Five Common Leak Detection Methods for HEPA Air Filters
Leak detection for HEPA air filters has become a standard requirement in cleanroom inspections, especially since the implementation of the 2010 version of GMP (Good Manufacturing Practice). Today, it is a mandatory procedure for pharmaceutical companies, and its importance is increasingly recognized across various industries such as healthcare, electronics, food processing, and cosmetics. Below are the five most commonly used methods for HEPA air filter leak testing.
1. Sodium Flame Method
The sodium flame method uses polydispersed sodium chloride mist as the test aerosol. The detection principle is based on the brightness of a hydrogen flame that changes when sodium chloride particles are present.
A saline solution is atomized by compressed air, forming fine salt crystals after drying. These particles are introduced into the air duct, and samples are taken before and after the filter. When the aerosol passes through a hydrogen flame, the flame turns blue and becomes brighter. The change in flame brightness indicates the salt concentration in the air, which is used to determine the HEPA air filter’s ability to remove salt particles.
The main detection instrument used in this method is a flame photometer. However, this method has relatively low sensitivity and is not suitable for ultra-HEPA filters.
2. Oil Mist Method
The oil mist method uses oil mist as the test aerosol, and the detection is based on the turbidity of the air containing the mist. The difference in turbidity between the upstream and downstream samples reflects the HEPA air filter’s performance in capturing oil particles.
In Germany, paraffin oil is typically used, with a droplet size of 0.3 to 0.5 microns. Although this method was once widely adopted, it has limitations. The test may damage the filter media, and since the results cannot be read directly, it is also time-consuming and less efficient.
3. DOP Method
The DOP (Dioctyl Phthalate) method was once a widely accepted standard for testing HEPA air filters worldwide. It uses monodispersed DOP droplets of 0.3 microns in size, often referred to as “hot DOP.”
DOP liquid is heated into vapor and then condensed under controlled conditions into fine droplets. Particles that are too large or too small are removed, leaving droplets around 0.3 microns. These particles are introduced into the airflow, and turbidity measurements are taken before and after the HEPA air filter to evaluate its filtration efficiency for 0.3-micron particulates.
4. Fluorescence Method
The fluorescence method uses sodium fluorescein particles generated by a sprayer as the test aerosol.
Samples are collected both upstream and downstream of the HEPA air filter. The sodium fluorescein on the sampling papers is dissolved in water, and the solution’s fluorescence intensity is measured under specific conditions. The brightness correlates with the amount of particulate matter, from which the filter’s efficiency can be calculated.
5. Particle Counting Method
This method is widely used in Europe and is also the basis for testing ultra-HEPA air filters in the United States. It is currently regarded as the most reliable and accurate method for leak detection of HEPA air filters.
The test aerosol can be a polydispersed liquid droplet or solid particles of known size. In some cases, filter manufacturers may be required to use atmospheric dust or other specific particles based on customer specifications.
When using a condensation nucleus counter (CNC), a monodispersed test aerosol with known particle size must be used. The primary instruments are high-flow laser particle counters or CNCs. These counters scan the entire downstream surface of the HEPA air filter, providing particle count data for each point and allowing local efficiency comparisons across the filter surface.
About E-FILT
E-FILT is a trusted air filter manufacturer specializing in the development and production of high-performance HEPA air filters and ULPA filters. Designed for use in demanding environments such as pharmaceutical cleanrooms, electronics production lines, and laboratories, E-FILT filters meet the highest international standards for air purification. With a focus on quality, reliability, and innovation, E-FILT offers both standard and customized filtration solutions backed by expert support and precise engineering.
