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Comment by jarvist | Hacker News Reader

You'd certainly expect the long-term failure for Helium filled to be lower. By displacing (moist) air, you've completely stopped any metal corrosion. Helium is also highly conductive to heat, so you will even out any hot spots.

Does anyone know to what sort of pressure helium the drives are filled?

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My thought when first hearing about them was that, having an extra point of failure (Helium leakage), they would have higher long-term failure rates.

If that leakage only brings the probability of failure back to previous rates, then you've still lowered the failure rate overall.

No, that's not what would happen. Helium drives enable higher data density, because helium decreases the amount of drag, which in turn reduces turbulence, platter vibration, and heat. This enables more more platters per drive and more accurate head positioning.

Helium leakage would make the drive substantially less reliable than air drives due to the higher data density.

But they're not comparing denser helium-filled drives to less-dense air-filled drives. They're comparing 12TB helium drives to 12TB air drives—equal density. (I have no idea how they're managing to make non-helium drives of that density, but apparently they can.)

Of course, for a drive density that can only be achieved using helium, a helium leak will kill the drive.

But right now, it seems that these drives might not actually need the helium, since equivalent drives exist without it.

FWIW a quick search suggests the 8TB drives have the same RPM too, so it's not like the helium-drives are running faster just because they can. (Helium-filled HUH728080ALE600 versus air-filled ST8000DM002.)

Technically, you could choose a 3.5 inch, 6 Gb/s, 4 Tb model that contains He and then compare to the non-He version.

I don't know for a fact about the pressure, but I assume it's at atmospheric pressure, to make sealing and such less of an issue than making the HDD a pressure vessel.

Helium gas being conductive? Usually fluid cooling works by convection, isn't conduction only for solids?

Fluid dynamicist here.

"Convective" heat transfer includes heat transfer by both advection (the bulk fluid movement) and conduction. You might think convection is just advection, and I will admit the terminology is used inconsistently.

Erm, I'm a lapsed physics graduate.

Thanks for the correction, I was also watching PBS Space Time on Red Dwarfs, and how small stars cool the core by convection. Larger stars use radiation in the core and so lose access to a majority of the mass of the star which could have been used for fuel.

Is the thermal conductivity of helium actually relevant to hard drives? Like what kind of hot spots and transfer rates are we talking about?

FWIW utility generators are cooled with hydrogen to reduce windage losses and promote cooling. Hydrogen is probably lower viscosity and higher thermal conductivity than helium. It's also very hard to contain due to the small molecular size and has the widest flammability range of any gas with which I'm familiar, making it an interesting gas to work with. ;)

The utility generators probably don't worry too much about leaks, since they can just refill with new, abundant hydrogen, too.

It would be amusing to have a hydrogen-cooled hard drive for which you'd have to regularly refill the water tank so that it could generate more hydrogen for itself. :)

Yeah, and it would have a little Bunsen burner to safely get rid of waste hydrogen!

Yes, it's the hydrogen that leaks out that's the problem. I was told that the way to find hydrogen leaks in plant piping was to walk the line holding a corn broom over the line. When it bursts into flame you found the leak. In an enclosed space it can form an explosive mix with air if it doesn't ignite before it accumulates.

Isn't there also a potential problem with hydrogen embrittlement (depending on what materials are exposed) that you avoid with helium?

I don't know, but it seems possible. The only references I recall about hydrogen embrittlement were with steam boilers at high temperature and pressure. I suppose like most engineering solutions it comes down to the various tradeoffs between cost and effectiveness for hydrogen vs. helium.

He has a conductivity coefficent K (W/mK) = .151 and air = .026. Since everything else in heat flux eq is linear, approximately 5x rate of heat movement per degree that the HD is heated beyond surrounding air is expected.

Yeah, I can read a table of figures too, but what does that mean in the context of a hard drive? What percentage of the heat generated by the write heads (or whatever) is conducted away via air?

That is a function of the design implemented. Since you can read a table of figures you can probably also plug them in the heat flux equation in with your own guesses at air gap distances and surface area. The relevant info is that 5 times the energy per degree difference are conducted, that is why any hot spots from friction are smoothed.

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