What does 17um technology mean here?

[CatalinaWOW]

[quote author=Ben321 link=topic=135251.msg1794869#msg1794869 date=1535968468

1 liter at 200PSI goes KABOOM if the container breaks. One liter is the size of a medium-sized soda/pop bottle. If that was filled with 200PSI of a gas, it would explode with an ear shattering BANG. You'd hear it a mile away. Standing next to it, it would probably be as loud as a 12 guage shotgun blast. And if the container was made out of metal like the coolers for the camera, you'd have pieces of metal shrapnel flying at lethal high velocities.

I can't see how you would ever say that a liter-sized container filled with 200PSI gas is safe.
[/quote]

As Fraser says safety depends on more than the pressure.  I depends on stored energy and containment method, exposure, maintenance and other factors.

You are right, a one liter bottle pops with an impressive bang.  Though your comments sound more like a two liter bottle to me.  I won't ask the source of your knowledge, but my experience is that the burst pressures of these bottles is closer to 300 psi.

So the first factor is stored energy.  Say 5 CC's for the cooler (which is significantly more than many coolers).  That makes the energy 200 times smaller than your one liter bottle.  Now consider the failure mechanisms.  Due to the small surface areas involved the tensile strengths of the tubes and structures won't be approached, so a general failure like the soda bottle is unlikely.  A deep Nick, or even a cross tube fracture as might result from metal fatigue after repeated bends is going to cause something closer to a rapid leak than an explosion.

A further example.  A truck tire and a bicycle tire will explode at roughly similar pressures, not greatly different from 200 psi.  One can easily kill you, the other is unlikely to injure you.

Pressurized systems must be treated with respect, but also with thought and knowledge.

[eKretz]

Nobody ever said there was a liter of helium at 200 psi. The generalized reference was to "fractions of a liter." The amount of helium actually in a camera cooler probably wouldn't be likely to hurt anyone or anything if it burst unless it was up against your skin. Maybe not even then. The amount of stored energy is directly related to the number of molecules of gas present. For instance, an air compressor with an 80 gallon air tank at 120 psi bursting might blow off the roof or wall of your garage. A camera cooler that holds a tiny bit of helium at 200 psi would likely barely even make any noise if it burst.

[CatalinaWOW]

Nobody ever said there was a liter of helium at 200 psi. The generalized reference was to "fractions of a liter." The amount of helium actually in a camera cooler probably wouldn't be likely to hurt anyone or anything if it burst unless it was up against your skin. Maybe not even then. The amount of stored energy is directly related to the number of molecules of gas present. For instance, an air compressor with an 80 gallon air tank at 120 psi bursting might blow off the roof or wall of your garage. A camera cooler that holds a tiny bit of helium at 200 psi would likely barely even make any noise if it burst.

No this is what was said-

"You said 200PSI for the helium fill? Doesn't that make the device itself an EXPLOSION hazard? Just a hairline crack in the case, and it would blow apart, like popping a balloon with just a tiny needle (only more violent than a balloon popping, because the pressure much is greater). Why not use normal pressure (15PSI above a vacuum) helium, or even ordinary air? I think adiabatic cooling doesn't require a high initial pressure, only high pressure during the compression phase. And the compression phase needs to happen slowly, while the decompression phase happens rapidly. So why would the initial fill pressure for such a cooler be 200PSI?

Subsequent simple explanations of why this isn't a serious explosion hazard weren't accepted, and further explanations have been provided.  I don't know if people are still fearful of these things or not, but realistically there are more serious dangers associated with these than explosion.  Things like burns from touching the hot end.  (Also not a serious problem).

A few numbers are always useful when calling the hazard card.  Things that are easy to find or calculate.  Like the stored energy at the target pressure plotted vs volume.  Or the tensile stress in a spherical tank as a function of wall thickness and volume.  Intuition based on you tube videos or a bad shop experience can be wildly wrong.  Just as intuition based on past shop success can also be wildly wrong.

[eKretz]

To my knowledge the pressure of the Helium is chosen to provide sufficient density to provide the right amount of heat flow.  The Helium is the physical transport mechanism.  The total amount of Helium in the coolers I worked with is measured in fractions of a liter so the stored energy (and hence the explosion hazard) is small.  Closer to Rice Krispies than dynamite.

I was referring to the "fractions of a liter" quoted here. Ben321 had started inserting his "liter of helium" into the conversation instead.

[Ben321]

More trivia for those interested.

