Determination of Initial Filtration Efficiency by the Oil Mist Method

The initial filtration efficiency determined by the oil mist method is a key performance indicator for air filters, filter media, and high-efficiency filtration products, measuring how effectively a brand-new filter captures submicrometer liquid aerosols at rated airflow. This test challenges the specimen with a controlled oil mist aerosol and compares upstream and downstream concentrations by light-scattering photometry, in accordance with recognized national and international test method standards. In this article, we explain the principle, sample types, testing procedure, calculation method, and typical applications of oil mist based initial filtration efficiency determination.

Technical Overview

The initial filtration efficiency, sometimes referred to as the clean-filter or original efficiency, describes the particle capture capability of a filter in its brand-new, unloaded state, evaluated at the rated airflow before any dust holding or loading process has taken place. Because a filter's efficiency changes as it accumulates dust during service, the initial value provides a reproducible baseline for comparing materials, designs, and production batches.

In the oil mist method, the test specimen is challenged with a stable, submicrometer liquid aerosol generated by atomizing oil with clean, dry compressed air. The mass median diameter of the challenge aerosol is typically controlled at approximately 0.3 micrometers, which is close to the most penetrating particle size for many fibrous filter media. Oil mist concentrations upstream and downstream of the specimen are measured with a light-scattering photometer, and the filtration efficiency is calculated from the concentration ratio. This methodology is specified in Chinese national test method standards such as GB/T 6165 and is applied alongside related international approaches from organizations including ISO, EN, and ASTM when evaluating high-efficiency filtration products in accredited third-party laboratories.

Testing Samples

Filter Media and Materials

Flat-sheet specimens cut from glass microfiber paper, melt-blown polypropylene nonwovens, PTFE membranes, composite laminates, electrostatically charged media, and activated-carbon-coated filtration materials can all be evaluated. Media testing is typically performed in a standardized filter holder at a defined face velocity corresponding to the intended application.

Filter Elements and Complete Filter Units

Finished products suitable for oil mist based initial efficiency determination include panel-type air filter elements, cylindrical engine intake air filter elements, cabin air filters, industrial dust collection cartridges, oil mist separator elements, and high-efficiency air filters used in ventilation and cleanroom systems. Complete elements are installed on a compatible test duct with verified sealing to prevent bypass leakage.

Specialty Filtration Components

Respirator filter layers, vacuum cleaner filter assemblies, compressor intake filters, and other custom-engineered components can also be assessed, provided that appropriate mounting fixtures and test flow rates are established.

Testing Items

Depending on the product standard and the client's technical requirements, the following parameters are typically measured:

• Initial filtration efficiency at the rated airflow, expressed as a percentage
• Initial pressure drop (initial resistance) across the clean filter at rated flow, when required as a companion measurement
• Penetration value, reported as the complement of efficiency for high-efficiency products
• Verification of challenge aerosol characteristics, including oil mist concentration and particle size distribution
• Efficiency at multiple flow rates or multiple particle size channels, when a fractional efficiency profile is required

Testing Methods

Test Principle

A controlled oil mist aerosol is produced by atomizing a suitable oil, such as DEHS, PAO, or a specified turbine-type oil, using filtered compressed air. The aerosol passes through a conditioning and mixing section to achieve a homogeneous and stable concentration and particle size distribution. The aerosol-laden air stream is drawn through the test specimen at the specified flow rate. A light-scattering photometer sequentially samples the oil mist concentration upstream of the filter, designated C1, and downstream of the filter, designated C2. The initial filtration efficiency is calculated as follows:

E = (C1 - C2) / C1 x 100% = (1 - C2/C1) x 100%

where C1 and C2 are the upstream and downstream oil mist mass concentrations, respectively. For high-efficiency filters, the result is often expressed as penetration, P = C2/C1 x 100%.

Test Procedure

• Specimen conditioning: samples are conditioned in a controlled temperature and humidity environment, and their dimensions, structural condition, and sealing surfaces are checked before installation
• System preparation: the test duct, photometer, and aerosol generator are warmed up, zeroed, and verified; blank readings are taken to confirm a clean system background
• Aerosol stabilization: the oil mist generator is adjusted until the upstream concentration and particle size distribution, with a mass median diameter typically in the range of approximately 0.28 to 0.34 micrometers, remain stable within the tolerance specified by the test standard
• Measurement: the rated airflow is established through the specimen, and upstream and downstream concentrations are sampled alternately until stable readings are obtained; pressure drop is recorded simultaneously where required
• Calculation and reporting: efficiency or penetration is computed from the mean concentration values, and the report documents the test conditions, flow rate, aerosol parameters, and individual as well as mean results

Quality Control Measures

To ensure validity, the test incorporates system leak checks, 100 percent reference checks on the photometer, replicate measurements, and verification that the aerosol generator output drifts within permissible limits. Sealing integrity around the specimen is critical, because any bypass leakage will understate the true efficiency of the filtration medium.

Applications

• Quality control of filter media and filter element production lines, including release testing of outgoing batches
• Research and development support for comparing media formulations, pleat geometries, electrostatic treatments, and manufacturing processes
• Qualification and classification of high-efficiency air filters used in cleanrooms, hospitals, laboratories, and precision manufacturing facilities
• Performance evaluation of automotive engine intake air filters, cabin air filters, and compressor intake filtration systems
• Verification of oil mist separators and industrial air purification equipment
• Regulatory compliance support, supplier qualification, and independent verification of manufacturer efficiency claims through accredited third-party laboratories

FAQ

What is the difference between initial filtration efficiency and cumulative efficiency?

Initial filtration efficiency is measured on a clean, unloaded filter at the beginning of its service life and represents the intrinsic capability of the filter medium and construction. Cumulative or average efficiency, by contrast, is determined after the filter has been loaded with a specified quantity of test dust, which reflects the evolving performance during use. Both values are complementary and are often reported together in a full performance assessment.

Why is the oil mist method used instead of a dust-based gravimetric method?

The oil mist method challenges the filter with submicrometer liquid droplets near the most penetrating particle size of fibrous media, providing a highly sensitive measure of filtration performance, particularly for high-efficiency products. Dust-based gravimetric methods emphasize dust holding capacity and efficiency over the loading cycle and are more relevant for coarse pre-filtration applications. The two approaches serve different evaluation purposes and are selected according to the applicable product standard.

How does the oil mist method compare with sodium flame or particle counting methods?

All three approaches quantify the filtration performance of a clean filter, but they differ in the challenge aerosol and detection principle. The oil mist method uses a liquid aerosol detected by light scattering, the sodium flame method uses salt aerosol detected by flame photometry, and modern counting methods, such as those referenced in ISO and EN standards for HEPA and ULPA filters, use discrete particle counting to identify the most penetrating particle size. Results from different methods are not directly interchangeable, so the test method must be agreed upon before testing.

What information should be provided when submitting a sample?

The client should specify the specimen type and dimensions, the rated airflow or face velocity, the applicable product or test method standard, the desired reported parameters such as efficiency, penetration, or pressure drop, and any special mounting requirements. For filter media, the number of replicate specimens required by the standard should be supplied from the same production batch.

How should the results be interpreted?

A higher initial filtration efficiency indicates better intrinsic capture of submicrometer liquid aerosols, which is generally desirable for fine and high-efficiency filtration stages. However, efficiency must always be interpreted together with the initial pressure drop, because overly restrictive media increase energy consumption. A balanced evaluation considers both filtration efficiency and airflow resistance relative to the intended application.