EEVblog® Electronics Community Forum
General => General Technical Chat => Topic started by: matseng on December 03, 2015, 08:48:07 am
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I was under the impression that helium tend to leak out of most things quite easily. How can they have a MTBF of 2.5M hours if the helium is a integral part of the disk?
https://www.hgst.com/products/hard-drives/ultrastar-he10 (https://www.hgst.com/products/hard-drives/ultrastar-he10)
Ultrastar He10
10TB & 8TB
3.5-inch Helium Platform
Enterprise Hard Drive
- Highest capacity – 10TB HDD, drop-in ready for mainstream enterprise
- Lowest Power – 56% more efficient than air-filled drives for lower TCO
- Most Reliable – Best-in-class reliability rating of 2.5M hours MTBF
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You need clean, solid seals to do it, but it's not by any means impossible.
Trivia: helium is used for testing vacuum leaks. A mass spectrometer(!) is connected to the vacuum system, tuned to the helium signature. A helium wand (just a tank leaking gas out of a tube..) is waved around the vacuum chamber. When a signal is generated (helium concentration rises suddenly, then falls slowly), you know there's a leak around that location.
So if the vacuum system is rejecting helium from the outside very well, it will retain helium inside very well by the same fact. You just need atomically good sealing, that's all. :)
Tim
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As far as I know, MTBF means nothing. They might as well make up the numbers, if they don't do already.
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Do these drives cost less to ship?
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I thought that helium like hydrogen will percolate through materials including glass and steel only a bit slower than hydrogen.
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As far as I know, MTBF means nothing. They might as well make up the numbers, if they don't do already.
You are correct. In general, MTBF is complete frogshit for an entity like a hard disk.
What is important is MTTF.
http://www.weibull.com/hotwire/issue94/relbasics94.htm (http://www.weibull.com/hotwire/issue94/relbasics94.htm)
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As far as I know, MTBF means nothing. They might as well make up the numbers, if they don't do already.
MTTFd is the term normally used, Mean time to dangerous failure. This is usually calculated from the B10d specification of a particular device or component. The B10d is the number of cycles until a component fails dangerously and should be quoted by the manufacturer of the component or device. They get this mythical B10d number by testing things until they fail (usually on the order of 20 million cycles). It's a statistical thing.
Just a ballpark guide to the reliability of a particular device or component. :)
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As far as I know, helium will go through stainless steel unless the material is absolutely defect free (and I mean absolutely). Otherwise it can diffuse out through the grain boundaries, etc.
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HGST launched (http://www.bit-tech.net/news/hardware/2013/11/07/hgst-he6/1) their first helium filled hard drive a little over two years ago, and there don't seem to be any reports of leakage from those.
Apparently (http://www.news.cornell.edu/stories/2008/09/cu-physicists-create-worlds-thinnest-balloon) a layer of graphene a single atom thick is completely impermeable to helium, so it's not impossible.
Do these drives cost less to ship?
:)
Actually, they might. The link above reports that HGST's 6TB, seven-platter helium filled drive was 50g lighter than their 4TB, five platter air filled model.
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So what effects would a user notice if the helium started to leak out?
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So what effects would a user notice if the helium started to leak out?
Everyone in the data centre suddenly sounding like Mickey Mouse, maybe ;D
Sorry, I'll get my coat :)
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So what effects would a user notice if the helium started to leak out?
Overheating - AFAIUI one reason the helium is used because of its better thermal conductivity. I suspect it will also allow the heads to fly closer to the disk surface as well.
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So what effects would a user notice if the helium started to leak out?
