Test method for determining the filtration efficiency in general (retention rate, filtering grade). This involves passing a specific mineral oil contaminated with quartz sand (Air Cleaner Fine Test Dust (ACFTD)) of various grain sizes through the filter to be tested several times. The measure of the retention rate is the β (Beta-) value, which shows the relationship between the number of particles of a specific size in front of the filter and those of the same size behind the filter.
The standard for the multipass test is set out under ISO 4572.