Liquid Nitrogen (LN2) Pour Filled Dewar Cameras

[MrSheep]

Just a teaser for my adventure into Liquid Nitrogen Pour Filled Cameras. One benefit of these cameras over a cryocooled stirling camera is that you can change the FPA's and filters are no longer "warm" filters but "cold" filters, which increases their performance.

These cameras require a lot of additional equipment to get them going. I had to build my own vacuum system which consisted of a turbo molecular pump, diaphragm pump, a few sensors and alot of high vacuum tubing. All just to evacuate the dewar. I did not want to take the chance of pouring LN2 into a dewar that was not properly evacuated. According to the manual u need at least 1.0x10^-5 Torr.

I also had to buy a LN2 dewar and the associated accessories, such as funnels and PPE.

Although it took me a good month just to get all the equipment. I can confirm that both cameras work! I was able to obtain good images. Will post more about this in the future.

[MrSheep]

This is the vacuum setup I have. Consist of a Pfeiffer Turbomolecular Pump and DCU, PKR-251 Full Range Gauge, Pfeiffer Diaphragm Pump, Edwards Pirani Gauge and Controller, isolation valves, and alot of reduces, tubes and bellows, all KF Style.

What all this ultimately attaches to is to something called a "vacuum operator". I had to look around for this as I had no idea what that was in the first place. I did a few days of searching just to figure out what this thing looked like. Essentially it is a plunger type device with vacuum seals inside. The plunger twists as well as moves in and out. This allows for the removal of the vacuum port on the back of the dewar. I was able to purchase one of these from a company called Cryofab. part number: 03V1045-3 "Operator, Vacuum Port, 1/2" w/NW-25 Flange"

I run the roughing pump first and gradually ramp up the speed of the turbo until I can easily pull out the plug. I don't want to open the dewar under high rpm because I don't know if it is under atmospheric pressure or not. I think for the future i'm going to add another gauge and isolation valve. That way I can rough down the system to about 5mbar first and check to see the vacuum pressure of the dewar. But so far my setup has worked.

[MrSheep]

Reserved

[KE5FX]

Interesting stuff.  Why do you have to evacuate the dewar before adding the LN2?  To avoid creating ice crystals?

[MrSheep]

Yes exactly. Think of a thermos you can drink out of. There is a vacuum on the inside of it. So the dewar is basically the same concept. You want to a) insulate the surrounding atmosphere's relatively high temperature 293K (20°C) from the cold 77K (-196 °C) LN2 cooled sensor. b) Avoid any impurities and water vapor from freezing on the sensor. I think if it is bad enough it can damage the FPA sensor inside. The manual even mentions this.

[Fraser]

One disadvantage with Liquid Nitrogen filled cameras, beyond the obvious management of the coolant supplies, was the issue of camera orientation. A camera that uses a liquid coolant that must NOT be sealed in the Dewar cannot, normally, be operated in the vertical or inverted plane. Whilst this may not sound like much of a problem, it was in some scenarios found in industry and labs. Because the camera could not operate in the vertical plane, specialist lenses were used that contained a 90 degree beam translation component (often an angled surface coated mirror) so that the lens looked down vertically onto the target whilst the camera remained horizontal. I have such an unusual lens that was made by Inframetrics for their liquid nitrogen cooled cameras. I also own an AGA 680 thermal microscope from the 1970’s that uses a similar beam translation method for its downward looking thermal microscope lens.

Inverted operation of a Liquid Nitrogen cooled camera was less of an issue as it was uncommon, unless the camera was installed in a military aircraft ! There were technical solutions for that application though

Fraser

[IR_Geek]

 

"Where Does He Get Those Wonderful Toys?" - Joker (Batman 1989)

Very nice.  Good rule is to record the date and Torr level on the Dewar.  Next time you pump it down this will give you an idea of the health of the Dewar.  Should last forever as long as it's not dropped and it was stored properly. 

Hopefully any filters you got with it were identified in some fashion.  Nothing worse than having a pile of filters with no identifying markings.

[MrSheep]

Fraser: Ah yes! The orientation is prob one of the biggest disadvantages. But yes as you said I have seen mirror contraptions that make it work. In some publications I have seen these cameras use an optical tables/benches and they use a bunch or mirrors to image whatever they were testing.

IR_Geek:
Haha! A lot of searching for deals, eventually things pop up from time to time.

