Initial Filtration Efficiency Sodium Flame Method Test: Principle, Procedure and Applications

The initial filtration efficiency sodium flame method test is a standardized aerosol-based performance evaluation that quantifies the particle capture efficiency of a new, unloaded air filter at its rated airflow. Guided by recognized procedures such as GB/T 6165 and related international practices, the method challenges the filter with a polydisperse sodium chloride aerosol and determines efficiency from upstream and downstream concentrations measured by hydrogen flame photometry. This article explains the test principle, applicable sample types, measured parameters, step-by-step methodology, calculation basis, typical applications, and frequently asked questions.

Technical Overview

The sodium flame method, also known as the sodium chloride aerosol flame photometry method, is a classical mass-concentration-based technique for evaluating the particulate filtration efficiency of air filters and filter media. It is widely referenced in Chinese national test standards such as GB/T 6165 (test methods for efficiency and resistance of high efficiency air filters), and it historically underpinned product standards including GB/T 13554 for high efficiency particulate air (HEPA) filters and GB/T 14295 for general ventilation air filters. Modern international equivalents, such as EN 1822 and ISO 29463, rely on most penetrating particle size (MPPS) counting techniques, so results from different methods must be compared with care.

Definition of Initial Filtration Efficiency

Initial filtration efficiency refers to the efficiency of a filter measured in its original, as-manufactured, unloaded condition at the rated airflow, before any artificial dust loading or aging. It is distinguished from the average or final efficiency determined after dust loading in long-duration cyclic tests. Because particle capture by a clean filter typically represents the worst case for many media, the initial value is a key acceptance indicator for factory quality control, incoming inspection, and certification.

Method Principle

Aqueous sodium chloride solution is atomized by compressed clean air through a spray nozzle, and the resulting droplets are mixed with dry dilution air so that the water evaporates and leaves solid polydisperse salt crystals, with a mass median diameter of approximately 0.4 to 0.6 micrometers. The aerosol is blended uniformly and passed through the test filter at the rated flow. Samples drawn upstream and downstream are fed into a hydrogen flame photometer: sodium atoms excited in the flame emit characteristic yellow light at approximately 589 nanometers, and the emission intensity, detected by a photomultiplier through an optical filter, is proportional to the sodium mass concentration. Penetration is the ratio of downstream to upstream concentration, and efficiency is calculated as one minus penetration.

• Aerosol type: solid polydisperse sodium chloride particles
• Particle size: mass median diameter of roughly 0.5 micrometers
• Detection principle: hydrogen flame photometry at the sodium D-line
• Suitable efficiency range: typically up to about 99.999 percent (penetration down to about 0.001 percent)

Testing Samples

High Efficiency Particulate Air Filters

Minipleat, separator-type, and compact HEPA filter units in standard frame sizes, for example 610 by 610 by 292 millimeter units commonly rated at 1,000 cubic meters per hour, are tested as complete filter assemblies installed in a sealed holding frame. The sodium flame method is particularly well suited to efficiency classes around 99.9 to 99.999 percent.

General Ventilation Air Filters

Panel, bag, pocket, and rigid-cell filters used in air handling systems can be evaluated in their initial state to establish the clean-filter baseline efficiency and resistance, which are then referenced in dust loading and lifecycle assessments conducted in accordance with standards such as GB/T 14295 or ISO 16890 concepts.

Filter Media and Laboratory Coupons

Flat sheets of glass fiber, synthetic polymer, or composite filter media are mounted in circular or rectangular media holders with a defined effective test area. Media-level testing supports research and development, formulation comparison, and incoming material quality control before pleating and assembly.

Special Filter Elements and Units

Terminal filter housings, fan filter units, laminar flow modules, biosafety cabinet filters, and other engineered air cleaning elements can be verified in the as-new state, provided the sample can be installed in the test duct with a verified leak-tight seal and operated at a controlled, representative airflow.

Sample preparation requirements are consistent across these categories: samples must be intact, clean, representative of production, conditioned to equilibrium with the laboratory environment, and clearly identified with dimensions, media type, and rated airflow. Testing is normally performed on multiple samples to assess consistency.

Testing Items

• Initial filtration efficiency against sodium chloride aerosol, expressed as a percentage
• Initial penetration, the complement of efficiency, useful for high efficiency grades
• Initial pressure drop (initial resistance) across the filter at rated airflow, expressed in pascals
• Repeatability verification through replicate upstream and downstream concentration readings
• Overall system integrity confirmation using a reference absolute filter blank check
• Ambient condition recording, including temperature, relative humidity, and barometric pressure

Testing Methods

Test Apparatus

A complete sodium flame test bench comprises an aerosol generator with solution reservoir and spray nozzle, a drying and mixing section, a horizontally arranged test duct with flow straighteners and mixing orifice plates, upstream and downstream sampling probes, a hydrogen flame photometer equipped with calibrated neutral optical attenuators, a primary filter for producing clean dilution air, a reference absolute filter for blank checks, a flow measurement device such as nozzles or a calibrated flowmeter, a precision micromanometer for pressure drop, and sensors for temperature and humidity. The photometer attenuators allow a very high upstream signal and a very low downstream signal to be compared within the linear response range of the detector.

