What does 17um technology mean here?

[Ben321]

I was just reading https://www.military.com/equipment/pas-13-thermal-weapon-sight about the US military thermal weapon sight. It says it uses 17um technology. Does that mean that it's sensitive to longer LWIR wavelengths than the standard maximum of 14um? Or does that mean the sensing pixels on the microbolometer array are 17um wide?

[Fraser]

It means the pixels are 17um in size.

Fraser

[coppice]

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

[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.

Most civilian thermal imaging technology works up to 14um, using VOx microbolometer arrays. How does the military get sensitivity to 20um?

I personally have had no problem with my Seek Compact Pro camera (which supposedly has an upper wavelength of 14um) looking through very thick smoke. I live in Seattle and recently some smoke has blown over the area from the massive wild fires in California, as well as from north in Canada. Well, it has gotten to the point on some days, the sun gets VERY dark red, as the sun nears setting in the evening. On the worst days, the sun even disappears (100% obscured in the visual spectrum) so I can't see it with my eyes, even before it reaches the horizon. However, using the Seek Compact Pro I can easily see the sun, and its intensity (even through the smoke) completely dwarfs the LWIR intensity of everything else in the field of view, so much so that when the Seek is set for automatic brightness control everything else on the screen turns black, and the sun stays white, just like what happens when I point it at the sun when there is NO SMOKE. So it seems that either my Seek can see wavelengths longer than 14um, or else 14um is easily a long enough wavelength to completely penetrate even the thickest smoke imaginable (because let me tell you, 100% blocking the sun's visible light is NOT an easy thing to do, but that smoke from the wild fires manages to accomplish exactly that).

[Fraser]

Fire fighting cameras are normally LWIR (up to 14um) and perform well in smoke of the type encountered in conventional combustion. Useable Thermal imaging wavelengths are dictated by the transmission properties of the earths atmosphere.

Fraser

[coppice]

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

Most civilian thermal imaging technology works up to 14um, using VOx microbolometer arrays. How does the military get sensitivity to 20um?

I personally have had no problem with my Seek Compact Pro camera (which supposedly has an upper wavelength of 14um) looking through very thick smoke. I live in Seattle and recently some smoke has blown over the area from the massive wild fires in California, as well as from north in Canada. Well, it has gotten to the point on some days, the sun gets VERY dark red, as the sun nears setting in the evening. On the worst days, the sun even disappears (100% obscured in the visual spectrum) so I can't see it with my eyes, even before it reaches the horizon. However, using the Seek Compact Pro I can easily see the sun, and its intensity (even through the smoke) completely dwarfs the LWIR intensity of everything else in the field of view, so much so that when the Seek is set for automatic brightness control everything else on the screen turns black, and the sun stays white, just like what happens when I point it at the sun when there is NO SMOKE. So it seems that either my Seek can see wavelengths longer than 14um, or else 14um is easily a long enough wavelength to completely penetrate even the thickest smoke imaginable (because let me tell you, 100% blocking the sun's visible light is NOT an easy thing to do, but that smoke from the wild fires manages to accomplish exactly that).

In military applications smoke is something specifically generated to provide a smokescreen. They generate smoke with large particles/droplets to extend the band over which it obscures EM radiation. This means the electronic systems are always trying to push the band over which they operate. Its 40 years since I worked on this stuff. I can't remember what type of sensor was used back then, but they probably have sensors with an even wider band now.

[eKretz]

Yeah that original link is referring to pixel size. Regarding the segue, this was an interesting read if none of you have seen it yet. Regarding the difference in long distance detection capability with MWIR and LWIR:

https://www.researchgate.net/publication/282328406_Comparison_of_medium_and_long_wave_infrared_imaging_for_ocean_based_sensing

[Bill W]

Fire fighting cameras are normally LWIR (up to 14um) and perform well in smoke of the type encountered in conventional combustion. Useable Thermal imaging wavelengths are dictated by the transmission properties of the earths atmosphere.

Fraser

Indeed, mostly it is water vapour.  No real problems with O2 and N2, a bit from CO2.
If it is a hot humid day do not expect the same results when calibrating cameras than you get the following morning.  10% over 8m !

Bill

[CatalinaWOW]

As the referenced paper says, intended application drives band selection.  No single band is perfect, and it is often too costly (dollars, mass, volume or other metrics) to provide multi band capability.  Systems are selected based on the best guesses at the bulk of applications.  There have been cases of egg on the face when actual use cases didn't fit the predictions. 

For very long wave infrared the limits are more often the atmosphere, the optics and the windows than I limits on bolometer sensitivity.  With a further note that cell size does start to matter as dimensions approach wavelengths.