After picking up these cameras I had to learn about high vacuum. That is a whole topic in itself haha! Very cool to learn about KF fittings and the proper way to startup and shutdown a turbomolecular pump. It is definitely easy to destroy these pumps if you are not careful (Like venting to atmosphere while under full rpm).  But now that I have a high vacuum system maybe I can mess around with Chemical vapor deposition (CVD).

And wow! Nice tip! I will do that from now on. I did record when I pumped the dewars down. I did the both on the same day so that makes it easy haha.
And yeah I have yet to see what filters are inside the filter wheel. If they aren't marked it's not the end of the world, just have to spend a bit more to get them.

I think the most interesting bands to look at in MWIR would be the Hydrocarbon/Methane (3.2 - 3.4 µm) and CO2 (4.3μm) bandpasses. I think I have a few ND filters as well. IIRC they are meant for the higher temps. Not sure how useful because my IR blackbody calibrator only goes to 500C haha.

[Fraser]

Somewhat off topic but may be of interest….. another cooling method for the thermal detectors was cooling by Argon gas or even compressed air !

The military could not cart around bottles of unsealed Liquid Nitrogen so other cooling methods were investigated. Many will be familiar with the use of Peltier stack coolers that could achieve a detector operating temperature of 203 Kelvin (-70 Celsius) for Lead Selenide (PbSe) “HOT” detectors (hot compared to other detectors requiring 77 Kelvin to operate). Another cooling technology that has interested me uses a highly compressed gas as part of the cooling mechanism. The gas itself is nothing very special. I have read of Argon being a favourite, but common air has also been used. The cooling system works by mounting the detector onto a special heat exchanger module through which the highly compressed gas passes and is allowed to expand and vent to atmosphere. This technology was based on the thermodynamics of gas expansion and its cooling effect. Military uses included thermal weapon and observation sights. The gas cooled sights were commonly called “Hissing Sid’s” as they hissed whilst in use due to the release of the cooling gas into the atmosphere. The technology was clever and a supply of compressed gas bottles could be carried with the thermal sight to extend operating time. The gas cylinders varied in size and pressure to meet the needs of the application. The Hughes Probe Eye thermal sight is a very well known gas cooled thermal imaging system but there were far more sophisticated designs used in the military fir battlefield applications.

Fraser

[Fraser]

Some pictures of gas cooled Thermal imaging systems……

The Hughes Probe Eye, documented on .. https://www.prc68.com/I/Probeye.shtml

The Thorn EMI DFOV gas cooled thermal scope detailed on.. https://www.prc68.com/I/ThermalIMagerDFOV.html

[MrSheep]

Very neat. Never knew you could use argon to cool IR cameras haha.

[Fraser]

Some pictures of my 90 degree beam translation lenses. First the Inframetrics unit that mounts on the front of a cameras lens as a supplemental lens so it may be moved between different powers of lens. It is shown attached to a 3X telescope.

The second lens is the primary lens of the AGA 680 thermal microscope. Note that it has both optical and thermal microscope lenses built into its design. The lens is 15X magnification.

Fraser

[MrSheep]

Nice that is pretty neat.

[MrSheep]

Just updated my previous post (post #2) with my vacuum setup. All of the parts were second hand except for the flanges, bellow tubes, and valves. If I were to buy everything new I think it would have cost like $23k . Luckily you can get most of this stuff second hand.

As for the vacuum port of these dewars. There is a black cap that goes over it. Originally I thought It was just to prevent damage or dust, which is true but it also protects the port from the LN2. If you pour LN2 over the vacuum port you can cause the o-ring seals to shrink and make the dewar lose its vacuum, which would be a bad day if the camera already had LN2 in it and the detector was cooled down to temp. Same thing with the large storage dewar I have you are suppose to pour the LN2 a certain way so u dont accidentally splash LN2 onto the vacuum port. (yes even my LN2 dewar has a vacuum port haha).

The plunger looking thing is called a "Vacuum operator". I think FLIR calls it a "Evacuation Adapter" in their manual. The plunger has a set of o-rings internally that allows it to move in and out while under high vacuum. There is also a threaded end (see second post) that you twist onto the vacuum port prior to the pump down. The operator uses a standard KF-25 flange. I've seen some with just a tube but I have no idea how they connect that to a high vacuum system. Having a KF-25 flange just seems more safe and less prone to accidental breakage of the high vacuum side.