Overheating - AFAIUI one reason the helium is used because of its better thermal conductivity. I suspect it will also allow the heads to fly closer to the disk surface as well.
less friction. platters moving through air cause massive drag. in helium less.
don't forget this monster has 7 (SEVEN) platters and 14 heads )
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First I thought that a (partial) vacuum would be both cheaper and more leakage-proof to make and reduce the air-drag/friction a lot, but then I realized that the heat transfer would suffer a lot... So there goes that good idea down the drain ;-)
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First I thought that a (partial) vacuum would be both cheaper and more leakage-proof to make and reduce the air-drag/friction a lot, but then I realized that the heat transfer would suffer a lot... So there goes that good idea down the drain ;-)
That and I think it's the air (or whatever gas is in there) that 'floats' the heads above the surface of the disk as it spins. I'd expect no atmosphere would be bad for that as well.
-Pat
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First I thought that a (partial) vacuum would be both cheaper and more leakage-proof to make and reduce the air-drag/friction a lot, but then I realized that the heat transfer would suffer a lot... So there goes that good idea down the drain ;-)
That and I think it's the air (or whatever gas is in there) that 'floats' the heads above the surface of the disk as it spins. I'd expect no atmosphere would be bad for that as well.
-Pat
That is a *very* valid point. It seems like I can't brain so good at midnight anymore - too old....|O
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As far as I know, MTBF means nothing. They might as well make up the numbers, if they don't do already.
You are correct. In general, MTBF is complete frogshit for an entity like a hard disk.
What is important is MTTF.
http://www.weibull.com/hotwire/issue94/relbasics94.htm (http://www.weibull.com/hotwire/issue94/relbasics94.htm)
MTBF for hard disk is equivalent to MTTF since a hard disk is not repairable. You could more accurately use MTBF on the computer system that the hard disk is installed in, but that is another discussion. In any case, MTBF is not a nonsense term but you need to understand what it means. It is the mean time between failures assuming that all disks are replaced at EOL (essentially end of warranty). MTBF of 2.5M hours does not mean that the disk is expected to operate for 2.5 million hours (285 years) before failing. It means more something like if you have 1000 such disks running, then you will see one failure per 2500 hours, or 104 days. After the lifetime of the disks is used up (~5 years), you need to replace them all, otherwise the failure rate will be expected to shoot up. Really, the industry should use the term MTBF_SR (MTBF with schedule replacements). The internet archive seems to keep good records their hard disk failures and publishes lots of data about them for those interested.
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First I thought that a (partial) vacuum would be both cheaper and more leakage-proof to make and reduce the air-drag/friction a lot, but then I realized that the heat transfer would suffer a lot... So there goes that good idea down the drain ;-)
That and I think it's the air (or whatever gas is in there) that 'floats' the heads above the surface of the disk as it spins. I'd expect no atmosphere would be bad for that as well.
-Pat
That is a *very* valid point. It seems like I can't brain so good at midnight anymore - too old....|O
:-DD I can't brain today - I have the dumb!
Seriously, as someone who's worked with vacuum systems in the past, evacuating the enclosure to reduce friction was my first thought, too. Then I remembered reading years ago that the head floats on a miniscule cushion of air that's dragged under it by the spinning platter, and that sank that idea right off the bat. I never thought of the heat transfer part, but it's very valid too. TANSTAAFL.
-Pat
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Guys, look at the price!!!
It is way more expensive per GB than 3TB or 4TB drives.
And, a simple mechanical failure kills all 10TB data.
BTW, MTBF means nothing. I have never had any HDD survived to their MTBF. They usually die in 2 years at 24/7 operation, which translates to 17.5k hrs, only 1.46% of their rated 1.2M hrs MTBF.
Because MTBF != expected lifetime before failure. It means only failure percentage during some predefined time.
(https://upload.wikimedia.org/math/d/a/b/dabf60c33e4ef9226fccb9604b7810b3.png)
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BTW, MTBF means nothing. I have never had any HDD survived to their MTBF. They usually die in 2 years at 24/7 operation, which translates to 17.5k hrs, only 1.46% of their rated 1.2M hrs MTBF.
Sounds like you need to take a hard look at your environment. I have had drives fail within 2 years, but certainly not all of them. Methinks you're abusing the hell out of your drives without knowing it. What kind of temperatures are they reporting? What is the vibration/shock environment like?
