Original Filtration Efficiency Testing System: Principles, Standards, Methods, and Applications

An Original Filtration Efficiency Testing System measures the particle-capture performance of new, unloaded filter elements under controlled laboratory conditions, establishing the baseline efficiency data required for product qualification and classification. Built in accordance with internationally recognized standards such as ISO 5011, ISO 16890, and ISO 16889, these systems combine precision aerosol or test-dust generation with calibrated upstream and downstream particle measurement. This article explains the working principles, applicable sample types, testing items, test methods, and typical applications of original filtration efficiency testing systems.

Technical Overview

An original filtration efficiency testing system, also known as an initial filtration efficiency test rig, is a laboratory installation designed to quantify how effectively a brand-new, uncontaminated filter element removes particles from an air or liquid stream. Because filtration performance changes as a filter loads with dust over its service life, the "original" or "initial" efficiency measured at the beginning of a test serves as the fundamental reference value for design verification, quality control, and performance classification.

The core measurement principle is straightforward: a challenge contaminant of known concentration and particle size distribution is introduced upstream of the test filter, and the concentration passing downstream is measured simultaneously. The original filtration efficiency is calculated as:

E = (1 − Cd / Cu) × 100%, where Cu is the upstream particle concentration and Cd is the downstream particle concentration. Depending on the standard, concentrations may be determined gravimetrically by weighing absolute (total) sampling filters, or optically by particle size counters that yield fractional efficiency values across discrete particle size channels.

A complete system typically integrates the following subsystems:

• Aerosol or test-dust generation: compressed-air nebulizers for KCl or NaCl solution aerosols, DEHS (di-ethylhexyl-sebacate) liquid aerosol generators, or dry-powder dust feeders dispensing ISO 12103-1 Arizona test dust (A1 ultrafine to A4 coarse) and ISO 15957 L-series test dust.
• Test duct and sample holder: a straight, stainless-steel duct with mixing and settling sections, plus holders for round, panel, or flat-sheet specimens with standardized sealing to eliminate bypass leakage.
• Upstream and downstream sampling: isokinetic sampling probes connected to optical particle counters (OPC), condensation particle counters (CPC), or gravimetric absolute filters.
• Flow control and measurement: variable-speed blowers or pumps, calibrated flow nozzles, and mass flow controllers to hold the rated test flow within tight tolerance.
• Differential pressure instrumentation: high-accuracy transducers to record the initial pressure drop (restriction) across the element.
• Environmental control and data acquisition: temperature and relative humidity conditioning of the test air or fluid, plus software that automates sampling sequences, calculates efficiency, and generates traceable reports.

Testing Samples

Engine Intake Air Filter Elements

Air cleaner elements for internal combustion engines, tested as complete elements or filter media coupons in accordance with ISO 5011 and the corresponding national standard GB/T 28949. These elements are typically evaluated with ISO 12103-1 coarse test dust at rated air flow.

General Ventilation and Cabin Air Filters

Panel, bag, and compact filters for HVAC systems, as well as automotive cabin air filters, tested to ISO 16890 (and historically EN 779) and national standards such as GB/T 14295. Flat-sheet media specimens are also tested for developmental purposes.

Hydraulic and Lubrication Oil Filter Elements

Hydraulic power filter elements evaluated with the multi-pass method of ISO 16889, using ISO medium test dust (MTD) suspended in a calibrated test fluid loop that continuously recirculates contaminant.

Fuel Filter Elements

Diesel and gasoline filter elements for particle retention, assessed with procedures aligned to ISO 19438, which specify defined test dusts and particle counting on both sides of the element.

High-Efficiency Filter Media and Elements

Flat-sheet filter media samples cut from production rolls are among the most common specimens, because material-level initial efficiency strongly predicts finished-element performance.

Testing Items

Typical measured parameters include:

• Original (initial) fractional filtration efficiency as a function of particle size, for example in channels from 0.3 µm to 10 µm or larger.
• Gravimetric (cumulative mass) efficiency against a specified test dust.
• ePM1, ePM2.5, and ePM10 efficiency values used for ISO 16890 filter classification.
• Initial pressure drop (flow resistance) at the rated flow rate.
• Beta ratio (βx) and associated efficiency ratings for hydraulic elements per ISO 16889.
• Verification of the challenge aerosol or dust concentration and particle size distribution.
• Repeatability runs to demonstrate measurement stability at the unloaded condition.

