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Test methods for high efficiency air filters
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As can be seen from the evolution of international HEPA filter testing standards, the main methods for testing high efficiency air filters are the sodium flame method, the oil mist method, the DOP method, the fluorescence method and the particle counting method. (1) Sodium flame method The sodium flame method originated in the UK in 1969, and was implemented in some European countries in the 1970s to 1990s, and is one of the current national standard methods in China. It tests the dust source for polydisperse phase sodium chloride salt spray, and the "amount" is the brightness of the hydrogen flame when burning with salt spray. The brine is stirred by compressed air and splashed, then dried to form tiny salt crystal particles and enter the air duct, which are sampled before and after the filter, respectively. The salt mist containing gas sample makes the hydrogen flame blue in colour and increase in brightness, and the brightness of the flame is used to judge the salt mist concentration of the air and to determine the filter efficiency of the filter for salt mist. The sodium flame method has the following standards: British BS3928-1969, European Eurovent 4/4, Chinese GB6165-85. The method can only detect the sensitivity is not high, can not be detected on the super HEPA high efficiency filter. (2) Oil mist method The oil mist method originated in Germany. Test dust source for the oil mist, "amount" for the turbidity of the air containing oil mist, before and after the filter to determine the difference in turbidity of the air sample filter efficiency of oil mist particles. The German regulations use paraffin oil, oil mist particle size of 0.3 ~ 0.5 μm. relevant standards are: German DIN24184-1990. oil mist method in the detection of filters, easy to damage the filter, and can not directly read the value, wasting time. At present the German oil mist method has become history, Germany took the lead in 1993 to promulgate the counting method as a national standard for testing methods, the European standard EN-1822 is based on the German standard. (3) DOP method The DOP method originated in the USA in 1956 and has been used in many countries. This method was once the most common method used internationally to test HEPA filters. The test dust source is a 0.3µm monodisperse phase dioctyl phthalate (DOP) droplet, also known as "hot DOP", and the "amount" is the degree of turbidity of the DOP-containing air. The DOP liquid is heated to vapour, which condenses into tiny droplets under certain conditions, leaving particles of around 0.3µm after removal of the oversized and undersized droplets, which enter the duct. The main measurement instrument is a light scattering photometer (photometer). The relevant standard is: MIL-STD-282-1956. (4) Fluorescence method The fluorescence method is only used in France. The source of the dust tested by the fluorescence method is the sodium fluorescein dust produced by the aerosol dispenser. The test method is to first sample the dust before and after the filter, then dissolve the sodium fluorescein on the sampled filter paper with water, and then measure the fluorescence brightness of the aqueous solution containing sodium fluorescein under specific conditions, the brightness responds to the weight of the dust, from which the filter filtration efficiency is calculated. France has long since not used the fluorescence method, they will also be the European Association for Standardization counting method as a national standard, and some nuclear industry systems currently on-site testing filters also use the fluorescence method. (5) Particle counting method This method is common in Europe, and the US super HEPA filter test method is relatively similar, and is currently the mainstream international test method. The dust source is polydisperse phase droplets, or solid dust of a defined particle size. Sometimes filter manufacturers have to use atmospheric dust or other specific dusts according to the specific requirements of the user. If a condensation counter is used in the test, a monodisperse phase test dust source with a known particle size must be used. The main measuring instrument is a high flow laser particle counter or a condensation nucleus counter (CNC). The entire air outlet surface of the filter is scanned and examined with the counter, which gives the number of dusts at each point and also allows the local efficiency of each point to be compared. European experience shows that for HEPA filters, the most easily penetrated dust particle size is at a point between 0.1µm and 0.25µm. The test conditions are first determined for the most easily penetrated dust particle size, and then the filter is continuously scanned to measure the filter effect on that particle size dust, a method the Europeans call the MPPS method [14]. The American standard specifies that only particles in the 0.1 to 0.2 µm range are measured. the MPPS method is in fact also a particle counting method, as the detection instrument used is a particle counter or condensation nucleus particle counter. The relevant standards for this method are: European EN1882-1998~2000 and American IES-RP-CC007.1-1992. |
