Stirling Cycle cooler with Helium charge..... anyone worked on these please ?

[Fraser]

Those who know me on this forum are aware that I have reasonable knowledge of thermal imaging cameras and their internal components. Well I am about to venture into an area with which I am unfamiliar...... the Stirling Cycle cooler.

I have just purchased a circa 2001 FLIR scientific thermal imaging camera that uses such a cooler to take the Quantum Well detector down to 70K which is pretty darned cold    Whilst I have used such cameras, I have never worked on the cooler as it is normally considered a 'return to base' job if it needs a service. The cooler service from FLIR was around $7500 so I will not be sending my camera back to them any time soon.

The cooler has a run time life of around 8000 hours but they often perform well beyond that point. The greatest issue is the leakage of the Helium gas charge over time. I believe it finds its way around (through?) the seals until the charge is insufficient to cool the detector to 70K. I am half expecting my latest purchase to arrive with a Stirling Cooler that needs a Helium recharge.

I have seen Air Conditioning systems being recharged using a vacuum pump and a charge cylinder containing whatever gas is used in the particular unit. I am considering whether it is possible to recharge the Stirling cooler in a similar manner using the appropriate vacuum pump, gas valves and one of those party balloon disposable gas cylinders.

No surprise that there is nothing on the NET detailing such as Stirling Cycle coolers are not the most common of devices. Does anyone on the forum have any experience of Stirling Coolers in terms of whether a gaseous charge is used or a high pressure liquid Helium fill is required ? My understanding is that Helium remains as a gas except at extremely low temperatures. much like liquid nitrogen. If a gas charge is used, I am uncertain how much pressure is required in the cooler for it to operate. I have seen some designs working at just 1 Bar. As I stated, this is very new territory for me so any thoughts or advice greatly appreciated.

For anyone unfamiliar with these types of coolers....they are like mini refrigerators that take a 'cold finger' down to temperatures more commonly seen when using liquid Nitrogen ! The cold finger can cool many types of semiconductor and not just thermal camera detectors. Such technology is to be found in very low noise amplifiers such as those used in radio astronomy.

I attach the datasheet for what I believe to be the cooler used in my camera.

[Fraser]

I just did a search on the FLIR and Inframetrics cryo-cooler patents...found some interesting documentation.

[IanB]

A Stirling engine used as a cooler in this way will be a completely sealed closed cycle unit, just like a refrigerator. So there should in theory be no mechanical seals for the helium to escape from (oh...except the motor?), but in reality helium gas is an escape artist to rival Houdini and it is very hard to keep it contained. (The reason is that helium has very small, very mobile molecules. It will find a way through the smallest of atomic sized holes.)

I think the reason helium is used in such a cooler is that it will not condense into a liquid or freeze at the temperatures involved. Consequently, the helium charge must be very pure, and therein lies the problem of recharging it. If you get any moisture, or carbon dioxide, or other impurities in the charge they will solidify and form a frost on the cold parts of the machine, which may interfere with its working.

This is all theoretical inference on my part, as I have no practical experience of such coolers. My suggestion would be first of all to test it and see what you have. Do you have a way of measuring 70 K? Maybe an IR thermometer or a thermocouple may be able to do this?

Mechanically, the machine should be very simple and there is almost nothing to go wrong.

[Fraser]

Thanks IanB,

Yes I am told that the Helium can find its way out over many years. Interestingly, I have only met one cooled camera that failed in service as most others went for calibration and re-gas regularly. The one that failed was an AGEMA Thermacam 1000 CCTV surveillance camera that had accidentally been left running for many months. Having read the Patents for FLIR and Inframetrics Stirling coolers it is apparent that wear in the mechanical bearing assembly is likely a bigger issue as these units have precision timing in order to operate efficiently. The Thermacam 1000 needed a new cryo cooler ($$$$) so mecahnical  wear is a likely scenario in that particular case. I have seen a couple of cooled cameras working after a decade in service, with no re-gas in their operational lives. I am hopeful that if the camera has a fault, it will not be in the cooler 

I have been hunting for one of these scientific grade cameras for ages but they were always well beyond my hobby budget. Even if the unit has a dead cooler, I can still make use of the parts and the price was right    It would also make an interesting paperweight on my desk at work 

Many thanks for the comments, and especially for the comment on Helium purity....I had not considered that. BOC in the UK could probably supply me with quality lab grade Helium if needed. I have confirmed via the Patents that the crankcase of the cooler is pressurised, but no mention of how many atmospheres of pressure    Some more reading may be needed to better understand the needs of these units.

I will advise on  the outcome of testing once I have received the camera. It will either be  or  depending upon the behaviour of the cooler over the 6 minute cool down period.

[ejeffrey]

I have used Cryomech pulse tubes.  They are basically a 2-stage stirling cycle cooler -- the first stage reaches around 50K, the second goes to 3k.  They need UHP helium gas and they need to be filled to about 230 psi.  It is possible to do yourself if you have a source of UHP helium and the proper regulators and fittings, but you have to be careful.  If you contaminate the gas, you pretty much have to send it back. That happened to one of the pulse tubes in our lab.  Cryomech gave us directions on how to flush and recharge it, but we were unable to do so and in the end had to return it for service (although it is possible contamination wasn't the real problem -- I don't know what they fixed).

Helium only liquefies below about 5 kelvin, but these cryo coolers normally operate above the critical pressure anyway so there is actually no liquid/gas phase boundary.