Testing Methods

Although details differ by product family, the general test sequence is consistent across standards:

• Condition the specimen to a defined temperature and humidity, then weigh and document it.
• Install the element or media coupon in the test holder with verified sealing integrity.
• Stabilize the rated flow rate and record the initial pressure drop.
• Generate the challenge contaminant at the concentration and size distribution required by the standard.
• Sample upstream and downstream simultaneously, either isokinetically with particle counters or with gravimetric absolute filters.
• Calculate efficiency values and repeat the initial measurement until stable, repeatable results are obtained.
• Report efficiency versus particle size, or mass efficiency, together with flow, pressure drop, and environmental conditions.

Standard-specific approaches include: ISO 5011 and GB/T 28949 specify feeding ISO 12103-1 A2 fine test dust into the intake stream and determining efficiency gravimetrically from the gain of an absolute filter downstream. ISO 16890 challenges the filter with a DEHS or KCl aerosol counted optically at approximately 0.3 µm to compute ePM values, after which the filter is loaded with ISO 15957 dust to verify conditioned performance. ISO 16889 uses a closed multi-pass circuit in which ISO MTD is recirculated through the element while inline particle counters track upstream and downstream counts, yielding the beta ratio at selected particle sizes. Japanese standards such as JIS B 9908, together with the JIS Z 8901 family of test dusts, define comparable procedures for ventilation filters, and GB/T 6165 covers sodium-flame and oil-mist methods for media efficiency in the national framework. For high-efficiency products, ISO 29463 and EN 1822 extend the counting method to the MPPS with scanning measurement for localized leaks.

Applications

Original filtration efficiency testing systems are used throughout the filtration industry and research community:

• Research and development of new filter media, including melt-blown, glass-fiber, electrostatically charged, and nanofiber materials.
• Design verification of engine air intake, cabin air, fuel, and oil filter elements for automotive, commercial vehicle, agricultural, and construction equipment manufacturers.
• Incoming material inspection and production quality control in filter manufacturing plants.
• Performance classification and labeling, for example ePM groups under ISO 16890 or efficiency grades under GB/T 13554.
• Verification testing commissioned with accredited third-party laboratories for regulatory compliance and customer qualification.
• Academic and industrial research on particle separation mechanisms such as inertial impaction, interception, diffusion, and electrostatic attraction.

FAQ

What is the difference between original efficiency and average or cumulative efficiency?

Original (initial) efficiency is measured on a clean, unloaded filter and represents the baseline capture performance. Average efficiency reflects performance integrated over a dust-loading cycle, while cumulative efficiency is computed progressively as contaminant is fed. Original efficiency is essential because a filter that allows excessive bypass of fine particles when new may compromise downstream components from the start of service.

Which test contaminant is used?

The contaminant depends on the standard: ISO 12103-1 Arizona test dusts (A1–A4) for engine air filters, ISO 15957 L-series dust for ventilation filters, ISO MTD for hydraulic multi-pass tests, and DEHS, KCl, or NaCl aerosols for optical counting methods. Certified reference dusts with documented size distributions must be used to ensure comparability between laboratories.

How is measurement accuracy ensured?

Accuracy is maintained through isokinetic sampling, zero-count and background checks, correlation of particle counters with traceable references, calibrated flow and pressure instrumentation, leak-tight sample mounting verified with a blank (high-efficiency reference) filter, and control of temperature and humidity. Interlaboratory comparisons and participation in round-robin programs further validate system performance.

Can the same system test both media and finished elements?

Yes. Most systems accept interchangeable holders: a flat-sheet media holder for 100 mm or 150 mm coupons and duct-mounted fixtures for complete round or panel elements. Flow capacity and duct size determine the maximum element size that can be tested at rated conditions.

Why does initial efficiency sometimes increase during the test?

As particles deposit on or inside the medium, they partially occlude the pores and form a dust cake, which raises capture efficiency while also increasing pressure drop. This is why standards prescribe strict timing and concentration limits for the initial measurement, so that the reported value truly represents the unloaded state of the filter.