Everyone I talk to who complains about their mechanical drives consistently failing after 1-2 years is always either using a laptop (laptops should never use mechanical drives, it's just begging for failures) or they're using some horrible sealed enclosure that's cooking the drives at 55+ C 24/7.
If you keep a mechanical HDD below 35C 99% of the time and always below 45C, in a vibration/shock-free installation, it should last for many, many years. I have about 150 mechanical HDDs operating under my care, and I typically lose about 1-2 a year. That's a MTBF of about 1M hours, which falls roughly in line with the advertised values. Most of the drives are over 6 years old and working just fine. The oldest are over 10 years old.
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G
BTW, MTBF means nothing. I have never had any HDD survived to their MTBF. They usually die in 2 years at 24/7 operation, which translates to 17.5k hrs, only 1.46% of their rated 1.2M hrs MTBF.
Suicidaleggrol beat me to it, just as I was replying to same quote .
Such a high failure rate, maybe unlucky. You have me worried now . I was considering a project where I need the HDD to be in constant use 24/7 writing multi TB a day and one of the things I was counting on at least a ~5 years lifetime (50% failure rate) to keep costs down.
I know manufacturers MTBF figures are not quoted for 24/7 continuous use, but I wouldn't have though that continuous use is going to be so much more stressful on HDD as to cause such a large discrepancy to manufacturers MTBF. need to research this further.
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The hard drives have an internal structure similar to traditional hard drives. with the body of the drive being cast aluminum. Cast aluminum is considered porous with Helium. Then two steel sheet metal casing pieces are placed around the drive and laser welded at the lip of the two pieces, electrical connections use an hermetic epoxy. Steel is considered non-porous over the life of the drive.
https://www.google.com/patents/US8279552?dq=seagate+helium+hard+drive&hl=en&sa=X&ved=0ahUKEwj4zovPicHJAhWC9R4KHZrcA_YQ6AEIHTAA (https://www.google.com/patents/US8279552?dq=seagate+helium+hard+drive&hl=en&sa=X&ved=0ahUKEwj4zovPicHJAhWC9R4KHZrcA_YQ6AEIHTAA)
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As far as I know, helium will go through stainless steel unless the material is absolutely defect free (and I mean absolutely). Otherwise it can diffuse out through the grain boundaries, etc.
Have you factored in the pressure inside the hard drive? How many defects could be reasonably expected? Calculated the diffusion rate? Calculated the resulting amount of time that it would take for the helium to diffuse out in a reasonable amount of time?
If the Helium is diffusing out, is that really a problem? Isn't it more the absence of oxygen/other stuff inside the HDD that is the goal here? If oxygen can't diffuse back into the HDD, what's the problem?
But no, you read once that helium at some unknown pressure will leak at some unknown rate through a grain boundary. I was going to make an LED blinky circuit, but then I remembered that leakage can occur on PCB surfaces. I probably shouldn't calculate whether those leakages will be significant right? I should just give up on the idea?
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LED blinkies work at voltages < 5 V, so unless your negative and positive contacts are within 20nm of each other, you should be fine with a normal PCB surface.
Yes, it was a silly example on my part on how failing to factor in quantities can lead to absurd leaps of logic. E.g.:
- "Current can leak over the surface of a PCB" --> "All PCB electronics is infeasible" (obviously ridiculous)
- "Helium can diffuse through stainless steel" --> "Helium-filled hard drives are infeasible" (implied by several people on this thread)
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My cryocooler uses helium as its working fluid and has been sealed for a decade with no loss of performance. It still happily liquifies air when i turn it on. And this is helium undergoing constant over and under pressure waves. (it is a split-pair stirling engine configuration, the fun stuff you find on ebay.).
If it can be solved for that, it can be solved for a hard disk.
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As far as I know, MTBF means nothing. They might as well make up the numbers, if they don't do already.
You are correct. In general, MTBF is complete frogshit for an entity like a hard disk.
