On 5/5/2018 2:24 PM, VanguardLH wrote:
Mark Perkins wrote:

VanguardLH wrote:

The heads need to fly over the platters. That cannot happen if there is
no gas inside the drive. Vacuum drives would have the heads sliding
across the surface of the platters, like how floppies worked. Heads fly
off the platter to eliminate wear.

Magnetic levitation? ;-)

(Easy for me to say since I'm not the one who has to develop the
technology.)

Not sure the heads would be close enough. Flying height with gas is
just a few nanometers. The drives are not sealed but a cleaned air
mixture is used inside and a sinter filter used to keep out particles
from outside air that might enter the drive. At high altitudes,
pressure drops outside (so air leaves the hard drive), so the air inside
can become too thin to support the minimal flying height.

http://www.dell.com/support/article/...high-altitudes
http://knowledge.seagate.com/article...language=ja_JA

There's probably some physics involved where if the gas molecules are
too few for flying height that there is too much resistance or
turbulence from molecular gas collisions. I suspect they won't ever
approach the atomic radius of molecular oxygen (O2). Atomic size of
nascent oxygen is 60 pm but it immediately bonds to an O2 molecule which
is ~120 pm. Nitrogen's atomic size is 65 pm. Clean room air gets into
the hard disk during manufacture. It is an air mixture. They could use
pure oxygen but you already know the explosive potential of pure oxygen,
plus pure oxygen is toxic. Maybe they went to helium (140 pm) because
the manufacture techs like talking in high-pitched voices.

Anything other than an air mixture means the techs assembling the hard
disk would have to wear breathing appartus, like a bunch of scuba divers
out of water. I was wondering why they didn't use nitrogen instead of
helium. It's because helium is an inert gas. For long-term storage and
to prevent corrosion, parts are stored in environments comprised of
inert gases: helium, neon, argon, krypton, or xenon. Nitrogen is not an
inert gas. However, it takes some extreme scenarios for nitrogen to be
corrosive. For example, to make ammonia requires directing nitrogen and
hydrogen over a hot iron plate. While not inert, dry nitrogen isn't
that active. Liquid nitrogen is used to super-cool high-density chips
(we built 3 mainframes on a 3" die which had to be in liquid nitrogen
when operating). So why not use dry nitrogen with its lower viscosity
and lower atomic size instead of helium? Nitrogen and oxygen are both a
poor conductor of heat. That's why air which is used as an thermal
insulator. See:

https://www.engineersedge.com/heat_t...vity-gases.htm

Helium conducts heat 8 times better than nitrogen or oxygen. Hydrogen
(which is reproducible) is also a good thermal conductor. Remember what
happened to the Hindenburg airship? When the limited supply of helium
gets more rare and its prices soar, we'll probably be seeing hydrogen-
filled hard disks (if rotating magnetic media is still used by then).

I doubt that hydrogen would ever be used for hard drive atmospheres.
Two phrases, "hydrogen wants to be free", and "hydrogen embrittlement"
come to mind.

BTW, there is still helium production in the USA. One of the Kansas
natural gas fields has a large portion of helium in it. They separate
it from the natural gas and sell it. I also suspect that helium could
be recovered from air should we want it bad enough. It would be a tough
separation though. According to wikipedia, helium is 5.2 ppm in the air.
https://en.wikipedia.org/wiki/Helium...and_production

Lynn