[Fraser]

It was the cost of appropriate hardware plus the Ultra High Purity Helium Gas that put pay to my hopes of recharging the Helium fill in my cryo coolers. Unless doing it as a professional business the relatively high cost for parts made no sense. That involved high pressures rather than high vacuum and specialist fittings were required. Added to that the fact that with high hours cryo-coolers it is not uncommon to refill them only to find that the cooler has other issues and needs a complete service in order to work properly

[MrSheep]

Yes Fraser, that is another thing! I want to know how to perform the Helium refill procedure you speak of. Honestly if I know how it is done I might be willing to eventually build a system to mirror what the bigger guys do when they manufacture the cryocoolers.

If I am not mistaken they use some sort of nut/screw on the side of the rotary coolers and some sort of indium seal?

And as you said this stuff isn't for the faint of heart but it is something I would like to attempt in this life  . Definitely doesn't make sense for an individual to do it considering how few people own cooled cameras. But I find it something that would be cool to do.

As for the high vacuum stuff. I think you need to have high vacuum as well. If you take a look at all IDCCAs (linear or rotary) the detector is under vacuum. See attached photo. So the detector side has a high vacuum and the cryocooler cold finger side has high pressure. Usually the vacuum port can be easily found since it is a copper tube that is pinched off. It is really like yin and yang with these intricate devices 

Also I heard that linear coolers can be saved! I talked to a specialist and he said the Amber cameras with the linear coolers might just need a cleaning and recharge. They aren't under the same stresses as rotary coolers.

Here is a link to the pdf I got the images from: https://support.flir.com/Answers/A4456/IR_Imaging_Radiometry_Handbook.pdf
I am still absorbing this material but it has been a blast reading it.

[MrSheep]

Fraser, This is the copper pinch off tube I was talking about. I think it is under the same high vacuum pressures (if not more) as my dewars

[Fraser]

I know a little about the cooler refill process that I learnt whilst considering a DIY version for my coolers.

1. The process requires UHP Helium, nothing of lesser purity is acceptable.
2. A dryer may be required for the UHP Helium but I am uncertain whether this is essential if quality UHP Helium is sourced.
3. Suitable Helium rated Helium cylinder regulator, gas gauges and hoses are required.
4. A special cryo-cooler fill port adapter is needed to access the sealing screw whilst the port is under pressure. more on that later.
5. A suitable Indium seal is needed to replace the original fitted in the cryo-cooler fill port.

The fill port adapter…..

The fill port on the cryo-cooler is a threaded hole at the bottom of which is a sealing surface against which the sealing screw compresses the Indium seal. The fill port May, or may not have a cover screw inserted to keep dirt out of the port. The cover screw and sealing screw are removed to access the fill port and the indium seal is extracted. At this point all Helium will obviously be vented to atmosphere. The Helium is replaced with whatever gas surrounds the cooler, most likely atmosphere in DIY applications. The atmospheric cases are considered a pollutant and should be purged as part of the refill process. No further disassembly of the cryocooler is required if a Helium refill is all that is to be done. If any other flanges are opened for inspection purposes, the seals must be renewed when reassembled. A full service is normally carried out in clean room conditions to limit the cryo-coolers exposure to contamination. The glass dewar containing the sensor array is under vacuum but these rarely leak and come up to atmosphere. Vacuum Dewar seal leaks are rare compared to those that have to deal with the Houdini of gases that is Helium.

To fill the cryo cooler with gas the adapter that mates with the coolers fill port must accomplish the following….

1. A Helium gas tight seal to the outer face of the cryo cooler port
2. A suitably sealed integrated tool for the insertion of the threaded sealing plug (the new Indium seals may be positioned in the fill port prior to attaching the fill port adapter)
3. A means to vent Helium gas as part of the cryo-cooler casing gas purge process and also used to flush out atmosphere from the Helium filling hoses and gauges.
4. A safety device to prevent overpressure events in the cooler. This is a safety device to safely release pressure if too great a pressure is applied to the fill system. The UHP cylinder will have a gas regulator and gauges but stuff can go wrong quickly at these pressures and safety is important.

The process to fill the cryo-cooler with UHP Helium varies with who you talk to. Some say that the Cryo-Cooler needs to be attached to a vacuum pump and all atmosphere removed with the vacuum pumping action before the Helium gas is injected into the coolers casing. Others use a Helium purge approach but this does require the deliberate loss of UHP helium through “venting” as part of the purge process. The “vac’ing dow” approach is common in HVAC but gas purging is equally common amongst those working in labs. The process of purging will be detailed here.

