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Sherlock Ohms

The Manometers Were Getting Light in Their Chambers

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Critic
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Platinum
Leaks
Critic   7/23/2015 11:55:28 AM
This is an interesting story, Russ, thanks for writing it up.

We all know that helium-filled balloons tend to lose their gas faster than air-filled balloons.  Balloon materials like latex rubber are relatively porous, allowing a significant volume of helium to escape in less than a day.  Mylar balloons, which are less porous, can retain helium much longer:  typically a few days.

Why does helium escape its balloon faster than air (or nitrogen)?  For one thing, helium atoms are smaller than nitrogen molecules.  Helium atoms have diameters of about 260 pm, versus about 370 pm for nitrogen molecules [www.kayelaby.npl.co.uk/general_physics/2_2/2_2_4.html].  The size of the molecules alone does not fully explain why the helium escapes faster. 

Gas molecules that are at the same temperature have roughly the same energy, which is proportional to mv2, where m is the mass of the molecule, and v is its velocity.  A helium atom has an atomic mass of 4, and a nitrogen molecule has a mass of 28.  Doing a little math, we find that the velocity of the helium atom is about 2.6 times that of the nitrogen molecule.  The helium atom's smaller size and higher velocity make it much more likely to escape through a balloon pore.

Many years ago, I needed to leak test a device that was designed for water cooling.  We didn't have an easy way to pressurize the device with water, but it was easy for us to pressurize it with air.  We ran the pressure test with air, and found that the device met its advertised leak rate.  I reported this to a manager, and an argument ensued because we hadn't run the test using water.  I used a helium balloon analogy to explain that if the device didn't leak air, then it wouldn't leak water.  The manager didn't understand the analogy, saying, "It's not a balloon!"  We were forced to run the test again with water, which resulted in considerable expense for test equipment and a significant delay.  The device passed the pressure test with water as well, of course.

When comparing leak rates of liquids and gasses, it is useful to employ viscosity [www.cincinnati-test.com/images/AB120%20Setting%20Acceptable%20Test%20Criteria.pdf].  Water is about 53 times more viscous than air, so the leak rate for air would be about 53 times that of water.

William K.
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Platinum
Re: Leaks
William K.   7/24/2015 8:55:07 AM
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"Critic's tale of the uneducated manager demanding a water test explains why a lot of organizations have problems. Water molecules, being larger than either oxygen or nitrogen molecules, will be even less prone to escape than air. This is intuitive to anybody who has been exposed to high school science, but that knowledge is evidently erased during MBA classes. 

I had not realized that helium would also diffuse through quartz, although my guess is that it diffused through the material that was used to seal the quartz tube, and not the tube itself. But that is conjecture on my part. AND it took me a while to realize the pun, "getting light" not meaning receiving photons but instead absorbing helium. That's a good one.

Bill21
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Iron
Re: Leaks
Bill21   7/24/2015 12:57:34 PM
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"that is conjecture on my part" might also be applied on the first paragraph.  

I did not see any description of the manager's education or degree attainment that required the leakage test using water nor that many organizations have issues to a certain degree.  I have worked with and for people that have had:  no degrees, BS, MS, MBA and even PhD.  From my perspective, there is little correlation (positive or negative) on degree earned, their success as a mgr over an organization or the problems 'their' organizations encounter.

One possibility why the manager demanded a water test:  the contract with a major customer required this to be performed per their specification (water and not any other substance) and if it was not performed with water, they could be sued or commit perjury signing false test documents.  

If another suitable substance could be used, then the supplier would need to prove this to their end customer that an alternative substance was faster, easier and cheaper and would guarantee meeting their specification.  Maybe the vendor tried this and the end customer refused an alternative test method.  Again, too many unknowns and conjecture....

Mgmt is not just about being technically smart but also ensuring that many other aspects also follow company policies/rules, federal/state laws as well as contractual obligations.

Just a different opinion.

 

Critic
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Platinum
Manager
Critic   7/24/2015 3:33:45 PM
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The manager was a college graduate, but not an enegineer, and he had very limited knowledge of scientific principles, or anything technical, for that matter.  He was not an MBA.  He was a nice guy, and brought us chocolate chip cookies or pizza when we had to work late.

