HDD Air Filter
During the performance of the HDD, the speed of rotation of a spindle is very high (in modern models it is up to 15000 revolutions per minute), and the distance between a surface of a disk and a magnetic head varies from units to several parts of a micron. The penetration of dust between a head and a disk can cause operation failure, and, most likely, the hard drive failure; therefore, the HDD case is made almost hermetic. To equalize the pressure using a barometric filter connecting the internal volume of the sealed zone to the atmosphere. It is designed for cleaning the atmospheric air from dust and aggressive substances and consists of the mechanical filter intercepting dust, chemical absorbent (the activated coal) absorbing gasses, and the dehydrator (silica gel) absorbing moisture, and sometimes a selective polymeric membrane. The barometric filter is capable of intercepting particles sized more than 0,3 microns that meet the standards of cleanliness of atmosphere in a storage device. As hard drives are not completely hermetic devices, manufacturers specify above sea level altitude range in which they can save workability (usually from -300 to +3000 meters) for them as the height of flight of a magnetic head depends on air rarefaction.
During the operation of the HDD the air stream circulating at a great speed due to the fast rotation of magnetic disks gets to the particular filter trap, which is designed for cleaning the internal "atmosphere" from small particles of a drive working layer; despite all the taken measures, those particles, nevertheless, are crumbled from disks at flying ups and landing of magnetic heads. The filter trap also intercepts any other small particles, which can get in the hermetic block.