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.