[Ben321]

There have been cases of egg on the face when actual use cases didn't fit the predictions. 

Can you post some examples?

[CatalinaWOW]

Best example is selection of LWIR for most military vision systems.  Clearly the best choice for the predicted war zones.  Places like the Fulda Gap or the Korean peninsula.  Only time it is not the best, or comparable choice is in very hot, humid environments.  Places like the Persian Gulf.

[Ben321]

Best example is selection of LWIR for most military vision systems.  Clearly the best choice for the predicted war zones.  Places like the Fulda Gap or the Korean peninsula.  Only time it is not the best, or comparable choice is in very hot, humid environments.  Places like the Persian Gulf.

I can't find the news story now, but a few years ago, there was a news story about them buying thermal PTZ cameras for the US/Mexico border. And according to the news story, these cameras failed to work, because they only are functional in a narrow range of operating temperatures. Outside of that range they don't work at all, and the US southwest desert was too hot for them to even generate an image.

[Fraser]

Long range thermal imaging cameras for borders were traditionally cooled cameras that used a Stirling mechanical cooler. Very high quality, low noise images were achievable. As uncooled thermal camera has improved, it has found its way into border protection as well. Still not normally as good as a cooled camera, but 'good enough'. The advantage of the uncooled cameras is long term reliability. The Stirling coolers are better than they were, but still have a limited service life that leads to high maintenance costs. In these days of austerity, buyers start to check the cost of ownership more carefully.

Ambient operating temperature has always been an issue with thermal cameras. In industries where high radiant thermal energy is involved, the camera and lens are protected behind a heat shield and air flushed cooling jacket. In external environments the cameras are normally protected inside a weather proof jacket of some sort that protects against fluid ingress and even heats the jacket in cold weather. High temperatures are a problem however. Most thermal cameras have very definite ambient temperature limits and they will no work correctly outside those limits. In the case of the cooled cameras it is dictated by the Delta T capabilities of the cooler and its thermal energy sinking capabilities. With uncooled cameras they are normally calibrated at an ambient temperature in the range 20C to 30C depending upon OEM. The microbolometer is either temperature stabilised using a Peltier module or unstabilised where it just rises to thermal equalibrium of the system. The operating temperature of the Microbolometer is usually around 32C, In a stabilised microbolometer system the TEC controller will do its best to maintain the correct centre temperature operating point but such is only possible if the TEC can cope with the ambient temperature around its heatsink. Unstabilised microbolometers can drift off their nominal centre temperature quite easily so the camera image processing electronics monitor the die temperature using diodes inside the microbolometer package and also the ambient temperature of the chassis. Offset tables are applied to compensate for a range of ambient temperature. Outside that range, the camera image processing program cannot compensate so either and error is generated "Overtemp alarm" or the measurements and image displayed suffer degradation. FLIR told me that the further away from 30C that the E4 microbolometer drifts, the greater the error in the measurements provided. Generic offset tables are OK, but buy no means perfect.

It does not surprise me at all that thermal cameras failed in the high temperatures of a desert environment. Sometimes it is down to deployment though. My military thermal cameras had a special "sun hat" accessory fitted to deflect the direct solar radiation and they were rated for very high ambient temperatures. They also had the advantage of often being mounted on moving vehicles that provided some air flow over them. Mil rated kit is normally built to survive in deserts so all components used are rated accordingly and any supplemental cooling requirements met at the time of deployment.  I have seen a 1.3 Million Dollar thermal camera that had its own fluid heating and cooling system to maintain its operating temperature in the heat of the Sahara desert or up at 65,000 feet above sea level ! Heating and cooling are essential considerations when deploying thermal cameras.

Fraser

[Ben321]

Interesting. I wonder if a TEC cooling system, in place of a mechanical sterling cooling system, would work well for VOx type LWIR imagers? Also, with the mechanically cooled units, when the cooler fails, is the entire unit replaced (camera and cooler together)? Or is the cooler just replaced? Or is it sent to a maintenance shop where mechanics fix the broken parts of the mechanical cooler?

[Fraser]

Many VOx Microbolometers already use Peltier temperature stabilisers. They do not use Stirling Coolers. With Stirling cooled sensors, a Peltier stack cannot replace the Stirling Cooler running down to -196C. The Peltier stack cooled cameras use sensor materials that do not cooling to the low Cryo temperatures.

When a Stirling Cooler fails, it can be due to one of several factors

1. Mechanical failure - motor, bearing, flexture or piston.
2. Loss of Helium Fill reducing cooling performance
3. Loss of vacuum in the sensor array dewar

When a Stirling Cooler is removed from a thermal camera, the sensor array comes with it. The sensor array is enclosed in a windowed vacuum flask/dewar to reduce the ambient heat load on the Cooler.