[Fraser]

@ejeffrey

230 psi UHP Helium    

Me thinks I will not be able to re-charge such a Stirling Cooler.

Many thanks for the comments. Most helpfull.

[ConKbot]

A Stirling engine used as a cooler in this way will be a completely sealed closed cycle unit, just like a refrigerator. So there should in theory be no mechanical seals for the helium to escape from (oh...except the motor?), but in reality helium gas is an escape artist to rival Houdini and it is very hard to keep it contained. (The reason is that helium has very small, very mobile molecules. It will find a way through the smallest of atomic sized holes.)

I think the reason helium is used in such a cooler is that it will not condense into a liquid or freeze at the temperatures involved. Consequently, the helium charge must be very pure, and therein lies the problem of recharging it. If you get any moisture, or carbon dioxide, or other impurities in the charge they will solidify and form a frost on the cold parts of the machine, which may interfere with its working.

This is all theoretical inference on my part, as I have no practical experience of such coolers. My suggestion would be first of all to test it and see what you have. Do you have a way of measuring 70 K? Maybe an IR thermometer or a thermocouple may be able to do this?

Mechanically, the machine should be very simple and there is almost nothing to go wrong.

Minor correction, since helium is a noble gas, its monoatomic, which means it can fit though damn near anything. Vs hydrogen (which tends to work its way into lots of stuff also) which has 2 atoms in its molecule, making it larger. 

Given that a cooler servicing is $7500, I'd imagine you'd be able to recharge it for less, but I'd expect it to still be pricy on the hobbyist scale.   You can probably get a bottle of UHP helium in a laboratory size bottle (I.e. something you can hold in one hand) and some cleaned&bagged regulators/hosing which would do the job.  Or you could buy a new oscilloscope for the same cost :p   Plus who knows what else they do in that $7500 service.

[IanB]

Minor correction, since helium is a noble gas, its monoatomic

Fair enough, although molecule tends to be the standard term for a gas particle, irrespective of what gas is being talked about:

http://en.wikipedia.org/wiki/Molecule

It would be confusing to have to keep switching between "atom" and "molecule" depending on which gas is being considered.

[amyk]

It will either be  or  depending upon the behaviour of the cooler over the 6 minute cool down period.

If it happens to be , a teardown video would be

[ganzuul]

Part 1:


Part 2:



Hydrogen has a tendency to weaken metal. Anyone know what can be done to prevent this?

[nunobs]

Only a comment about the hellium pressure:
I have an axsys fieldpro 5x camera that has a cryo-cooler from RICOR Fn Harod Ihud Israel
(PN:705C118A) that has a sticker on the cooler stating a pressure of 35 Atm... (about 510 PSI).

[SeanB]

Don't worry, go to your local Tesco and look at what they use as refrigerant, CO2 at supercritical pressure, or basically at "Yougottabefuddunkidding" pressure, where you can have the equipment explode if you have liquid trapped in a valve.

The problem with H2 IIRC is that it is not only a small molecule but is also very reactive, wanting to bond to the edges of the crystal lattices in metals, thus disconnecting the bonds that keep the crystals strongly bound to each other. This weakens the crystal structure such that only the mechanical shape keeps it together, and the crystals can slip over each other. Thus you have a very limited lifetime for metalwork that is exposed to Hydrogen at any pressure, as it will weaken with time along with losing ductility and becoming brittle. There is no remedy other than choosing things like pure nickel and thick steel, and replacing it at regular intervals before it weakens to the point of failure. You recycle it by melting it, which drives the Hydrogen out of the solid solution.

Helium, while a smaller molecule, which tends to treat most materials as a minor obstacle in the way to getting to outer space, does not react chemically much, which keeps the metal intact.

A nasty gas is Fluorine, which reacts with pretty much everything, including noble gases. The trick is to choose containers where the fluoride layer is well bonded to the base metal, thus passivating it. Thus every thing is made from pure nickel. You know it is nasty when the fire fighting instructions are "wear running shoes, and get far away" and putting out the fire is simply "Don't bother, let it burn out".

[Dan_o]

@ejeffrey

230 psi UHP Helium    

Me thinks I will not be able to re-charge such a Stirling Cooler.

Many thanks for the comments. Most helpfull.

You can get 999 (99.999 % pure) helium cylinders charged to 300 bar - then you would just need a gas regulator to bring it down to 15bar. Purge the lines of air, and connect to the fill port (i assume it has one) of the cooler.
We use this technique on MRI machines to keep the liquid helium in the magnet at 4k. The gas is available from BOC if you are in the UK.
The regulator is something like this: Regulator - but you wouldn't need such high pressure on the fill side.

[Fraser]

Hi Dan O,

Many thanks for the comments.

I have not had to face the recharge situation as yet as I traced the fault in the thermal camera cooler to main microprocessor PCB that controls power to the Cooler. I could hear the cooler pulse when power was applied but that was just a switch on glitch. The cooler is not being supplied with power and the camera id not booting so the fault likely lies elsewhere.

I have never recharged a cooling system and would have to think carefully before attempting such as I old enough to know that some operations are best left to those who do it as a daily job. Hopefully the cooler is still fit for purpose and working within spec. It does appear to have been serviced in the past as their are witness marks around the filler port but it is still fitted with the FLIR seal.

I have not had time to play with the camera recently but that will change shortly so I will advise here if the cooler runs OK or not. Thanks again to everyone who has offered comment and advice. Very interesting.

Aurora