What is important is MTTF.
http://www.weibull.com/hotwire/issue94/relbasics94.htm (http://www.weibull.com/hotwire/issue94/relbasics94.htm)
MTBF for hard disk is equivalent to MTTF since a hard disk is not repairable. You could more accurately use MTBF on the computer system that the hard disk is installed in, but that is another discussion. In any case, MTBF is not a nonsense term but you need to understand what it means. It is the mean time between failures assuming that all disks are replaced at EOL (essentially end of warranty). MTBF of 2.5M hours does not mean that the disk is expected to operate for 2.5 million hours (285 years) before failing. It means more something like if you have 1000 such disks running, then you will see one failure per 2500 hours, or 104 days. After the lifetime of the disks is used up (~5 years), you need to replace them all, otherwise the failure rate will be expected to shoot up. Really, the industry should use the term MTBF_SR (MTBF with schedule replacements). The internet archive seems to keep good records their hard disk failures and publishes lots of data about them for those interested.
Than if I have 10 disks running, I will see one failure per 250.000 hours, or 28,5 years?
Bollocks...
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Than if I have 10 disks running, I will see one failure per 250.000 hours, or 28,5 years?
If you preemptively replace the disks every 5 years, keep them in a cool, shock-free data center, etc, etc -- then yes.
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Helium filled hard drives.... that's going to have a fast data transfer rate, it will make MP3s sound like Alvin and the chipmunks.
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In comparison, in the recent 5 years, I own and owned at least 14 SSDs, and none of them failed till the point I sold them. The oldest one I'm current using is 2 years old, with its temperature constantly at 50C (in NUC, its SSD bay is hotter than its HDD bay), not even a single SMART warning.
My point is HDD will eventually die out. During black friday, the cheapest 960G SSD goes down to $150-ish. $800/10T=$0.08/G, $150/1T=$0.15/G. Not too much cost advantage to me.
Most SSDs don't have temperature sensor and just report some fake temperature.
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Most SSDs don't have temperature sensor and just report some fake temperature.
All of mine do...Samsung, Intel, and Sandisk.
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As previously noted, the drive is a normal cast aluminum base with a laser welded sheet over it. The helium (actually a mixture of various gases that they won't reveal but mostly helium) is at a very slight positive pressure but the total amount is really small. The primary reason for the helium is to reduce turbulence when flying the heads over the platters. Reduced turbulence means the heads can safely fly closer to the platters which means fitting 7 slightly thinner platters in the same physical envelope.
Regarding the amount of helium, when HGST first started talking to my team at work about these drives over 2 years ago, I quickly pointed out that we crush drives on a near continuous basis as part of our data security program. They did a report for us to show that crushing 1000+ drives a day in a 20'x20'x10' room would be perfectly safe and no one would have high-pitched voices at the end of a shift.
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As far as I know, MTBF means nothing. They might as well make up the numbers, if they don't do already.
You are correct. In general, MTBF is complete frogshit for an entity like a hard disk.
What is important is MTTF.
http://www.weibull.com/hotwire/issue94/relbasics94.htm (http://www.weibull.com/hotwire/issue94/relbasics94.htm)
MTBF for hard disk is equivalent to MTTF since a hard disk is not repairable. You could more accurately use MTBF on the computer system that the hard disk is installed in, but that is another discussion. In any case, MTBF is not a nonsense term but you need to understand what it means. It is the mean time between failures assuming that all disks are replaced at EOL (essentially end of warranty). MTBF of 2.5M hours does not mean that the disk is expected to operate for 2.5 million hours (285 years) before failing. It means more something like if you have 1000 such disks running, then you will see one failure per 2500 hours, or 104 days. After the lifetime of the disks is used up (~5 years), you need to replace them all, otherwise the failure rate will be expected to shoot up. Really, the industry should use the term MTBF_SR (MTBF with schedule replacements). The internet archive seems to keep good records their hard disk failures and publishes lots of data about them for those interested.