The metal seal for the the Stirling Cooler fill port slug seal is Indium. A very soft metal that forms an excellent gas tight seal. Helium is the Houdini of gases due to its small molecule size. It can even leak through high quality Aluminium so Aluminium cased Stirling Coolers normally have a special metal coating applied to their Aluminium interior surface to better seal against Helium leakage.

https://en.m.wikipedia.org/wiki/Indium

Are you saying helium can leak through solid matter? Not around tiny nano-scale cracks, but directly THROUGH solid matter itself?

[eKretz]

Yes it can. Leaks out of heat sealed mylar balloons all the time. Helium is a very small molecule, thus it can fit between larger ones like going through a sieve.

[Fraser]

Ben321,

Not wanting to get into deep physics here but all that surrounds you is made up of molecules and atoms. As has been stated, Helium is a VERY small molecule that can pass through materials that have large enough holes in them at the molecular level.

Not applicable but Hydrogen is another gas that can be used in Coolers BUT it also has a bad 'habit' Over time it adversely effects the Aluminium making it brittle. Metal fatigue in moving parts then becomes a real issue.

Fraser

[eKretz]

Same for steel. Known as "hydrogen embrittlement."

[Fraser]

This thread has certainly covered some interesting ground. I will resist driving it further off topic though so if Ben321 wants to discuss Coolers etc further, I think a new thread may be wise.

Fraser

[Ben321]

Yes it can. Leaks out of heat sealed mylar balloons all the time. Helium is a very small molecule, thus it can fit between larger ones like going through a sieve.

I should point out that helium is an atom, not a molecule. Molecules are composed of atoms. Some gasses naturally exist as molecules like oxygen diatomic molecule. However, helium does not. It is a noble gas, and this means it is completely nonreactive. It cannot form atomic bonds with any other atoms, including other helium atoms.

[Fraser]

Ben321,

Thanks for the atomic physics lesson.

[eKretz]

Yes it can. Leaks out of heat sealed mylar balloons all the time. Helium is a very small molecule, thus it can fit between larger ones like going through a sieve.

I should point out that helium is an atom, not a molecule. Molecules are composed of atoms. Some gasses naturally exist as molecules like oxygen diatomic molecule. However, helium does not. It is a noble gas, and this means it is completely nonreactive. It cannot form atomic bonds with any other atoms, including other helium atoms.

Yes. That is true. I used the wrong term. The point still stands. And it's nice to see you DO know how to use Google after all.

[Ben321]

Good military sights respond to wavelengths over 20um. 14um can be obscured by thick smoke. Above 20um you see through most forms of smoke.

Ok, so what technology does the military use to sense over 20um? Civilian cameras use VOx for the sensor, and Germanium for the lens, and maximum wavelength is about 14um. So what materials does the military use for their sensors and lenses?

[Fraser]

Ben321

Wave length limits in this case are set by materials used in the optics, Detector capsule window and AR coatings.

A microbolometer is just a thermally responsive variable resistor after all ! The pixel size can be selected to suit the application requirements.

The spectral response of our atmosphere has a significant effect on the best areas of the spectrum for general thermography. Specialist needs, such as those of the military, and they sometimes use very narrow areas of the EM spectrum that would not be very useful in general applications.

You need to do some googling and reading on thermal camera and sensor theory if you want a more detailed understanding of this topic.

[Ultrapurple]

Whilst I'm aware that Fraser is talking about an older, somewhat larger Stirling cooler, here is a re-post of one I saw at a show recently. The (linear type) Stirling cooler is the stainless steel-coloured cylinder at the top and the lower, gold-plated cylindrical object is the dewar that contains the cold finger and sensor. The optical window is facing the camera. This tiny little MWIR core has 640x512 resolution and up to 180Hz frame rate.

The spectacles are for scale; the whole cooled core fitted very easily into the palm of my hand and it weighs about 300g.

I doubt there is more than a couple of cc's of high pressure helium in the assembly. I was interested to note that the fill seals appeared to be crimps rather than the complex indium washer arrangement Fraser describes for older coolers. Domestic refrigerators and the like have for a long time been using crimps to seal off their refrigerant gases. Whilst I'm aware that high pressure, high purity helium is a completely different animal from what's in my fridge, it's not too hard to imagine that the inside of the pipe could be plated with something like indium with a thickness that's adequate to form a reliable seal.

As I understand it, linear Stirling coolers are much more reliable and have a much longer life than their rotary predecessors so it's possible that an engineering decision was taken that the life is good-enough (>18,000 hours in this case) that it's not worth making the cooler and dewar assembly repairable (ie refillable).

[Fraser]

Hi Ultrapurple,

Just to clarify something in your picture.

