What is meant by the lower and upper passage width of a protective filter?

In accordance with german DIN 13443-1, a lower and an upper passage width were specified. This is not measured optically, but is determined under test conditions.

The lower passage width is defined as the condition in which at least 90 % of a substance with a grain size of 80 to 120 μm (0.08 – 0.12 mm) must pass through the filter screen.

The upper passage width is defined as the condition in which a material with a grain size of 100 to 150 μm (0.1 – 0.15 mm) is passed through the filter screen to a maximum of 10 %.