Hardware Reference
In-Depth Information
designed never to be changed for the life of the drive, unlike many older mainframe hard disks that
had changeable filters.
A hard disk on a PC system does not circulate air from inside to outside the HDA or vice versa. The
recirculating filter permanently installed inside the HDA is designed to filter only the small particles
scraped off the platters during head takeoffs and landings (and possibly any other small particles
dislodged inside the drive). Because PC HDDs are permanently sealed and do not circulate outside
air, they can run in extremely dirty environments (see Figure 9.15 ).
Figure 9.15. Air circulation in a hard disk.
The HDA in an HDD is sealed but not airtight. The HDA is vented through a barometric or breather
filter element that enables pressure equalization (breathing) between the inside and the outside of the
drive. For this reason, most hard drives are rated by the drive's manufacturer to run in a specific
range of altitudes, usually from 1,000 feet below to 10,000 feet above sea level. In fact, some hard
drives are not rated to exceed 7,000 feet while operating because the air pressure would be too low
inside the drive to float the heads properly. As the environmental air pressure changes, air bleeds into
or out of the drive so internal and external pressures are identical. Although air does bleed through a
vent, contamination usually is not a concern because the barometric filter on this vent is designed to
filter out all particles larger than 0.3 microns (about 12 μ-inches) to meet the specifications for
cleanliness inside the drive. You can see the vent holes on most drives, which are covered internally
by this breather filter. Some drives use even finer grade filter elements to keep out even smaller
particles.
I conducted a seminar in Hawaii several years ago, and several of the students were from one of the
astronomical observatories atop Mauna Kea. They indicated that virtually all the HDDs they had tried
to use at the observatory site had failed quickly, if they worked at all. This was no surprise because
the observatories are at the 13,796-foot peak of the mountain, and at that altitude, even people don't
 
 
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