Servicing Stirling Crycocoolers - Page 1
Forum section: electronics / projects

author: adammunich
author_rank: Contributor
timestamp: October 19, 2019, 01:01:24 am
content: Hello Forum,I recently purchased pieces of an MWIR camera for my CO2 camera project, and it has a stirling-cycle cryocooler from Ricor.I'm currently trying to understand how they service these cryo-coolers.Best I can tell is that there is a plug on top that they would unscrew, fill the thing with UHP 200-250psi helium, then screw the plug back in using indium as a sealing gasket. I can't imagine any other reason why this plug would have its hex socket slightly deformed if it wasn't serviced before in this way.It seems to me that it would be easier to just machine a new plug and leave a valve connected to it, but I guess I'll cross that bridge when comes to it, as supposedly, this one was last re-filled 3 years ago and might still be good for a while.Still, I am interested in getting in touch with anyone who has experience with these sorts of cryocoolers.

author: coppercone2
author_rank: Super Contributor
timestamp: October 19, 2019, 04:46:29 am
content: I want to know this too. I am guessing valves for helium are not so good. Would look at valve cross sections and seal it extra tight.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 19, 2019, 05:05:09 am
content: Not really an expert but my understanding is the that there are several reasons for the plug approach.First, as is mentioned is the difficulty of sealing helium. But also cost of valves that are almost as good as solid metal seal and size of such valves.

author: Gyro
author_rank: Super Contributor
timestamp: October 19, 2019, 09:11:32 am
content: You would do well to look through the Thermal Imaging subforum. There are several threads relating to cyrocoolers. I believe Forum member Fraser has a good deal of experience with them.You might want to relocate your thread (or ask a mod to do it for you) to that subforum.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 19, 2019, 06:02:33 pm
content: You may find the screw is seated in a indium seal and is not a simple screw shape. You'll also learn the need for "ultraclean" and baking the instrument to drive off water of any kind, vapor or adsorbed. Then pull a really good vacuum on it before filling, while still baking, with a "cold trapped" pump.The tool used is called a "valve actuator" and the prices are astronomical. If have access to a really really good lathe you can make them.Otherwise a copper or nickle tube is used and it is "cold welded" shut. The only way any one would get my cold weld tool is to pry it from my cold dead hands.I don't work on cryopumps, but I work on something that needs similar ultra-hermetic sealing technology.Note for newbies, the pinch tool used for refrigeration is not a cold weld pinch tool. Cold Weld Pinch tools have carbide or tungsten jaws and usually but not always are hydraulically driven. Real tools seal with > then 20,000 PSI.Dont waste your money on the "hope and solder closed" method.Steve

author: adammunich
author_rank: Contributor
timestamp: October 19, 2019, 06:34:35 pm
content: You are right, it is very likely not a simple screw shape at all. But if it can be removed without too much mayhem, it might be possible to drill through it and solder in a copper pipe that can be then connected to a (very expensive) valve, or pinched off as you say. Fortunately there's not much need to worry about virtual leaks in a positive-pressure system like this, so solder is likely O.K.@lasersteve, I do imagine the gas would need to be sourced from some specialty company at upwards of $200 / bottle. I do not think it will be possible to bake the instrument too much though, as it is a permanent part of the camera assembly. Unfortunately it seems they glue these focal plane arrays to the cold finger!For anyone familiar with these Ricor coolers, is this plug on top (image in first post) actually the fill port? Iif anyone has a broken cryocooler on this forum of similar construction, maybe we should tear it downQuote from: Gyro on October 19, 2019, 09:11:32 am@gyro, thank you for the pointer. If there are any mods reading this thread, do feel free to move it to the thermal sub-forum.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 19, 2019, 08:03:18 pm
content: Given the forces that build up inside, that would be one miracle of a soldering job. Think Tig Weld or Oven Braze. Or thread it and install a Swagelok connector with the compressive tube attached. Either way, please do not loose the critical finish on the bore.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: October 19, 2019, 08:06:23 pm
content: Current price in US for a good stock of Helium of high purity is 550 Dollars. I just bought a D2 sized bottle, and a lecture bottle is probably too small for you to work with. Beware of Balloon grade helium with deliberate 10% air in it. Welding grade may be 30-40 % cheaper for a non contract customer in the US. At work where we buy in bulk, you may get to 250$ for a small tank.My opinions on the insane sell off and depleation of our national reserve here are well known.My two coolers have pinches, so I cant help you there.Steve

author: ejeffrey
author_rank: Super Contributor
timestamp: October 19, 2019, 09:13:33 pm
content: I haven't worked with smaller devices like this, only on bigger systems used for cryopumps or superconducting magnets. In the cryocoolers I have used helium replacement is not something you do as any kind of maintenance. If properly sealed the helium leak rate should be negligible. The only reason you recharge is if you either screwed up, or you have to change the delivery lines between the compressor and the cold head. Every time you crack the seal is just another chance to get contamination and it is really hard to clean out if you have to.Do you actually know that the pressure is low or if there is contaminated helium you have to purge? Unless you have a reason I wouldn't mess with it.

author: coppercone2
author_rank: Super Contributor
timestamp: October 19, 2019, 09:28:39 pm
content: what is the pressure that you need to plug ? why is it too difficult to solder?keep in mind brazing pipework messes it up because the heat causes something like 'zone purification' to occur in the thin pipe (at least in pictures that I have seen).

author: LaserSteve
author_rank: Super Contributor
timestamp: October 19, 2019, 11:32:12 pm
content: Oven or "Bottle" brazing is done in H2 or Forming gas. Your parts come out bright, shiny, oxide free, and annealed in many cases. At least for the processes I've contracted for or witnessed.There are silver based brazes we use in the lab with minimal damage to copper. The trick is proper surface prep , the proper oxidation state of tbe flame, and a flux that you really never want to be around on daily basis. They are not exactly cheap, either. I doubt you'd see them in Europe due to small amounts of Cadmium in them.On a production basis the braze is in the form of a "preform" or near net shape. For pipes and flares I make my own preforms.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: October 19, 2019, 11:42:57 pm
content: Several hundred Psi of Helium under compression by a free piston at like 30 to 60 Hz generally calls for something other then lead based solders, which are subject to brittle failure. Especially if the mating substrate is Steel.You might want to find a modern edition ofHandbook of Electronic and Vacuum Tube Techniques by Fred Rosebury. It covers some of the "old" ways. The one free copy I found online was very early and incomplete.Steve

author: coppercone2
author_rank: Super Contributor
timestamp: October 19, 2019, 11:49:47 pm
content: yea that sounds vicious on plumbing, not what I had in mind. sounds very temprementalI was imagining torch brazing

author: adammunich
author_rank: Contributor
timestamp: November 14, 2019, 11:12:55 pm
content: Seems I missed the new reply bell, sorry about that!I wouldn't consider soldering to the cooler itself, just the plug that keeps in the gas. Vibration seems to be relatively low on this machine, it is not a free piston type (i wish it were though, it would last longer!).I did get my hands on a different cryocooler though, a DS 1.5 from sunpower. It supposedly works if you give it a 60HZ feed signal in the two internal voice coils, but I tried this and didn't get much cooling, just a warm hot end. I wonder if the displacer is stuck in the cold finger somehow.