<story><title>6TB helium-filled hard drives bump capacity, decrease power use</title><url>http://www.computerworld.com/s/article/9243751/6TB_helium_filled_hard_drives_take_flight_bump_capacity_50_</url></story><parent_chain><item><author>todayiamme</author><text>I would like to understand why they are using helium at all instead of a several other plausible alternatives. The article offers the following rationale for using helium in this capacity;&lt;p&gt;&amp;gt;&amp;gt;&amp;gt;At one-seventh the density of air, helium produces less drag on the moving components of a drive - the spinning disk platters and actuator arms -- which translates into less friction and lower operating temperatures.&amp;lt;&amp;lt;&amp;lt;&lt;p&gt;This appears to be a slight misunderstanding as the density of any gas is variable and we have found that it is directly proportional to the unit weight of the substance, the pressure and inversely proportional to temperature. It is not fixed. They could have easily achieved the result in multiple ways. For instance, to reduce density it might have been easier to simply reduce pressure and then create a hermetically sealed chamber with a reasonably inert gas with low atomic or molecular mass such as nitrogen. This should have saved them a lot of trouble as helium is actually pretty hard to contain. An alternative nitrogen based solution would also have been good enough for a long enough period of time and it could have been achieved in a much less expensive manner. (they say in the article that successfully trapping helium for a long enough period of time required a decade of research. This is a relatively solved problem for gases such as nitrogen)&lt;p&gt;I am sure that the people at Western Digital can see something that I don&amp;#x27;t - I&amp;#x27;m just curious as to what that insight is. What makes helium worth all of the trouble?</text></item></parent_chain><comment><author>Lagged2Death</author><text>&lt;i&gt;... to reduce density it might have been easier to simply reduce pressure and then create a hermetically sealed chamber...&lt;/i&gt;&lt;p&gt;Disk heads &amp;quot;fly&amp;quot; above the surface of the disk on a cushion of gas that&amp;#x27;s pulled under the head by the spinning disk. If there isn&amp;#x27;t enough gas pressure inside the disk enclosure, this method of locating the heads won&amp;#x27;t work. Wikipedia actually says:&lt;p&gt;&lt;i&gt;If the air density is too low, then there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation, above about 3,000 m (9,800 ft).&lt;/i&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Hard_disk_drive#Integrity_and_failure&quot; rel=&quot;nofollow&quot;&gt;http:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;Hard_disk_drive#Integrity_and_f...&lt;/a&gt;&lt;p&gt;Atmospheric pressure on the top and bottom faces of a 3.5&amp;quot; drive case (~22 square inches) is in the neighborhood of 300 pounds each, and there&amp;#x27;s still more on the front, back, and sides. A square box of thin aluminum is probably not a great starting point for building a pressure vessel. If the box is filled with a gas at close to atmospheric pressure, you don&amp;#x27;t have to worry about it.&lt;p&gt;Helium&amp;#x27;s specific heat is five times that of air, so it probably offers better heat conduction than air at 1&amp;#x2F;7th atmospheric pressure would.</text></comment>
<story><title>6TB helium-filled hard drives bump capacity, decrease power use</title><url>http://www.computerworld.com/s/article/9243751/6TB_helium_filled_hard_drives_take_flight_bump_capacity_50_</url></story><parent_chain><item><author>todayiamme</author><text>I would like to understand why they are using helium at all instead of a several other plausible alternatives. The article offers the following rationale for using helium in this capacity;&lt;p&gt;&amp;gt;&amp;gt;&amp;gt;At one-seventh the density of air, helium produces less drag on the moving components of a drive - the spinning disk platters and actuator arms -- which translates into less friction and lower operating temperatures.&amp;lt;&amp;lt;&amp;lt;&lt;p&gt;This appears to be a slight misunderstanding as the density of any gas is variable and we have found that it is directly proportional to the unit weight of the substance, the pressure and inversely proportional to temperature. It is not fixed. They could have easily achieved the result in multiple ways. For instance, to reduce density it might have been easier to simply reduce pressure and then create a hermetically sealed chamber with a reasonably inert gas with low atomic or molecular mass such as nitrogen. This should have saved them a lot of trouble as helium is actually pretty hard to contain. An alternative nitrogen based solution would also have been good enough for a long enough period of time and it could have been achieved in a much less expensive manner. (they say in the article that successfully trapping helium for a long enough period of time required a decade of research. This is a relatively solved problem for gases such as nitrogen)&lt;p&gt;I am sure that the people at Western Digital can see something that I don&amp;#x27;t - I&amp;#x27;m just curious as to what that insight is. What makes helium worth all of the trouble?</text></item></parent_chain><comment><author>tryp</author><text>The density of helium is less than 1&amp;#x2F;7 of the density of nitrogen, which is a good approximation for air.&lt;p&gt;Two ways that this is helpful come to mind:&lt;p&gt;1. Aerodynamic effects like drag and vorticity are largely governed by the Reynolds Number, which holding all else constant, varies linearly with density. Changing the density of the fluid can drastically change the aerodynamic situation. See 
&lt;a href=&quot;http://www.grc.nasa.gov/WWW/k-12/airplane/dragsphere.html&quot; rel=&quot;nofollow&quot;&gt;http:&amp;#x2F;&amp;#x2F;www.grc.nasa.gov&amp;#x2F;WWW&amp;#x2F;k-12&amp;#x2F;airplane&amp;#x2F;dragsphere.html&lt;/a&gt; for an interesting and accessible example.&lt;p&gt;2. &amp;quot;Flow-induced vibration&amp;quot; is caused by the swirling gases crashing like stormy seas, making platters vibrate and heads flutter. Switching to helium cuts the momentum in these turbulent flows to a seventh of what they would be with air.</text></comment>