frame leakage and interface sealing.

If there are abnormal particles on site but the filter efficiency is acceptable, the sealing and installation should be prioritized rather than just looking at the reported numbers. Related analysis can refer to Does higher filtration efficiency necessarily lead to improved site ratings?。

The test air volume should correspond to the actual air volume used

Efficiency reports usually indicate the test air volume. If the test air volume is different from the actual air volume used, the efficiency data cannot be directly applied. The efficiency and resistance of the filter may change under different air volumes.

When selecting a model, you should confirm whether the test air volume in the report is close to the actual working point of the project. If you are not familiar with the pronunciation of working points, you can read Why should we look at the complete performance curve when selecting FFU? , first confirm the target air volume and system resistance.

For the matching of filter rated air volume and actual operating air volume, please refer to How does the filter's rated air volume match the actual operating air volume? , learn how to check when selecting.

Don’t just take a percentage comparison

When comparing two reports, don’t just look at efficiency percentages. Should also check:

• Whether the test standards are the same;

• Test whether the particle size and caliber are consistent;

• Test whether the air volume is close;

• Whether the reporting objects are the same;

• Efficiency is overall efficiency or partial efficiency.

Only when these conditions are unified can the percentages be compared. If the conditions are different, the standards should be unified first or the supplier should be asked to supplement the corresponding data.

Efficiency reporting versus site grade

The filter efficiency report describes the performance of the filter under specified test conditions, not the cleanliness level of the room or area. The cleanliness level must be comprehensively judged based on air supply volume, air flow organization, sealing, pollution load and operating status.

If you are still unclear about the difference between space rating and filter efficiency, you can read What is the difference between cleanliness level and filtration efficiency ? Avoid confusing the two concepts. The cleanliness level must also indicate the measured particle size. For details, please refer to Why must the particle size be indicated in the cleanliness grade?。

Sealing and installation leak detection cannot be omitted

If the high-efficiency filter is not properly installed and sealed, bypass leakage will allow unfiltered air to enter the clean side. At this time, no matter how efficient the report is, the expected results will not be achieved on site. After installation, the frame seal, installation interface and leak detection conditions should be checked.

If there are abnormal particles on site but the filter efficiency is acceptable, priority should be given to checking the sealed bypass and airflow short circuit. Can be referenced What particle problems can airflow short circuits cause? and What should be checked when cleaning equipment is accepted?。

Initial resistance and final resistance are used for different judgments

The initial resistance usually describes the pressure drop of a new filter under a specified air volume, and the final resistance is used to judge maintenance or replacement during the agreed use period. The final resistance is not the only life criterion. Damage, moisture, process risks and actual air supply capacity must also be considered.

For specific judgment, please refer to What are the initial resistance and final resistance used to judge? . If the operating pressure differential is abnormally low, a breakage or bypass should be investigated rather than a performance improvement.

The rated air volume must match the actual operating air volume

The rated air volume of the filter is used to describe its specified test or design conditions. The actual operating air volume should fall within the usage range confirmed by the manufacturer. When selecting a model, you need to check the efficiency, resistance, and fan operating point. You cannot determine replacement based on the same size.

For specific judgment, please refer to How does the filter's rated air volume match the actual operating air volume? . If the air volume drops after replacing the filter, you should first check the operating point instead of directly increasing the rotation speed.

Project judgment example

A certain project compared two filter reports, one said 0.3μm efficiency was 99.99%, and the other was written H14 according to MPPS classification. The original plan was sorted directly by percentage, thinking that the first one was better. After checking, it was found that the two reports had different test calibers and could not be directly compared. Adjustment method: Recheck after unifying the test standards, and select a model that matches the sensitive particle size of the project.

Another project saw no drop in on-site particulates after replacing a high-efficiency filter. The investigation revealed that the reported data were the results of filter material samples, and the actual finished filter frame was poorly sealed. Adjustment method: Replace the complete filter and recheck for leaks. After the seal is improved, the particles on site will decrease.

These two situations illustrate that the test particle size in the filter efficiency report must be read in conjunction with the standards, methods and report objects, and cannot be compared with just a percentage.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

The filter efficiency report involves multiple variables such as standard, particle size, air volume and test object. It is difficult to judge whether it is suitable for the project by just looking at a percentage. Aili Industrial also does air filters, fans, motors, control systems and complete machine assembly. It can match the test conditions in the filter efficiency report with the actual working point of the project, sealing method and complete machine resistance, helping to judge whether the report data can be directly used for model selection, or whether it is necessary to supplement performance data under corresponding working conditions. Specific filter parameters can be viewed HEPA and ULPA filters , the complete machine package can be viewed FFU clean air supply products and EFU equipment side fan filter unit。