Environmental and Aerosol Conditions

Testing is performed under controlled laboratory conditions with temperature and relative humidity maintained within the limits specified by the test standard. Humidity control is critical because sodium chloride is hygroscopic and deliquesces at high relative humidity, which changes particle size and invalidates efficiency readings. The challenge aerosol concentration is stabilized and kept within the range defined by the method, and its stability is continuously monitored through upstream sampling.

Test Procedure

• Inspect and condition the sample, record its identification and dimensions, and install it in the test duct with a verified leak-tight gasket seal.
• Adjust the fan and flow control elements to achieve the rated airflow, and allow the flow and photometer flame to stabilize.
• Measure and record the initial pressure drop across the clean filter using the micromanometer before aerosol challenge.
• Verify the blank penetration of the system with a reference absolute filter installed, confirming that duct and seal leakage is below the required threshold.
• Start the sodium chloride aerosol generator, allow the upstream concentration to stabilize, and perform zero and span checks of the photometer using the calibrated attenuators.
• Alternately sample upstream and downstream air, record the photometer readings, and apply the attenuation factor to obtain the true concentration ratio.
• Repeat the measurement sequence at least three times, verify that the upstream concentration variation and reading scatter remain within permissible limits, and calculate the arithmetic mean penetration and efficiency.
• Compile the test report with all raw data, calculated results, environmental conditions, and apparatus calibration status.

Calculation of Efficiency

Penetration is calculated as P equals the downstream photometer signal divided by the upstream signal, multiplied by the attenuation factor of the calibrated neutral filter used during the upstream measurement. Filtration efficiency is then E equals one minus P, multiplied by one hundred percent. For example, if the mean penetration is 0.005 percent, the initial filtration efficiency is reported as 99.995 percent. Pressure drop is reported directly in pascals at the stated actual airflow.

Quality Control Measures

Reliable results depend on rigorous quality control: periodic calibration of the photometer with certified neutral density filters, calibration of flow and pressure instruments, verification of aerosol generator stability, leak testing of the entire sampling circuit, use of freshly prepared sodium chloride solution at the specified concentration, and participation in interlaboratory comparisons. Tests intended for certification purposes are typically performed by accredited third-party laboratories operating in conformity with the referenced standard.

Applications

• Factory quality control and batch release testing of HEPA filters for cleanroom applications
• Type testing and certification of high efficiency and general ventilation air filters
• Pharmaceutical and biotechnology cleanroom supply air filtration verification supporting good manufacturing practice requirements
• Hospital isolation rooms, operating theaters, and laboratory containment ventilation systems
• Electronics, semiconductor, and precision manufacturing environments requiring controlled particulate levels
• Nuclear and radiological facility air cleaning systems, where sodium flame testing has a long historical record
• Food and beverage aseptic filling areas and other hygienic air supply systems
• Filter media development, benchmarking, and incoming inspection in research and industrial laboratories

FAQ

What does "initial" mean in initial filtration efficiency?

Initial refers to the as-new, unloaded condition of the filter. The efficiency is measured on a clean filter at rated airflow, without any prior dust loading, which provides the baseline performance value used for acceptance and classification.

What particle size does the sodium flame method use?

The method uses a polydisperse solid sodium chloride aerosol with a mass median diameter of approximately 0.5 micrometers, which is smaller than the particles used in gravimetric dust loading tests and therefore exercises the filter in a more demanding size range.

How sensitive is the method?

With a properly calibrated hydrogen flame photometer and stable aerosol generation, penetrations down to approximately 0.001 percent can be resolved, corresponding to efficiencies up to about 99.999 percent. For ultra-low penetration filters beyond this range, an MPPS particle counting method per EN 1822 or ISO 29463 is generally more appropriate.

How does the sodium flame method differ from counting methods?

The sodium flame method measures the mass concentration of sodium reaching the flame and reports a single integrated efficiency value, whereas counting methods size and count individual particles to locate the most penetrating particle size. Results are related but not numerically identical, so the test method must always be stated alongside the reported efficiency.

Why must relative humidity be controlled?

Sodium chloride particles absorb moisture and grow, or dissolve entirely, at elevated humidity. Particle growth alters the challenge size distribution and measured efficiency, so the test standard defines temperature and relative humidity limits that must be maintained and documented throughout the test.

Can the method be used for leak scanning of filter surfaces?

The classical sodium flame method is an overall (total filter) efficiency test on the installed assembly. Local leak scanning of the filter face and frame is typically performed with scanning counting probes per the applicable standard, and the two approaches complement each other in a full qualification program.

What information is included in the test report?

A compliant report identifies the sample and standard used, states the rated and actual airflow, records environmental conditions, lists individual and mean penetration and efficiency values, reports the initial pressure drop, describes the apparatus and its calibration status, and is signed by authorized laboratory personnel.