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Quality testing of filter papers

How can we test the quality of filter paper? You will find an explanation of how the bubble test works and what the differential pressure is in this article.

The filter paper is subjected to strict quality controls. One of the most important test procedures is the so-called bubble test. Put simply, this test consists of soaking the test paper with a precisely defined liquid and subjecting it to different test pressures. 

The first air bubble can be mathematically assigned to the largest pore. When the test piece is completely covered with air bubbles, this indicates the average distribution of pore sizes. This is because: "Large pores require low pressure loads, small pores require high pressure loads." This method is also used to determine the differential pressure. 

The test is relatively simple to execute, but its results are still very exact. It is important to remember, however, that the test only provides comparative values with other papers. In addition, separation tests with test particles are carried out (direct method according to DIN ISO 5011). The following illustration shows the test set-up for the determination of the filtration efficiency and the dust capacity of filter elements. 

The correct filter paper for each application can be defined precisely from the results.


1 Inlet pipe
2 Dust injector
3 Housing for dust addition
4 Outlet pipe
5 Pressure meter
6 Absolute filter
7 Volume flow meter
8 Volumetric flow control device
9 Suction blower
10 Filter