The copper tube extending out of the side wall of the gold coated detector chamber is for the sensor Dewar Vacuum only and is not part of the Stirling Cycle Cooler Helium circuit. The vacuum is drawn down to the required level, the copper tube crushed using a specialist high performance tool and very hard epoxy lime coating (white in the image) is applied as a secondary seal. The vacuum Dewar surrounds the sensor and associated cold finger to improve cooling efficiency, reduce thermal load and avoid icing issues.

It would be unusual to use such a crush seal for the Helium Circuit, especially in military applications. I know miniaturisation has brought change in Stirling Cooler formats but I thought the fill port remained a threaded assembly for ease of production and serviceability. Military coolers have a routine service cycle as you want the weapon to work when needed.

It us true that the service life of both rotary and linear stroke coolers has increased greatly with advances in the technology. It is a machine after all and machines continue to be improved in terms of precision, miniaturisation and performance. Early rotary type Stirling Coolers had a service life of around 2000 hours ! When I say “early”, for thermal cameras, I am talking about the 1990’s and early 2000’s. With their use in Space platforms for science and imaging operations, the coolers have continued to be developed. Modern rotary Stirling Coolers commonly have a stated service life in excess of 10,000 hours and often more. Linear Stroke Coolers have fewer moving parts to fail and operate on a resonant oscillation of a floating piston acting against a long life precision spring. Compared to a rotary type cooler that resembles a piston engine or standard piston based compressor, the Linear stoke unit is far simpler and more reliable. The precision of the pistons and bores are amazing and negates the need for piston seals that can wear over time. A recent test of several Linear stroke Stirling Coolers designed for camera use showed them to have amazing service life. The accelerated test simulated real world use but what that means in practice I do not know. The test was ended at a calculated 100,000 hours run time and all coolers were still operating within specification.

Fraser

[Ultrapurple]

Thanks Fraser

I'm looking to see if I have any other photos that might shed light on my assumptions.

You are of course quite right about the vacuum seal on the dewar - thanks for pointing that out.

[Fraser]

Just for interest, here is a web page with pictures of several cooled core models, both rotary and linear coolers.

They look pretty conventional to my eye with just the miniaturised HOT models standing out as an advanced format. I could not see the Helium ports in the pictures but have not studied them closely yet.

There are some lovel imaging cores from this company 

http://www.scd.co.il/SCD/Templates/showpage.asp?DBID=1&LNGID=1&TMID=108&FID=1293&IID=1853

Fraser

[Ultrapurple]

Yes, that's the one. They even offer a better-than-full-HD 3Mpix InSb MWIR sensor that must be a joy to behold (and a pain to finance).

None of my other photos shed any additional light on the Sparrow's cooler.

[frogg]

You said 200PSI for the helium fill? Doesn't that make the device itself an EXPLOSION hazard? Just a hairline crack in the case, and it would blow apart, like popping a balloon with just a tiny needle (only more violent than a balloon popping, because the pressure much is greater). Why not use normal pressure (15PSI above a vacuum) helium, or even ordinary air? I think adiabatic cooling doesn't require a high initial pressure, only high pressure during the compression phase. And the compression phase needs to happen slowly, while the decompression phase happens rapidly. So why would the initial fill pressure for such a cooler be 200PSI?

1. Helium is gaseous at room temperature and therefore a 200psi system pressure is not surprising.
2. A small volume of gas at 200psi, especially in a properly designed container, is not dangerous.
3. 200psi is not "high pressure". Period.

R134a refrigeration systems commonly have 235psi head pressures at the compressor outlet when operating at 150 deg F, and they house a significantly larger amount of working fluid than a Stirling cooler.

Just a point of trivia:

Helium and Hydrogen are often used as the working fluids for stirling cryocoolers. Among other reasons, this is because both Helium and Hydrogen have extremely high thermal conductivities - much higher than other gases (another useless fact: Helium is used preferentially over pure Argon for some welding processes because Helium shielding gas cools the electrode much better than Argon.)

Despite hydrogen having higher thermal conductivity than Helium, Helium is used preferentially over Hydrogen because it becomes a liquid at a much lower temperature (therefore allowing much colder temperatures when used as a cooling device.)

[Fraser]

Some more trivia .......

1. Helium is preferential to Hydrogen in some systems due to the risk of Hydrogen induced embrittlement in some metals, including steel.

https://en.m.wikipedia.org/wiki/Hydrogen_embrittlement

The constant exposure to Hydrogen can cause some susceptible metals to fail unexpectedly.