Than if I have 10 disks running, I will see one failure per 250.000 hours, or 28,5 years?
Bollocks...
Absolutely, if you replace them when their warranty is up, regardless of whether they appear to be working fine. For cheap consumer disks, this means every 1 to 2 years. For expensive enterprise disks, once every 5 years (or so).
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If by 2.5M hours they mean 2.5 million, that's close to 300 years.
I didn't kno they had Hard disks in 1715,
Make you wonder. :-//
Or maybe they tested 300 disks for a year, I guess.
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If by 2.5M hours they mean 2.5 million, that's close to 300 years.
I didn't kno they had Hard disks in 1715,
Make you wonder. :-//
Or maybe they tested 300 disks for a year, I guess.
I heartily encourage you to read the rest of this thread :)
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Apparently (http://www.news.cornell.edu/stories/2008/09/cu-physicists-create-worlds-thinnest-balloon) a layer of graphene a single atom thick is completely impermeable to helium, so it's not impossible.
Would make a neat party balloon.
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As previously noted, the drive is a normal cast aluminum base with a laser welded sheet over it. The helium (actually a mixture of various gases that they won't reveal but mostly helium) is at a very slight positive pressure but the total amount is really small. The primary reason for the helium is to reduce turbulence when flying the heads over the platters. Reduced turbulence means the heads can safely fly closer to the platters which means fitting 7 slightly thinner platters in the same physical envelope.
Regarding the amount of helium, when HGST first started talking to my team at work about these drives over 2 years ago, I quickly pointed out that we crush drives on a near continuous basis as part of our data security program. They did a report for us to show that crushing 1000+ drives a day in a 20'x20'x10' room would be perfectly safe and no one would have high-pitched voices at the end of a shift.
[bold added to quote]
Interesting! I was guessing it was because helium is a noble gas and would not react with anything thereby eliminating oxidation and other degeneration induced by chemical reactions.
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Helium is usually chosen for thermal conductivity, e.g. in turbines (Brayton cycle?), bearings, etc.
I don't know that the fluid properties, such as turbulence, are significantly different (i.e., the flow regime at the surface of a disk spinning that fast, will still be largely turbulent). I doubt it's eliminated, but reduced is possible.
If chemical inertness were a priority, dry nitrogen, or at worst, argon (only needed for the most reactive metals, like welding titanium for example), would do the job.
Indeed, argon is used for this reason in incandescent lamps: reduced convection loss, compared to other gasses. Krypton is even better (heavier, slower), and xenon still more; but they are relatively expensive, so don't see widespread use, except where very small quantities are needed ("krypton" flashlight bulbs), or where the properties are directly required (xenon flash and HID lamps are excellent because xenon has a broad "white" response with few bright spectral lines; obviously, the properties of ionized xenon being somewhat more important than its gaseous properties!).
Tim
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HGST talks about this in their helium technology brochure (http://www.hgst.com/sites/default/files/resources/HGST-Helium-Technology-BR03.pdf (http://www.hgst.com/sites/default/files/resources/HGST-Helium-Technology-BR03.pdf)). Helium, being less dense than air, reduces turbulence and allows the platters to spin more easily at the same speeds. Someone asked about what happens if a drive leaks. There is no built-in sensor for pressure change or chemical detection. The drive just slows down and performs poorly if enough helium is lost.
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HGST talks about this in their helium technology brochure (http://www.hgst.com/sites/default/files/resources/HGST-Helium-Technology-BR03.pdf (http://www.hgst.com/sites/default/files/resources/HGST-Helium-Technology-BR03.pdf)). Helium, being less dense than air, reduces turbulence and allows the platters to spin more easily at the same speeds. Someone asked about what happens if a drive leaks. There is no built-in sensor for pressure change or chemical detection. The drive just slows down and performs poorly if enough helium is lost.