1.The first step in filling a Cryo Cooler is the removal of all pollutant gases from the refill gas lines, gauges and, of course the coolers casing. The fill lines may be flushed with the UHP Helium to push atmosphere out of them. The coolers casing is more complex…. It is filled with Helium and then the Helium permitted to leak out whilst more UHP helium is being pushed in to prevent atmosphere re-entering the casing. It’s sounds pretty crude to me and it uses a lot of UHP Helium in the fill-vent-fill-vent process. More research is needed on the exact purge process to achieve the enquired expulsion of atmosphere from the cooler casing. Personally I think the vacuum pump method more professional if the cooler seals are designed to cope with vacuum as well as pressure, which I believe they are. The vacuum process literally sucks the atmosphere and pollutant gases out of the coolers casing and components within. There is the risk of pollutants coming back from the vacuum pump so I believe a special pump is needed, but I know nothing more on that topic.

2. Once the coolers casing has been purged of atmosphere it is “just” a case of injecting the UHP Helium at the correct pressure for the particular Cryo-Cooler. IIRC the common fill pressure is around 200psi. I have previously been told that 200psi is not a very high pressure but having seen the results of a 115psi air compressor reservoir tank explosion in safety lectures I have the utmost respect for any gas at this pressure and all that is exposed to it’s forces. The Cryo cooler casing is a very small volume so not a lot of UHP Helium gas is needed. Far more gas is used in the fill hose and casing purge stages than in the final fill.

3. The integrated seal screw tool is used to seal the cryo-cooler once the correct UHP Helium fill pressure has been achieved. The Indium seal is squashed between the seal screw and the fill port seal surface. These must both be carefully cleaned before the fill process begins to avoid microscopic gas leaks past the Indium seal. Helium gas is called the “Houdini of gases” for good reason. It’s molecule size is so small that it can even escape through some metals if a special sealing coating is not used to prevent such. Soft Indium seals are essential as other seals would present little resistance to Heliums efforts to get past them. It is an amazing gas for Cryo-cooling due to its low freezing point and cooler efficiency improvement, but it is also a PITA due to its small molecule size.

4. A dust cover is normally inserted into the cooler fill port to prevent dirt ingress. This often take the form of a large nylon bolt that is screwed into the port. This is not a secondary gas seal as some might think. If it was missing from the cooler at the start of the process, it is of no great concern in terms of gas leakage but the fill port threads should be carefully cleaned and flushed with IPA prior to removing the sealing screw so as to avoid any solid contaminants entering the coolers casing under the pressure of the new Helium gas fill. It is a very good idea to clean the fill port threads before refilling the cooler anyway.

Testing of the refilled cooler is just a case of listening to the cooling cycle and timing how long it takes to drop into the “maintain” mode. Monitoring of sensor temperature is also possible with some cameras. The cameras manufacturers usually state the nominal time for cooling down to operating temperature but ambient temperature has a small effect on this. A cool down time of 5 minutes is not unreasonable for a freshly filled Cryo-cooler but some take longer (up to 10 minutes) It is important to listen to the cryo-cooler as it operates as its sound can provide a further indication of mechanical health. I love linear type coolers as they are quieter than rotary type coolers so sound more “refined”  They also tend to have a longer mechanical life expectancy due to their lower stresses on the internal components. Linear coolers tend to “buzz” with little to no “rattle” in my experience. Rotary coolers are noisy blighters ! There is a lot going on in a rotary cooler in terns of mechanical movement and changes in forces on bearing surfaces. They have a distinctive raucous rattle that is loud at start and moderates once the cooler reaches operating temperature and the motor RPM reduces. A rotary coolers noise signature is very distinctive and I can listen to one operating and normally tell when it sounds “off”. When they start to get more noisy than normal and/or take too long to reach operating temperature, it is time for a service. Long cooling down times can be due to loss of Helium gas fill pressure but an increase in noise can mean a cooler is approaching the end of its mechanical components life and new components will be required during a service. Inspection of a cryo cooler is not something I am qualified to carry out but I suspect it involves careful inspection of the moving parts and sliding surfaces for evidence of wear and damage. Motor and crank bearings are obvious candidates for replacement during a service but the compressor piston seals and the regenerator may also require replacement due to wear. If the special gas tight coating of the cylinder walls has been damaged, the coolers casing would require specialist refurbishment.

Well I think that is enough for now

Fraser

[MrSheep]

Thanks for the extensive write up. All of this makes sense. Now the question is has anyone seen or even have a picture of the fill port adapter. I'd imagine it would be something similar to a vacuum operator, but has the ability to withstand high pressures.

Also check this article out! After testing the linear compressor they even used the old cold head without cleaning it and got the same cooling performance. This is why I think linear coolers offer the best chance of being "refurbished" rather than being replaced.

