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All membranes are subjected to some level of visual inspection. In some cases the visual inspection is done online, using cameras that capture the transmission of light through the structure. Finding a bright spot above some threshold indicates a pinhole in the membrane or, in the case of wovens, areas where the fibers are so dispersed that the material will not be effective as a filter. Defective areas can be marked as the material is produced and culled out of finished product at a later time.
There are additional types of visual inspections that can be done off-line. Sections of the membrane are placed on a light box or under a lamp and inspected for other defects that can compromise filter performance. Although the inspection is subjective, there is a variety of visual defects that indicate areas where the filter will not function properly. Typically, visual defects are related to problems with the casting process. For example, solvent spots are areas in a membrane where the polymer did not precipitate properly. By inspecting for and removing areas with visual defect, the uniformity of structure is assured.
Depending on the filter type, the thickness of membrane filters ranges from 10 to 170 µm. Conversely, depth filters can range between 100 and 500 µm in thickness.
Thickness of membranes is measured using gauges with a resolution of 1 to 2 microns. Care is taken to ensure that the membrane is not crushed during the measurement.