Al Dykes said:
The altitude limit is common for lots of sophisticated electronic
equipment. Circulating air's ability to remove heat is a function of
density (ie altitude.)
Heat has to be disapated from the spinning disk. ISTR that at 10K rpm,
for instance, essentially all the wattage that goes into the motor
gets turned into heat via aerodynamic drag on the surface of the disk.
I've also heard that, for some disks, a certain amount of air density
is necessary to float the flying heads.
There is essentially zero airflow thru the filter. It just keeps the
pressure equal to the ambient.
There is another reason for altitude restrictions, but they apply
for higher voltages such as those found in a switching power supply.
This has to do with arcing. When the air separating two electrical
potentials is dense, any ionization caused by cosmic rays will
quickly be quenched before the ionized gas particles can accelerate
to a speed (i.e. before they can attain a kinetic energy high enough)
to cause more ionization in an effect known as "avalanche discharge".
When the air gets rarified, though, this quenching effect diminishes,
and at some point of rarification, it is not enough to prevent
avalanching, and the electrical circuitry is effectively short-circuited.
This avalanching and hermetic seals to prevent it (along with cooling
requirements) are important engineering problems for designers of
electronic gear used in high-altitude airplanes and spacecraft. In the
case of a hard disk drive, there is the added requirement of air density
high enough to sustain separation of the flying read/write head above
the surface of the spinning platter. Which requirement - cooling or
head levitation - is really responsible for the 10,000 ft. requirement
may have been lost in the translation between Maxtor's engineering
staff and its technical support supervisor. The website for the
Keck observatory on Hawaii's 13,800 ft. high Mauna Kea does
mention that air pressure there is only 60% of that at sea level. An
interesting statement is this:
"A four-wheel-drive vehicle is required beyond the 9,200 foot
level as the air is too thin to adequately cool a vehicle's brakes
upon descent."
*TimDaniels*