Surely the spindle is driven by a motor with feedback, would it not just push harder to attain the engineered speed? I'd be surprised if the change caused by a 10% infiltration of nitrogen would be significant compared to the lubrication being cold, or a tiny bit of grit in the bearings, etc. They wouldn't cut the design that fine.
They talk about allowing the heads to fly closer to the disk due to reduced turbulence, based on this my conclusion would be that there would be an increased risk of a head crash if a) the drive leaked and b) the driver had no way of detecting the leak and explicitly slowing down in response. Do you have a source to justify your claim that the platter will spin more slowly if there's a leak? Or are you referring to the arm swinging around more slowly?
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I'd be surprised if the change caused by a 10% infiltration of nitrogen would be significant compared to the lubrication being cold, or a tiny bit of grit in the bearings, etc. They wouldn't cut the design that fine.
If the enclosure is filled 100% pure helium and it is solid enough (aluminium?) to not let "regular" gasses diffuse through it but permeable enough to have the helium escape, what would then happen? Would the pressure inside just drop over time until all helium is gone and there's just a vacuum there? Or would an equilibrium point be reached after a while?
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I'd be surprised if the change caused by a 10% infiltration of nitrogen would be significant compared to the lubrication being cold, or a tiny bit of grit in the bearings, etc. They wouldn't cut the design that fine.
If the enclosure is filled 100% pure helium and it is solid enough (aluminium?) to not let "regular" gasses diffuse through it but permeable enough to have the helium escape, what would then happen? Would the pressure inside just drop over time until all helium is gone and there's just a vacuum there? Or would an equilibrium point be reached after a while?
Presumably it would leak until the vapour pressure of helium inside is equal to that outside.
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Presumably it would leak until the vapour pressure of helium inside is equal to that outside.
Which would correspond to an almost perfect vacuum -- I'm skeptical, I feel like I can maybe build a perpetual motion machine if this were true.
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Presumably it would leak until the vapour pressure of helium inside is equal to that outside.
Which would correspond to an almost perfect vacuum -- I'm skeptical, I feel like I can maybe build a perpetual motion machine if this were true.
Not possible for it to leak to almost perfect vacuum. Once the pressure inside is less than the outside world, air will leak in to fill the void.
Unless of course if the drive is operating in space in which case it could indeed leaks down to near vacuum...
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Not possible for it to leak to almost perfect vacuum. Once the pressure inside is less than the outside world, air will leak in to fill the void.
That doesn't sound right. You're saying that it's impossible to keep a vacuum (or near vacuum) inside a sealed canister? Lucky for us born before the advent of flat-TVs that the CRT tubes didn't know about that. :-D
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Not possible for it to leak to almost perfect vacuum. Once the pressure inside is less than the outside world, air will leak in to fill the void.
Unless of course if the drive is operating in space in which case it could indeed leaks down to near vacuum...
Right, but we're talking about a (hypothetical?) material here which allows Helium to diffuse through, but not N2 or O2. It seems weird to claim that the helium would leak out and leave a vacuum behind, I agree. But it also seems weird to assert that the high pressure of N2 and O2 on the outside would somehow prevent He from diffusing out, even though the N2 or O2 can't diffuse into the material in order to provide this preventative force?
What are you claiming will happen to a vessel constructed of a material permeable to Helium, but impermeable to everything else?
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HGST talks about this in their helium technology brochure (http://www.hgst.com/sites/default/files/resources/HGST-Helium-Technology-BR03.pdf (http://www.hgst.com/sites/default/files/resources/HGST-Helium-Technology-BR03.pdf)). Helium, being less dense than air, reduces turbulence and allows the platters to spin more easily at the same speeds. Someone asked about what happens if a drive leaks. There is no built-in sensor for pressure change or chemical detection. The drive just slows down and performs poorly if enough helium is lost.
Surely the spindle is driven by a motor with feedback, would it not just push harder to attain the engineered speed? I'd be surprised if the change caused by a 10% infiltration of nitrogen would be significant compared to the lubrication being cold, or a tiny bit of grit in the bearings, etc. They wouldn't cut the design that fine.