[coppice]

The military could not cart around bottles of unsealed Liquid Nitrogen so other cooling methods were investigated.

The military never liked the hassle of needing to refill with liquid nitrogen before every flight, but there have been plenty of military thermal imaging systems which worked that way. The ones I developed used liquid nitrogen. The filler for the tank was positioned to be easy for ground staff to fill up just before take off.

[Fraser]

Mr Sheep,

I believe it looks much like your vacuum adapter with its central “screwdriver” tool but it is mounted to the face of the fill port surround using a clamp rather than the nut and seal that your adapter uses. I will see if I can find the picture I have of a cooler being refilled in a clean room. From memory, the fill adapter is quite small but well engineered.

Fraser

[Fraser]

Coppice,

Yes, I am aware that aircraft had Liquid Nitrogen cooled thermal cameras but, as you say, they presented challenges My comment regarding karting around Liquid Nitrogen was more about battlefield use on the ground with Recon teams using hand held thermal observation scopes. Liquid Nitrogen could be used but alternatives were investigated. When no alternative is available, you have to work with what you have and as your comments support, you had to be inventive to get the job done

As a side note, I was contacted by a very pleasant USA based scrap dealer some years ago. He had in his possession a thermal imaging camera pod from a US Military fighter jet. I forget the exact model of fighter but it had two pods beneath its fuselage, one was terrain radar and ten other was thermal imaging of the ground via a scanning system. The thermal camera was a Stirling Cooler based design but what was odd was the pipe work that fed the pod from the fighter aircraft. I surmised that it was heating or cooling fluid for the pods power supplies and camera. The scrap dealer wanted to know more about what he had so provided many pictures. I was able to determine thathe camera could not be returned to operation (which was a good thing in the circumstances) as part of its essential control systems were within the airframe and not the pod. The very interesting (for me) part of the thermal camera was its lens design. It used two contra-rotating wedge profile circular lenses to create the scan of ten ground below the aircraft. The two lenses must have had some sort of synchronisation system and presumable rotated at a fair rate for a decent scanned image. I had never before seen such a scene scanning system. I ended up advising the scrap dealer to either contact a museum of aviation or break up the pod for its metal value. I was most certainly not going to try to help getting it running !

[coppice]

As a side note, I was contacted by a very pleasant USA based scrap dealer some years ago. He had in his possession a thermal imaging camera pod from a US Military fighter jet. I forget the exact model of fighter but it had two pods beneath its fuselage, one was terrain radar and ten other was thermal imaging of the ground via a scanning system.

Line scanning thermal imaging for terrain scanning beneath a military plane or a helicopter moving steadily forwards were quite common in the late 70s and early 80s. I'm not sure what cooling or heating such a thing would need, apart from the actual camera sensor. Perhaps the plumbing you saw was for heat extraction from the stirling cooler. In hot climates you need to do your best to make life easy for those things.

[Fraser]

Coppice,

I just checked. The unit was a LANTIRN terrain navigation pod from an F-15. I attach some pictures. It was the upper of the pod parts with the Orange (ZNSE ?) window.

LANTIRN

https://www.af.mil/About-Us/Fact-Sheets/Display/Article/104582/lantirn/

The scanning “lenses” were, in fact Risley Prisms.

[coppice]

I just checked. The unit was a LANTIR navigation pod from an F-15. I attach some pictures. It was the upper of the pod parts with the Orange (ZNSE ?) window.

That's not a ground scanner. That's a target acquisition imager. Its forward facing. It will be a full sensor imaging sensor. rather than the kind of line scanner that is used for ground scanning reconnaissance.

[Fraser]

Interesting. As you can tell, this is not the sort of thermal imager I normally deal with. It definitely had the Risley Prism system though.

[coppice]

Interesting. As you can tell, this is not the sort of thermal imager I normally deal with. It definitely had the Risley Prism system though.

Interesting. I thought LANTIRN was late enough that it would use a full image sensor. Either way, its not like the ground mapping line scanners. They only have a single pixel sensor, spinning on the edge of a vertical disc, sweeping the ground below. As the plane moves forward they achieve a raster scan. The electronics needs to manipulate the geometry of the scan, to allow for the actual forward speed of the plane, and may do the image manipulations needed to correct for the plane's attitude as well.

