Leak detection for HEPA filters is a standard testing requirement for cleanrooms and a mandatory procedure for pharmaceutical companies; an increasing number of facilities in sectors such as healthcare, electronics, food processing, and cosmetics are also adopting these testing protocols.
The following section details the specific methods used for HEPA filter leak detection.
I. Sodium Flame Method
The test aerosol source for the sodium flame method is a poly disperse sodium chloride salt mist; the measured parameter is the brightness of a hydrogen flame when exposed to the salt-laden mist.
Saltwater is atomized by compressed air and dried to form tiny salt crystal particles, which enter the airflow duct; air samples are collected both upstream and downstream of the air filter.
The salt-laden air causes the hydrogen flame to turn blue and increase in brightness. Flame brightness is used to gauge the salt mist concentration, thereby determining the filter's filtration efficiency.
The primary testing instrument is a flame photometer. This method has relatively low sensitivity and is unsuitable for testing HEPA filters.
II. Oil Mist Method
The test aerosol source for the oil mist method is oil mist; the measured parameter is the turbidity of the air containing the mist. Filtration efficiency is determined by comparing the turbidity of air samples taken upstream and downstream of the air filter.
German standards specify the use of paraffin oil, with oil mist particle sizes ranging from 0.3 to 0.5 micrometers. This method carries a risk of damaging the filter during testing, does not provide direct readings, and is time-consuming.
III. DOP Method
This method was once the internationally standard approach for testing HEPA filters.
The test aerosol source consists of monodisperse dioctyl phthalate (DOP) droplets with a diameter of 0.3 micrometers—also known as "Hot DOP"—and the measured parameter is the turbidity of the DOP-laden air.
DOP liquid is heated into vapor, which condenses into tiny droplets under specific conditions. After filtering out droplets that are too large or too small, particles of approximately 0.3 micrometers remain and enter the airflow duct. Filtration efficiency for 0.3-micrometer particles is determined by measuring the turbidity of air samples upstream and downstream of the air filter.
IV. Fluorescence Method
The test aerosol source for the fluorescence method is sodium fluorescein dust generated by a sprayer. The testing procedure involves first taking samples upstream and downstream of the filter sponge, then dissolving the sodium fluorescein from the sampling filter paper into water. Next, the fluorescence intensity of the resulting solution is measured under specific conditions; since this intensity correlates with the mass of the captured dust, the filter's efficiency can be calculated.
V. Particle Counting Method
This method is widely used in Europe, and the testing procedure for ultra-low penetration air (ULPA) filters in the United States is similar; it is currently the mainstream international method for testing sponge filters.
The dust source consists of polydisperse liquid droplets or solid dust particles of a specific size. In some cases, filter manufacturers may use atmospheric dust or other specific types of dust to meet a user's special requirements.
If a condensation nucleus counter is used for the test, a monodisperse test dust source with a known particle size is required. The primary measuring instruments are high-flow-rate laser particle counters or condensation nucleus counters.
A counter is used to scan the entire downstream face of the filter; it records the number of dust particles at each point, allowing for a comparison of local efficiencies across different locations.
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