Knowledge!Five common methods for efficient filter leakage detection

Efficient filter leakage detection, as one of the inspection projects in cleanrooms, has been increasingly valued since the implementation of the 2010 version of GMP. It has become a mandatory inspection project for pharmaceutical companies, and more and more industries such as hospitals, electronics, food, and cosmetics are conducting efficient filter leakage detection.

The following paragraphs will provide a detailed introduction to the methods of efficient filter leakage detection.

 1. Sodium Flame Test:
The sodium flame test uses a test dust source composed of polydisperse sodium chloride mist, and the “measurement” is the brightness of the hydrogen flame during the combustion of the mist.
The saltwater is splashed and dried under the agitation of compressed air, forming tiny salt crystal particles that enter the air duct and are sampled before and after the filter.
The sodium chloride mist in the air sample changes the color of the hydrogen flame to blue and increases its brightness. The brightness of the flame is used to determine the concentration of salt mist in the air, and to determine the filtering efficiency of the filter for salt mist.

The main detection instrument is a flame photometer. This method has low sensitivity and cannot detect super-efficient filters.

 2. Oil Mist Test:
The oil mist test uses oil mist as the test dust source, and the “measurement” is the turbidity of the air containing oil mist. The difference in turbidity between the air samples before and after the filter is used to determine the filtering efficiency of the filter for oil mist particles.
In Germany, it is specified to use paraffin oil with oil mist particle sizes ranging from 0.3 to 0.5 micrometers. The oil mist test is prone to damaging the filter during the detection process and cannot provide direct readings, resulting in time wastage.
3. DOP Test
This method was once a commonly used method for testing the leakage of efficient filters internationally. The test dust source is 0.3-micrometer monodisperse dioctyl phthalate (DOP) droplets, also known as “hot DOP,” and the “measurement” is the turbidity of the air containing DOP particles.
DOP liquid is heated to vaporize and then condensed into tiny droplets under specific conditions. Droplets larger or smaller than 0.3 micrometers are removed, and the remaining particles are sent into the air duct to measure the turbidity of the air samples before and after the filter. This is used to determine the filtering efficiency of the filter for 0.3-micrometer particles.
4. Fluorescent Particle Test
The test dust source for the fluorescent particle test is fluorescent sodium powder generated by a sprayer. The samples are collected in front of and behind the filter sponge. The sodium fluorescein on the sampling filter is dissolved in water, and the fluorescence brightness of the solution under specific conditions is measured. The brightness reflects the weight of the dust, from which the filtering efficiency of the filter can be calculated.
5. Particle Counting Test
This method is commonly used in Europe and has similarities with the ultra-efficient air filter testing method in the US. It is currently the mainstream sponge testing method internationally.
The test dust source is polydisperse droplets or solid dust particles of a determined size. Sometimes, filter manufacturers are required to use atmospheric dust or other specific dust according to user requirements.
If a condensation nucleus counter is used in the test, a monodisperse 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.
The counter scans the entire outflow surface of the filter for inspection and provides the number of particles at each point. Local efficiency comparisons can also be made.

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