[Fraser]

I just found the detail on the LANTIRN system and I can confirm that the unit I advised on was part of the navigation pod and provides a forward thermal view of the terrain that is presented to the pilot on the HUD. It all gets very confusing with the various similar looking pods, some with fixed heads, others with moving ball heads and modern versions doing the job of both the navigation and targeting pods in a single unit. As I said, not my area so this is uncharted territory for me beyond the basic design of the thermal imaging camera core.

https://www.oocities.org/capecanaveral/5415/lantirn.html

From the provided information I can also confirm that the pod contained an “environmental control” section so that was likely why it had fluid pipes in the camera section.

Fraser

[coppice]

I just found the detail on the LANTIRN system and I can confirm that the unit I advised on was part of the navigation pod and provides a forward thermal view of the terrain that is presented to the pilot on the HUD. It all gets very confusing with the various similar looking pods, some with fixed heads, others with moving ball heads and modern versions doing the job of both the navigation and targeting pods in a single unit. As I said, not my area so this is uncharted territory for me beyond the basic design of the thermal imaging camera core.

https://www.oocities.org/capecanaveral/5415/lantirn.html

From the provided information I can also confirm that the pod contained an “environmental control” section so that was likely why it had fluid pipes in the camera section.

Fraser

If its specifically described as terrain imaging, it must have been for ground hugging missions at night. Mechanically scanned imagers usually have a rather low refresh rate. I wonder what this one achieves? A pilot needs something good if they are following the terrain at high speed.

[Fraser]

The image presented to the pilot appears decent but no idea of the frame rate. I attach an example image and, as you say, it appears to be for low level operations. We had better shut down this interesting ‘conversation’ now as we are way off topic for this thread

Thanks for your insight into military aircraft thermal imaging systems 

Fraser

[coppice]

Thanks for your insight into military aircraft thermal imaging systems 

If you are interested in thermal imaging, this is the killer application. 

[CatalinaWOW]

I just found the detail on the LANTIRN system and I can confirm that the unit I advised on was part of the navigation pod and provides a forward thermal view of the terrain that is presented to the pilot on the HUD. It all gets very confusing with the various similar looking pods, some with fixed heads, others with moving ball heads and modern versions doing the job of both the navigation and targeting pods in a single unit. As I said, not my area so this is uncharted territory for me beyond the basic design of the thermal imaging camera core.

https://www.oocities.org/capecanaveral/5415/lantirn.html

From the provided information I can also confirm that the pod contained an “environmental control” section so that was likely why it had fluid pipes in the camera section.

Fraser

If its specifically described as terrain imaging, it must have been for ground hugging missions at night. Mechanically scanned imagers usually have a rather low refresh rate. I wonder what this one achieves? A pilot needs something good if they are following the terrain at high speed.

Frame rate, scan speed and sensitivity are a trade space.  Many scanners of the era achieved frame rates well above the effective frame rate of a human pilot (20-60 fps range).

The rotating wedge system can generate a large number of scan patterns with the interesting property of widely varying scan rates over the field.  Thus information can be gathered over a large field of view with a small number of detectors, and very high quality, data information provided over a small region. 


For MrSheep - Just about every cooling gas imaginable has been used for cooling infrared systems.  Helium for systems that need really low temperatures, Neon for slightly less demanding cases.  Nitrogen and argon are widely used, and for roughly similar temperatures.  Nitrogen is widely available and cheap, argon has desirable thermodynamic properties.  Oxygen is rarely used because of fire danger, but compressed and dried air is sometimes used in cryostats.  There are even systems that use Freon and CO2.

[Fraser]

I will see whether I kept the images of the F-15 navigation pod and start a new thread about it as such might be interesting to forum members. It is not often that we get to see the inner workings of military thermal imaging systems, albeit older technology in this case.

Fraser

[Fraser]

I have found the images of the LANTIRN Thermal navigation pod internals  I am having some doubts about posting such in the public domain though ! The unit was dated 2001 so not exactly ancient technology. Posting technical pictures and details of an F-15 related thermal imaging technology may not be the brightest of ideas. I was surprised that the scrap dealer obtained a complete pod that was actually unused stores stock with only 4 hours on its cooler. When discussing the more advanced thermal imaging technologies we do have to consider why such is not already documented in the public domain.

