CRYOGENIC COOLER problem Cooled thermal camera

[geggen]

Hi i think there is a bearing problem with my cooler,  it runs 2-3 minutes normal.  Then it runs slower and slower untill it stops.  It looks like the bearings heats up/cools down and siezes up.

Is it possible to change the high speed bearings inside this cooler?  and fill it with new helium gas

[Fraser]

This thread may be of interest to you.........

https://www.eevblog.com/forum/thermal-imaging/thermal-camera-purchasing-the-perils-of-buying-a-cooled-camera/

When a Rotary Stirling Cooler fails it usually either sounds pretty normal but never achieves operating temperature at the cold finger, or it sounds like a tin can full of bolts being shaken  Rarely have I seen much in between these two states in terms of failure. In your case I would suspect that you are actually experiencing piston lock rather than bearing or flexture failure. The Pistons are precision parts and run in a very pure Helium environment that will not freeze at the intended operating temperature. If the Helium fill has been lost through leakage past a seal or deliberate act, then the cooler can contain atmosphere or pollutants and that gas environment can very easily freeze causing the pistons to become locked in their cylinders.

Are such coolers repairable ? Yes, but at very high cost. Something around $10K by a manufacturer. Remember that the imaging array is integrated into the coolers cold finger Dewar so either the original cooler must be rebuilt or a new/rebuilt cooler bought from the camera manufacturer. Both are very expensive options.

Can a Stirling Cooler be DIY rebuilt ? In my opinion, no. Such a rebuild would be beyond most home workshops as these really are very high precision items and are normally rebuilt in conditions similar to a hard disk construction or repair lab. To truly rebuild such a cooler, all worn parts are changed, all new specialist soft metal seals are replaced (Helium seals are specialist metals) and finally the Helium Gas must be introduced to the cooler and the sealing plug fitted without any Helium gas contamination occurring. That includes the presence of atmosphere in the housing before fill or any lubricants that would contaminate the gas fill or freeze at 77K. Not a DIY job.

Can a cooler be just refilled with Helium gas ? Yes but with caveats ....

1. The Cause of the Helium gas loss must be addressed or the new gas fill will escape.
2. The Helium gas is specialist Ultra High Purity Helium and not just “Balloon Helium”. Any contamination of the gas with atmosphere or pollutants can lead to freezing and piston lock, just as you are already experiencing !
3. The atmosphere within the cooler should be purged to remove any contaminants in preparation for the Helium gas fill.
4. Special fittings and tools are required to use the coolers fill port and seal it after filling.
5. Specialist soft metal gas seals are used in the cooler, including a single use (non reusable) seal on the fill plug. A new fill port seal will be required.
6. Even those who have addressed the above issues have found the process a complete failure. A cooler may have suffered wear or failure that cannot be seen without a complete rebuild. Basically, you are taking a risk spending money on a re-gas without a full cooler rebuild.

Fraser

[Fraser]

Another thread that may be of interest....

https://www.eevblog.com/forum/thermal-imaging/inframetrics-flir-pm280-cooled-thermal-camera-teardown-for-repair/

Can you tell us more about the camera that the cooler was used in please ?

Fraser

[Fraser]

For information, these coolers usually achieve operating temperature in 5 to 10 minutes. As they get older and more worn, or suffer Helium gas leakage, the time taken to reach 77K increases until eventually 77K cannot be achieved and the cooler never drops into the slower rpm ‘maintain’ mode of operation.

The expected life of a camera rotary type Stirling Cooler was around 2000 hours of operation but some failed sooner, whilst others lasted a lot longer. This life normally relates to wear in the machine but loss of Helium gas through a leaking seal can lead to premature failure or failure over time as opposed to hours run. Newer Rotary type Stirling coolers have a run time life of around 6000 hours but thanks to improved design they do last a long time in service. The linear type coolers, as used in cameras like my FLIR SC4000, have a much longer predicted life of 10000 hours but have been found to be working after 100K ‘simulated’ hours in NASA tests.

Fraser