Hello Dimitri!

If you are talking about an ESR900 cryostat,  you will be able to open the black housing of the cryostat body, but unfortunately find a system of tubings and other things that basically cannot be disassembled without destroying it (it is completely welded/soldered). Did you try to blow the whole thing just from the He inlet? You can also rinse the whole system (also from the He inlet) with different solvents as long as they are not corrosive. Maybe this is sufficient to remove a possible blockage. Otherwise, I'm afraid you have to send it to Oxford for repair, unless you have a workshop  with good people (pref. experts in vacuum system construction/maintainence) at your university."

But let me know if the problem persists...maybe we can resolve it without involving Oxford. I know a person in our department who replaced the heater in one of these cryostats some years ago...he has a bit more experience with them).

All the best,

Johann.

We have an ESR 900 Oxford cryostat.

http://www.oxford-instruments.com/products/cryogenic-environments/optical-cryostats-for-spectroscopy/liquid-helium-cryostats-%284k%29/esr-epr-spectroscopy-x-band-esr900-esr910

1 Check if the lever is not clogged (I assume you did it)

2. Check if you can create vacuum in the system. You will be shure that the glas part that is inside of the resonator is not broken and properly sealed with apiezon in the metal socket.

3. In this glas part there are holes that need to fit to the same holes in the metal part to have vacuum in the glass part.

4. Maybe the glass part is not put inside correctly. You can take it out and put new apiezon and put it back. The glass part is expensive if you brake it.

5. If that won't work than the round black box can be open. It is vacuum sealed with imbus screws cup and a gasket. I think that a hole somwhere there would force problems with vacuum. But who knows...

Dear Johann and Krzysztof, thank you for your replies. I will only be able to confirm that my cryostat is ESR900 on Monday tomorrow, when I am back at work but the picture on the website seems identical to what I remember. The quartz part is OK, and I am confident I put it in place the same way I did numerous time before, after cleaning, so that should not be an issue. Also I don't think this is a vacuum problem and the gage at the pump shows a good vacuum, although a slight concern is that the cryostat body (the black cylindrical part) is cold when touched, after running helium at a fullly open valve for 0.5 h)  - not ice/frost cold but still. Maybe this is normal? 

Yes, I did blow air through the cryostat before use but if the path I think is involved is blocked, I would not notice it.

I have a new hypothesis since I posted my message. It could be not a blockage but a leak just before the point where helium should be coming to the thermocouple/sample path. I so, helium will be sucked, through the leak, straight into the out path, with no resistance from 'sample path'.

One annoying thing is that the trouble is intermittent: on Friday late afternoon it got itself sorted and I managed to set 10 K on the thermocouple. I switched the pump 3 times off and on, I the temperature would still come to 10 K with no problem. I really look forward to tomorrow - to see how it is going to behave after being warmed up to room temperature over the weekend.

The most annoying thing is that this sort of troubles (as well as the chances to investigate them) only happen once you have paid a grand for the helium...

Hi Dimitri,

I believe that your new hypothesis is the correct one. In fact, I'd wager that your intermittent problem is caused by a leak between the helium jet and the quartz tubing that is "sealed" to it by Apiezon grease. This is the piece of quartz tubing into which your EPR tube is inserted during use. The problem is that you have two materials, quartz and metal, with two different coefficients of expansion, and you are hoping that the grease will somehow maintain its seal all the way from room temperature to 4 K. This caused me so much grief as a graduate student that I machined a new piece to hold the quartz tube.

Thanks, Donald. Yes, I think you are right!