Anyway, apologies to the OP for the off topic discussion. Now back to the topic of LN2 cooled cameras 

I own two LN2 cooled cameras, the AGA 680 thermal microscope and the AGA 780 series portable thermal imaging scanner. Neither will ever see LN2 again as they are museum pieces. I did look into sourcing LN2 and what the costs would be. Sadly in the UK, unless you are well connected with a Technical University, it is actually quite hard to obtain small quantities of LN2 on an ad-hoc basis. The storage Dewars are quite expensive as well. It quickly became apparent to me that the cost of providing LN2 for my two antique cameras far exceeded any benefit of getting them running. The situation with the LN2 cooled Amber cameras is very different though. Those cameras are worth the cost and effort of the LN2 sourcing and associated equipment. I was not previously aware of the need to refresh the vacuum on the dewars of those detector housings and was similarly unaware of the extensive equipment requirement to pull that vacuum to the required level. As has already been stated, these cameras are well suited to science roles where LN2 and associated vacuum pumps are more common. I will stick to Stirling coolers and can understand why the Stirling mechanical cooler was such a welcome development for many industrial users of cooled thermal imaging systems. We actually had an AGEMA THV880 scanning thermal camera upgraded to a Stirling cooler by AGEMA in Sweden. We were one of the first to have that upgrade installed and it cost a small fortune.

Pictures added…

AGEMA 880 LN2 model
AGEMA 880 with Stirling Cooler upgrade mounted on rear
AGA 780 series portable LN2 cooled system
AGA 680 LN2 cooled camera …. Early thermal imaging ! Not exactly small or light in weight !

Fraser

[MrSheep]

Fraser, yeah I didn't know that I needed high vacuum equipment either until I read the manual. You learn something new everyday haha 

Also yeah it is kind of a hassle to source LN2. It is easier if you live near colleges or universities as the always get it by the truck load. However not sure how willing they are to give it to you when you are not part of the department. I have however been able to source some near me via welding supply shops. Not all of them have it but some do so you have to do some research and some calling to find some. I know Airgas and Praxair sell it. But the prices near me are a bit too high $10/L. Luckily I found a store that was less at $5/L

I will prob not use the LN2 cameras as much as well but will from time to time because I do want to play around with them more.

[MrSheep]

Cool video on servicing a LN2 Detector system 
Although I do question is it safe to just pull the vacuum port like that. I at least pump mine down to rough vacuum (5mbar) before I attempt to remove the plug.

[DaJMasta]

A little bit of a necro, but a very similar topic probably relevant to very similar people:

I've recently got my hands on a very interesting cooled camera that uses an LN2 dewar with a fill port I'm having trouble making sense of.  I've talked to the company that made it and while they were quite helpful, they ultimately couldn't provide software or documentation for it - they claimed it was a legacy product out of support and that they had no software for, but I suspect it was a precursor to a current product and it never ran on published software or with guaranteed specifications.

In any case, that means it's time to poke around and see what I can get working, and the fill port on the dewar has a lot of fittings and seals that I wouldn't expect to see.  I've included some pictures, but can anyone with more experience than I make sense of it?  I was expecting a non-screw-down plug, since the boiling nitrogen would need an escape route, but instead I've got a hard plastic insert into the neck that seems to be stuck in place, a large grub screw as a cap for the center, and a pair of smaller pipe fittings on the side of the neck of the dewar that have plugs with small holes.

Best guess, the grub screw in the plastic insert covers the fill port and the side pipe fittings are the vents for the gas, but it also doesn't seem like they're currently opened - a brief low pressure test makes it seem like no gas can enter/leave (maybe the fillport is still closed?)

In any case, can someone shed some light on how this should operate before I risk further disassembly?

[MrSheep]

Ah yes I have an Amber Dewar that houses their 256x256 ROICs with a very similar port. So the top port is where you put the LN2. The side ports are pressure relief valves. Since the top is screwed on, LN2 expands very rapidly (700x in volume) when it evaporates. So in order to not create a bomb. The pressure has to be released somehow. The other LN2 dewars I have, Do not have screw on caps, but rather a loose fitting plug is used periodically letting gas escape.

Are you able to pull on the brass part of the valve. You should feel some spring pressure. This is to check if those valves are not stuck

Also do you happen to have a turbo molecular pump or high vacuum system? This is need to pump down the dewar. If not I wouldn't risk pouring LN2 into the dewar until you can confirm the vacuum is good.

[coppice]

The military could not cart around bottles of unsealed Liquid Nitrogen so other cooling methods were investigated.

Really? It never stopped them. The 70s thermal imaging I worked on and that I am aware of all used dewars of liquid nitrogen. The main problem with liquid nitrogen is its a liquid. It sloshes around. Most thermal imaging applications are mobile, even in something that doesn't do many wild manoeuvres, like a helicopter doing simple observation work. In another message you talked about orientation issues. Although its a long time ago, and I can't remember what short of slosh control measures were in our systems, sloshing was definitely a big deal, whatever the orientation.