The Cooler can be rebuilt or a new one fitted depending upon the OEM and labour costs etc. Rebuilding involved full disassembly and inspection, followed by reassembly with new parts where deemed appropriate. New seals are fitted and the Cooler refilled with Ultra Pure Helium at a pressure of 200PSI.
The Cooler is then tested for performance. Once the Cooler is re-installed in the camera, a full calibration is carried out.

How much does a Cooler rebuild cost ? FLIR quoted £7000 for this service on an Inframetrics PM280 Sterling Cooler.

The other option is to fit a brand new Stirling Cooler / sensor array assembly. If such an assembly is still available, it may be cheaper to fit such instead of a rebuild. Most Stirling Cooler / Sensor assemblies are very expensive however. £5K is not unusual. Once the new assembly is fitted, the camera must be calibrated at additional cost.

Fraser

[Ben321]

Many VOx Microbolometers already use Peltier temperature stabilisers. They do not use Sterling Coolers. With Sterling cooled sensors, a Peltier stack cannot replace the Sterling Cooler running down to -196C. The Peltier stack cooled cameras use sensor materials that do not cooling to the low Cryo temperatures.

When a Sterling Cooler fails, it can be due to one of several factors

1. Mechanical failure - motor, bearing, flexture or piston.
2. Loss of Helium Fill reducing cooling performance
3. Loss of vacuum in the sensor array dewar

When a Sterling Cooler is removed from a thermal camera, the sensor array comes with it. The sensor array is enclosed in a windowed vacuum flask/dewar to reduce the ambient heat load on the Cooler.

The Cooler can be rebuilt or a new one fitted depending upon the OEM and labour costs etc. Rebuilding involved full disassembly and inspection, followed by reassembly with new parts where deemed appropriate. New seals are fitted and the Cooler refilled with Ultra Pure Helium at a pressure of 200PSI.
The cooloer is then tested for performance. Once the Cooler is re-installed in the camera, a full calibration is carried out.

How much does a Cooler rebuild cost ? FLIR quoted £7000 for this service on an Inframetrics PM280 Sterling Cooler.

The other option is to fit a brand new Sterling Cooler / sensor array assembly. If such an assembly is still available, it may be cheaper to fit such instead of a rebuild. Most Sterling Cooler / Sensor assemblies are very expensive however. £5K is not unusual. Once the new assembly is fitted, the camera must be calibrated atvadditional cost.

Fraser

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?

[eKretz]

Not trying to pick nits, but isn't it St(i)rling cooler? Or is my memory going?

[CatalinaWOW]

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.

[Fraser]

eKretz,

Correct, my spell checker keeps changing it ! Oooops

I have gone back and corrected the spelling Auto correct has its place but can be a PITA on technical words etc.

Fraser

[Fraser]

Regarding the Ultra High Purity Helium (UHP) gas used in the Coolers. It also needs to be remembered that the 'Cold Finger' is lowered to a temperature approaching 77K (-196C approx) Any air or fluid within the Coolers cold finger piston cylinder will freeze and potentially impede the pistons movement.
Stirling Coolers come in many forms but the units used in thermal cameras are miniature pieces of precision engineering that require such precision to reach the very low cold finger temperature. Any wear or loss of gas in the system usually leads to either excessive operating noise or failure to reach The intended operating temperature. The Cooler in one of my PM280 cameras still gets down to temperature but it sounds like a can of ball bearings being rolled down a hill ! It is not long for this world I may do a teardown on it.

Talking to HVAC engineers, the 200PSI Helium fill did not phase them. Such pressures in such a small volume are apparently no big deal. I want to refill one of my Stirling Coolers, but that is a lot harder than it sounds and obtaining UHP Helium at an affordable price has been challenging. Balloon Helium can be very impure but I have even considered trying that with a regulator set to 200PSI. Not high on my priority list at the moment and a project fraught with problems.

Fraser

[Ben321]

Regarding the Ultra High Purity Helium (UHP) gas used in the Coolers. It also needs to be remembered that the 'Cold Finger' is lowered to a temperature approaching 77K (-196C approx) Any air or fluid within the Coolers cold finger piston cylinder will freeze and potentially impede the pistons movement.
Stirling Coolers come in many forms but the units used in thermal cameras are miniature pieces of precision engineering that require such precision to reach the very low cold finger temperature. Any wear or loss of gas in the system usually leads to either excessive operating noise or failure to reach The intended operating temperature. The Cooler in one of my PM280 cameras still gets down to temperature but it sounds like a can of ball bearings being rolled down a hill ! It is not long for this world I may do a teardown on it.