They talk about allowing the heads to fly closer to the disk due to reduced turbulence, based on this my conclusion would be that there would be an increased risk of a head crash if a) the drive leaked and b) the driver had no way of detecting the leak and explicitly slowing down in response. Do you have a source to justify your claim that the platter will spin more slowly if there's a leak? Or are you referring to the arm swinging around more slowly?
I wasn't clear. By using helium, the energy required to spin the platter at a constant speed is lower. If the helium leaks (and is replaced by air), the motor controller will compensate to keep a constant speed but will use more energy.
The other major effect of a helium leak is that the increased turbulence causes the drive to fly the heads higher to avoid impacts. This causes a higher rate of read errors which means longer read command times (drives internally retry up to 10+ times before returning failure to the host). The net effect is that data throughput decreases once enough helium has leaked out.
I can't point you to a source as this was discussed in person with the product manager and lead engineer of the helium project.
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Not possible for it to leak to almost perfect vacuum. Once the pressure inside is less than the outside world, air will leak in to fill the void.
That doesn't sound right. You're saying that it's impossible to keep a vacuum (or near vacuum) inside a sealed canister? Lucky for us born before the advent of flat-TVs that the CRT tubes didn't know about that. :-D
You misunderstood what I wrote. To better express what I mean, I have to use all the words however long the sentence: to leak air from inside that sealed space to the outside, leaking enough to a point that the inside is almost perfect vacuum. That is impossible. The inside cannot have lower air pressure than the outside and still be leaking air outward.
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The other major effect of a helium leak is that the increased turbulence causes the drive to fly the heads higher to avoid impacts.
I may be nitpicking on your precise choice of words here, but the drive doesn't fly the heads, nor does it have any control over the height of the heads, right? The head spacing is maintained entirely through the wing-on-ground effect, at least that was my understanding.
If you're saying that the increased density causes the head to simply be pushed further away, then yeah everything you say makes sense.
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The inside cannot have lower air pressure than the outside and still be leaking air outward.
There's still something weird going on here, as I expressed in my earlier question to you. Consider a 0.5 atmosphere Helium atmosphere, in a Helium-only-permeable vessel, sitting in normal 1 atmosphere air. So we've got a Helium atom, that's diffusing happily through the bulk of the vessel wall. It reaches the outside interface of the vessel wall, where it meets atmospheric air. Does the Helium get turned back; prevented from actually completely leaking out of the vessel by some unknown force that is omnipotently aware that the vessel from which it came is at a lower pressure than the air which it is about to enter? Or does it escape, leading to a situation which asymptotically leads to the vessel becoming more and more depressurised? I totally get that both possibilities I present here seem absurd, but I can't tell what kind of third story I can tell about any given Helium atom that makes any sense.
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As far as I understand it is the partial pressure of the individual gasses which matter. So, if the vessle is permiable to Helium, it will leak out until the pressure of helium inside is equal to the partial pressure of helium outside (not very much for atmostpheric helium).
Of course O2 and N2 mollecules are trying to leak in at the same time.
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The other major effect of a helium leak is that the increased turbulence causes the drive to fly the heads higher to avoid impacts.
I may be nitpicking on your precise choice of words here, but the drive doesn't fly the heads, nor does it have any control over the height of the heads, right? The head spacing is maintained entirely through the wing-on-ground effect, at least that was my understanding.
If you're saying that the increased density causes the head to simply be pushed further away, then yeah everything you say makes sense.
The drives do fly the heads. The heads are an airfoil shape that changes profile (and thus lift) as it rotates. There are microactuators from the arm to the head that rotate the head to adjust the fly height. Normally, the heads fly fairly high during idle to avoid impacts and lower for reads and writes. Writes happen much closer to the platter then reads to limit the bleed-over of the write to adjacent sectors.
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The inside cannot have lower air pressure than the outside and still be leaking air outward.