[DaJMasta]

@MrSheep very much appreciated!  The brass sections come out very easily and when I pull one out I can confirm that now blowing air into the chamber from the fill port completes the circuit so to speak.  There is mention in the literature that one design goal was long (8h+) duration operating time on a fill, and I suppose the smaller port and sealed system is a way to insure that.

As this camera purportedly has some operating uses at room temperature, I will probably try LN2 as a check to see if the dewar sweats, but I've got a full vacuum system, so if it needs to be evacuated, it will take some fittings, but can be done.

[MrSheep]

Nice! Yeah the only part that you need then is the vacuum operator, it is shown earlier in this thread. It cost around $300ish and can be KF or CF.

One thing to note on these operators is to not screw them on too much. Once I evacuated a dewar and ended up not being able to separate the screw plug from the operator shaft, It kept spinning inside the vacuum port. I had to re-evacuate the system again . While not under vacuum test to see if the operator shaft can unscrew off of the plug without having it spin in place.

[Fraser]

Coppice,

An interesting insight, thank you. How did the Army & Special forces recon units, that spent days away from base, cope with the finite LN2 life before it boiled off ? As I read your post, I had mental images of our guys yomping across the Falkland Islands and finding that their thermal surveillance scopes were useless because the LN2 would have boiled off during the journey. The fact that LN2 boils off whether or not the scope/camera is used can be an operational support problem. The Argon Gas / Air cooled scopes/cameras did have a finite run-time but the gas was only used when the unit was actually in use.

Fraser

[coppice]

Coppice,

An interesting insight, thank you. How did the Army & Special forces recon units, that spent days away from base, cope with the finite LN2 life before it boiled off ? As I read your post, I had mental images of our guys yomping across the Falkland Islands and finding that their thermal surveillance scopes were useless because the LN2 would have boiled off during the journey. The fact that LN2 boils off whether or not the scope/camera is used can be an operational support problem. The Argon Gas cooled scopes/cameras did have a finite run-time but the gas was only used when the unit was actually in use.

Fraser

The only thermal systems I saw in the 70s for army use were fixed to vehicles. The stuff I worked on was for airborne use. The fire services had hand held mechanically scanned thermal cameras, but I never saw one of those close up. I think they were pretty basic, for cost reasons. The Falklands war was sufficiently after I did that work that I expect things had moved on. Things were quite bulky and heavy in those days, though, so anything being carried long distances would have had to be very basic.

[CatalinaWOW]

 As far as I know, in that era the army had no foot mobile infrared.  But they did have image intensifiers, which were was pretty new still.  About like uncooled IR a decade ago.  Well developed for well heeled players, but out of reach of those limited to the budgets
Of smaller nations.

[MrSheep]

Found an interesting article while looking for information on Amber Raytheon Cameras. Saw some information on Liquid nitrogen cooled arrays on page 33

"U.S. Air Force and Navy aircraft of the 1950s and 1960s, such
as the F-101B Voodoo, F-104 Starfighter, F 106 Delta Dart,
F-8 Crusader, and F-4B Phantom, were equipped with these
early sensors. Air Force interceptors carried nose-mounted,
nitrogen-cooled, lead selenide (PbSe) cross-array IRST sets
to detect high flying Russian bombers, such as the Bear and
Bison. "

https://dsiac.dtic.mil/wp-content/uploads/2021/10/DSIAC-Monograph-FLIR.pdf

P.S. Worth it to read the entire thing, learned a good amount on the history of IR in this one read

[Fraser]

This is the beastie that I was thinking of when discussing the gas cooled surveillance  thermal imaging systems used in the military. It was made by Thorn EMI.
I note that this example has a date code of 1991 and documentation for it shows 1987. As has been stated, this is after the Falklands conflict of 1982. The units internal design principals appear similar to that of the Hughes Probeye Argon gas cooled thermal imager that was available in 1974 !

https://www.prc68.com/I/ThermalIMagerDFOV.html

Hughes Probeye….

https://www.prc68.com/I/Probeye.shtml

Fraser

[Fraser]

MrSheep,

That document you referenced is a very interesting read. Thank you 

The Javalin AT system is detailed…. I expected more thermal pixels in the missiles seeker head (64x64 pixels FPA) for a system designed as an anti-tank direct and indirect fire weapon. It just goes to show that it is not always about how many thermal pixels you have, but rather how you use them 

Fraser