2. Helium is used and abused....... Helium is a non replaceable, finite resource. It is wasted in things like party balloons and the world is just starting to realise the stupidity of such waste. Hydrogen is both common, easily obtained and not a finite resource in the way that Helium is. Sadly Hydrogen Is a flammable gas, unlike Helium so some care is needed in its use.

https://www.geolsoc.org.uk/Geoscientist/Archive/December-January-2017-18/Helium-Hero-or-Houdini

3. Helium is often called the ‘Houdini” of gases. It’s molecular dimensions permit it to pass into, and through, many materials over time. This can cause loss of Helium gas at a rate dictated by the porosity of the containment material. Aluminium is porous to Helium to the point that in Stirling Coolers, a layer of less porous metal is normally applied to the surfaces exposed to the Helium gas fill. The piston cylinders are effectively ‘sleeved’.
If there is any easy path for Helium egress in a Stirling Cooler, it will quickly fail due to the small volume of gas contained within the unit. More modern miniaturised coolers have even less ‘reserves’ of Helium than the older, larger units. From what I have read, the greatest risk of Helium leakage comes from containment boundaries. This includes flanges and ports. The thicker the metal used for containment, the slower the Helium loss but it is an exponential loss curve as the metal containment gets thinner. The most likely areas of leakage in a common Rotary Stirling Cycle cooler are the Helium fill port, interconnecting pipe work, electric motor wiring seals and any joints in the design. Linear stroke coolers remove the potential Helium loss path associated with the motor wire seals as the piston is driven by induction so no wires penetrate the coolers Helium comtainment.


Fraser

[eKretz]

You said 200PSI for the helium fill? Doesn't that make the device itself an EXPLOSION hazard? Just a hairline crack in the case, and it would blow apart, like popping a balloon with just a tiny needle (only more violent than a balloon popping, because the pressure much is greater). Why not use normal pressure (15PSI above a vacuum) helium, or even ordinary air? I think adiabatic cooling doesn't require a high initial pressure, only high pressure during the compression phase. And the compression phase needs to happen slowly, while the decompression phase happens rapidly. So why would the initial fill pressure for such a cooler be 200PSI?

1. Helium is gaseous at room temperature and therefore a 200psi system pressure is not surprising.
2. A small volume of gas at 200psi, especially in a properly designed container, is not dangerous.
3. 200psi is not "high pressure". Period.

R134a refrigeration systems commonly have 235psi head pressures at the compressor outlet when operating at 150 deg F, and they house a significantly larger amount of working fluid than a Stirling cooler.

Just a point of trivia:

Helium and Hydrogen are often used as the working fluids for stirling cryocoolers. Among other reasons, this is because both Helium and Hydrogen have extremely high thermal conductivities - much higher than other gases (another useless fact: Helium is used preferentially over pure Argon for some welding processes because Helium shielding gas cools the electrode much better than Argon.)

Despite hydrogen having higher thermal conductivity than Helium, Helium is used preferentially over Hydrogen because it becomes a liquid at a much lower temperature (therefore allowing much colder temperatures when used as a cooling device.)

I wonder if his head would explode if he knew how many airgun enthusiasts had 88 ft³ 4,500psi air tanks sitting in their homes. Talk about a potential bomb...

[bap2703]

Helium trivia continued: it also makes for funny voices

BTW it's really not easy to link pressure to danger.
From a home (wife) perspective I got the same level of danger/noise/cleaning involved with the elastic energy stored in the walls of a vacuum vessel imploding and a soda bottle exploding at 150 psi.

[Ultrapurple]

Some more trivia .......

2. Helium is used and abused....... Helium is a non replaceable, finite resource.

It depends how you define 'non-renewable'. According to Wikipedia and other sources (with my emphasis),

Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements (thorium and uranium, although there are other examples), as the alpha particles emitted by such decays consist of helium-4 nuclei. This radiogenic helium is trapped with natural gas in concentrations as great as 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation. Previously, terrestrial helium—a non-renewable resource, because, once released into the atmosphere it readily escapes into space—was thought to be in increasingly short supply. However, recent studies suggest that helium produced deep in the earth by radioactive decay can collect in natural gas reserves in larger than expected quantities, in some cases, having been released by volcanic activity.

One day, of course, the universe's final radioactive decay will take place and at that point there will be no more chance for helium to be produced. But that day is so far into the future that not even the most pessimistic environmentalist is particularly worried.

So you can have a squeaky-voice party with a clear conscience. And balloons.

[Ben321]

Is the method by which LWIR cameras can be made to see longer wavelengths than 14um simply a top secret military technology? Is that why I can't find anything about it in Google searches?