The manager was assigned to micromanage the project we were working on, because it was late.  We had requested more technicians to get work done faster, but instead, we were micromanaged, which slowed us down considerably due to numerous status reports and meetings.

The leak test was not a required test; we intended it to be a quick check to ensure that water would not leak at the customer's facility later on, where accpetance testing was to be performed.

It was not the manager's place to decide how the test should be run, or even if it should be run.  This was typical of how we were managed, and this sort of behavior stifled creativity and ultimately killed the company.

Bill21
User Rank
Iron
Re: Manager
Bill21   7/24/2015 3:46:45 PM
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that clarifies the situation, not contractual obligations but internal test to prove sealed with no leaks.  this person was brought in to project manage a project that was off the tracks from many factors (wrong plan, wrong skills in team, wrong funding).  Given the clarity that you provided, you probably could name many aspects that were contributing factors.  but this poor manager was picked for a difficult situation by higher up mgmt.  

 

curious if this was a rare occurence in this company, if this project ever completed or if company is still in business.....

GSKrasle
User Rank
Gold
Gas Diffusion
GSKrasle   7/24/2015 1:09:02 PM
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Critic,

Thanks for a very informative explanation!

I once worked where we used Helium to check the seals of hermetic devices. We would back-fill the cases with it before welding them shut, and then send them to another building (to avoid errant whiffs) where a "sniffer" could detect any leakage. Many of these devices were intended to be operated in high-vacuum, and that is WHY they had to be sealed: small amounts of residual or leaking gas is easily ionized into a conductive state. That plasma might or might not carry enough current to directly cause damage, but it messes-up delicate signals and circuits and sensors, at least. Outgassing made the 'Space Tether' fail: search for 'The Space Tether Experiment'

I also recall that back when my hair was short, you could 'recondition' a HeNe laser by putting it in a Helium atmosphere for a long time, to allow He to diffuse into the tube, replacing some that had previously diffused away, leaving the mixture unbalanced. That's exactly like what happened in the manometer case above.

Think about Hydrogen. It's comparably 'diffusey' and leaks through seals, tubes and walls, and also reacts with metals, embrittling them by forming hydrides. And what happens when it diffuses down-gradient and accumulates at structural imperfections (voids in fiberglass tanks)? Once it gets loose, it forms explosive mixtures with air over the widest range of any substance, so leakage is dangerous. These are reasons there is so much interest in storing hydrogen cryogenically, or as a solute or compound rather than as a pressurized gas like methane, etc. Deployment of a 'Hydrogen Economy' is fraught. 

Critic
User Rank
Platinum
Re: Gas Diffusion
Critic   7/24/2015 3:20:53 PM
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Another way to leak test hermetic devices is to "bomb" them.  After sealing, the devices are placed in a "bomb" (a pressure vessel filled with He) and the bomb is pressurized to 45 - 75 PSIG for several hours.  If there is a leak in a device, then He seeps into the device.  Then, the devices are placed in a vacuum chamber to which a mass spectrometer is connected.  If He is detected, then there is a leak from one or more of the devices.  This testing is described in MIL-STD-883 Method 1014.

A similar test can be run using a radioactive tracer gas.  If a device leaks, it essentially becomes radioactive because of the trapped radioactive tracer gas.  Detection is simplified in this test, and no mass spectrometer is necessary.

Hermetically-sealed devices are typically also subjected to gross leak (bubble chamber) testing, because the fine leak tests may not detect gross leaks!

Hydrogen is only a little more "diffusy" than He at molecular flow rates.  Hydrogen is a diatomic gas, so the molecules are a little bigger than He atoms.  H2 does have less mass than He, though, so its velocity is higher.  At higher flow rates, H2 leaks more because it is less viscous than He.

patb2009
User Rank
Gold
Re: Gas Diffusion
patb2009   7/24/2015 3:23:34 PM
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it's why i think Hydrogen Fuel cell cars are silly.  7,000 PSI systems, 

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