There's still something weird going on here, as I expressed in my earlier question to you. Consider a 0.5 atmosphere Helium atmosphere, in a Helium-only-permeable vessel, sitting in normal 1 atmosphere air. So we've got a Helium atom, that's diffusing happily through the bulk of the vessel wall. It reaches the outside interface of the vessel wall, where it meets atmospheric air. Does the Helium get turned back; prevented from actually completely leaking out of the vessel by some unknown force that is omnipotently aware that the vessel from which it came is at a lower pressure than the air which it is about to enter? Or does it escape, leading to a situation which asymptotically leads to the vessel becoming more and more depressurised? I totally get that both possibilities I present here seem absurd, but I can't tell what kind of third story I can tell about any given Helium atom that makes any sense.
RS20,
I agree with Grumpydoc's assessment (quoted below). As you directed your question directly to me, I will expanded Grumpydoc's assessment a bit and see if you agree.
Air pressure is a macro behavior. Quantum Tunneling alone would say that anything can escape when we leave the macro world into the quantum world. The empty space of the wall is huge comparing to the size of atoms.
Staying with macro behavior, the air pressure is the frequency and force of air molecule collision. Keeping the Helium-only-permeable wall in your scenario/question. As the helium atom is about to leave, it may get hit by the collision from atoms in the outside. This helium atom doesn't get to leave. (Non-collision events discussed in later paragraph.)
Some atoms from the outside (if it isn't a vacuum) will come the other way. Only helium is permeable in your preposition. So, the helium atoms from the outside world all has a chance to come into the sealed space. The less helium is inside the space, the less likely the in-coming helium will get hit back outside.
Collision or not, the total density of the atoms (on either side) decides how frequent a atom will try to cross that border. More atoms on one side, more random moving atom will be "running the border" from that side, so to speak.
Such two way travel continues until the probability of going-in = probability of coming-out. That is equal vapor gas pressure.
If you actively remove the helium from the space immediately surrounding the helium disk-drive balloon, you can of course suck out until it reaches near vacuum. That would not be leakage as leakage is molecules escaping by random motion and not by active removal forces.
Rick
[EDIT: I mean gas pressure, not vapor pressure.]
As far as I understand it is the partial pressure of the individual gasses which matter. So, if the vessle is permiable to Helium, it will leak out until the pressure of helium inside is equal to the partial pressure of helium outside (not very much for atmostpheric helium).
Of course O2 and N2 mollecules are trying to leak in at the same time.
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Rick -- I completely agree with you now. When you said "The inside cannot have lower air pressure than the outside and still be leaking air outward", I thought you were implying that a vessel containing 0.5 atm of Helium would not leak Helium. But I think we're all in agreement now that that container would tend towards a near perfect vacuum; because the partial pressure of He in atmospheric air is minuscule.
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Rick -- I completely agree with you now. When you said "The inside cannot have lower air pressure than the outside and still be leaking air outward", I thought you were implying that a vessel containing 0.5 atm of Helium would not leak Helium. But I think we're all in agreement now that that container would tend towards a near perfect vacuum; because the partial pressure of He in atmospheric air is minuscule.
Good to see eye to eye. I expressed myself adequately this time. I actually had a typo there. I mean gas pressure and not vapor pressure. Vapor pressure would relate to evaporation more than to what is applicable here.
No need to look more stupid than I really am, so I edited that part.
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Helium ( and to a lesser extent Hydrogen, simply because it is a molecule) do diffuse very well through most materials that otherwise appear to be solid. That is why it is used as a leak detector agent, it will very easily diffuse through even the smallest spaces in a measurable concentration, which for N2 or other large molecule gases might only be a few grams per year.
Thus it is very easy for it to diffuse out of the container with outside air unable to enter, leaving a low pressure inside. Will still leave a measurable pressure of He, and the fill will probably not be a pure gas fill, just enough that it has an effect on head height, so likely is going to be some N2 inside as well even after all the He has leaked out.