Hardware Reference
In-Depth Information
the drive and are 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 per-
manently sealed and do not circulate outside air, they can run in extremely dirty environ-
ments (see
Figure 9.14
)
.
Figure 9.14
Air circulation in a hard disk.
The HDA in an HDD is sealed but not airtight. The HDA is vented through a barometric
orbreatherfilterelementthatenablespressureequalization(breathing)betweentheinside
andtheoutsideofthedrive.Forthisreason,mostharddrivesareratedbythedrive'sman-
ufacturer 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, con-
tamination 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 specifica-
tionsforcleanliness insidethedrive.Youcanseetheventholesonmostdrives,whichare