Talking to HVAC engineers, the 200PSI Helium fill did not phase them. Such pressures in such a small volume are apparently no big deal. I want to refill one of my Stirling Coolers, but that is a lot harder than it sounds and obtaining UHP Helium at an affordable price has been challenging. Balloon Helium can be very impure but I have even considered trying that with a regulator set to 200PSI. Not high on my priority list at the moment and a project fraught with problems.

Fraser

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.

[Fraser]

It is "safe" if done properly. Standard BOC ballon gas cylinders are pressurised to 2900PSI, yes 2900PSI. Most pressure vessels are safe provided they are designed for purpose and tested. Unsafe is when an inappropriate vessel of associated accessories are used with high pressure gasses. "Safe" is likely the incorrect term to use here. Hazardous is likely a better word. High pressures, no matter what gas is involved can be hazardous if not correctly handled.

Standard domestic air compressor reservoirs are commonly holding around 120 PSI and so they are safety tested with water and licenced to act as a pressure vessel. You would not be allowed to sell an unsafe pressure vessel to the public so it is "safe" if unmodified and within test.

One of the reasons I have not 'experimented' with refilling a cooler with Helium is safety. If I cannot get the filling rig safe for use at 200PSI, I am not going to risk such a task as it is not that important to me. "Bush Engineering" can lead to injury where high pressure gases are involved, small items like valve parts and screws can become bullets if something goes wrong.

Fraser

[Ben321]

It is "safe" if done properly. Standard BOC ballon gas cylinders are pressurised to 2900PSI, yes 2900PSI. Most pressure vessels are safe provided they are designed for purpose and tested. Unsafe is when an inappropriate vessel of assiciated accessories are used with high pressure gasses. "Safe" is likely the incorrect term to use here. Hazardous is likely a better word. High pressures, no matter what gas is involved can be hazardous if not correctly handled.

Standard domestic air compressor reservoirs are commonly holding around 120 PSI and so they are safety tested with water and licenced to act as a pressure vessel. You would not be allowed to sell an unsafe pressure vessel to the public so it is "safe" if unmodified and within test.

One of the reasons I gave not 'experimented' with refilling a cooler with Helium is safety. If I cannot get the filling rig safe for use at 200PSI, I am not going to risk such a task as it is not that important to me. "Bush Engineering" can lead to injury where high pressure gases are involved, small items like valve parts and screws can become bullets if something goes wrong.

Fraser

Are you sure you actually meant a liter of gas? That the cryo cooler was actually a liter sized volume? If so, that would make all MWIR camera's pretty big devices (not to mention the hazard of having a full liter of gas at 200PSI). Or do you mean they take an amount of gas that would be a liter at normal pressure/temperature, and then pressurize it to 200PSI to make it fit in a small space (the miniature cooler that is used with MWIR cameras)?

[Fraser]

Who mentioned a litre of gas as the capacity of the Cooler ? You have lost me there. The standard Coolers contain a few cc of UHP Helium gas. Some confusion has crept in here me thinks

For info, the challenge in refilling a camera Stirling Cooler is in safely and effectively coupling to the Cooler itself as the fill port is not some simple car tyre valve type. It is a threaded hole with a gas seal seat at the bottom. Into that hole is inserted a threaded sealing slug with special soft metal ring gasket. To refill the cooler the slug must be removed, cooler vacuumed to remove any air, then pressurised with the UHP Helium at 200PSI and the sealing slug reinserted whilst not losing the Helium pressure or introducing any impurities into the UHP gas. I suspect a special 'dock' attaches to the Cooler that enables these operations to be performed whilst always sealed against the atmosphere. Making such a 'dock' is possible and I have the parts to do it. The problem would be ...... is a DIY "bush engineering" dock safe to use at 200PSI when not formally tested ? Likely I would be able to reduce the risk with good engineering practices, but, as you say, a 200PSI explosion involving metal fragments is not something you would want to be near ! I would rather stick with a dead Cooler than risk serious injury trying to refill it.

Fraser

[Fraser]

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

[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?

[CatalinaWOW]

Perhaps some better Google Fu.  Try a search on VLWIR.  Gives me 38000 results.  Use your imagination and you should be able to come up with others.  Think about the interchangeability of frequency and wavelength.  Think about stoichiometry in HgCdTe detectors.  And while you are at it, look up atmospheric transmission.

Have you looked up the book I suggested?

[Bill W]

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?

Not really, just not having any air in the way

http://blair.pha.jhu.edu/spectroscopy/figures/trans.gif

[bap2703]

And also optics made of mirrors can help --> totally telescope compatible