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kazinator

Interesting: found this table:

http://www.engineeringtoolbox.com/thermal-conductivity-d_429...

Big difference between different gases.

But, check this out. Helium also has a high heat capacity. Table on same website:

http://www.engineeringtoolbox.com/specific-heat-capacity-gas...

Six times the conductivity of nitrogen (0.142 to 0.024) and five times the heat capacity: 5.19 to 1.04.

Big conductivity and capacity should translate to "convective cooling monster gas". :)

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This is misleading. Helium has a higher conductivity and heat capacity than nitrogen per unit mass, but since helium has such low density, the heat capacity and conductivity per unit volume at atmospheric pressure is less than nitrogen.

While it's true that helium will have a lower practical heat capacity due to lower density, it still has far higher thermal conductivity than air at the same pressure.

http://www.engineersedge.com/heat_transfer/thermal-conductiv...

You're right, my mistake.

Why does this chart show nitrogen < helium while the other is opposite?

That's relevant to the specific heat table, but not the conductivity table, no?

Is it at atmospheric pressure though?

Why would the platters be generating heat? By definition there's no physical concact with the heads, ergo minimal friction. I thought the primary reason for using a monatomic gas was it would allow the heads to fly closer to the metal.

Gases have viscosity...

There is that factor, but how much heat do the heads generate, that's what I'm asking.

The servo and associated electronics surely generate many orders of magintude more heat.

Air shear can generate a significant amount of heat. Reducing drag will also reduce the power requirements for the motor and voice coil. HGST claim a 23% reduction in operating power over air-filled drives.

http://www.hgst.com/company/media-room/press-releases/HGST-H...

As I understand it, the crucial factor motivating helium-filled drives is the reduction of vibration due to internal turbulence. Reducing turbulence inside the drives allows platters and heads to be made thinner and packed more closely together without affecting reliability.

Right, so it's the nature of the gas in terms of its viscosity and turbulence that's the factor here, not in terms of friction per-se.

About ~3/4 of the power a drive uses goes into the spindle motor. Only a relatively small part of that is dissipated in the bearings (a quarter or so), all the rest is gas-platter friction, turbulences due to the heads etc.

Fun-fact aside: The head fly height in helium drives is ~1 nm.

Related trivia: Technical SCUBA divers use a helium-oxygen breathing mixture for deep dives. In cold waters, most will carry a small cylinder of air or argon for inflating their drysuit, because a heliox-filled drysuit has significantly less insulative value.

One time the gas supplier accidentally gave us helium in the argon supply cylinder and so my dive buddy ended up with a drysuit inflation tank full of helium. He couldn't figure out why he was freezing his ass off during the dive until he returned to the dive shop and checked that tank with a helium analyzer.

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