Author Topic: EDS LN2 Dewar failure  (Read 1372 times)

neko

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EDS LN2 Dewar failure
« on: September 27, 2017, 11:00:40 am »
So we seem to have a sudden issue with the LN2 dewar that cools our EDS (an older PGT Prism-2000). I haven't been able to figure out what this type of dewar is specifically called (in the event we need a replacement).... assuming they're even still made. We're waiting for the LN2 to finish evaporating since it seems to be warm enough to boil slightly in there (you can see the frost around the cap in the picture), even though it's been cold for weeks or months straight.

Does anyone know what this dewar type is called, or some way to describe it to google to get relevant results?

Cheers,
Nick

[Update Sep 28: The LN2 has finished boiling out completely over the course of 6 hours or so. Upon visual inspection there seems to be a seam around the middle of the tank that isn't completely aligned all the way around (I can make a drawing but haven't been able to get a picture) and the parts where you can see the rim of the seam, it's shinier than the surrounding metal, leading us to believe that the seam may have parted with a loss of vacuum insulation causing the system to boil.

Either that or... ? If anyone has any ideas or has had a similar experience, we'd love to hear about it. Looks like we're going to have to purchase a replacement dewar if we can find one.
« Last Edit: September 28, 2017, 10:03:41 am by neko »

jrminter

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Re: EDS LN2 Dewar failure
« Reply #1 on: September 28, 2017, 01:19:46 pm »
Can you describe what happened immediately before this happened? Could something have hit and ruptured the ultrathin window on the detector or was your dry gas vent pressure too high? This occured on a TEM in our lab several years ago when the column was vented to change apertures (gas pressure too high). The boil-off from the dewar on the EDS was as you described - it blew the plug out of the dewar.

neko

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Re: EDS LN2 Dewar failure
« Reply #2 on: October 19, 2017, 02:28:12 pm »
Hi jr,

Thanks for your reply - We think it was indeed a thin window rupture - refilling it while the vacuum is sealed makes things work again until we vent, in which case it will start boiling off.

The venting that caused the rupture is, as far as I know, the same as it's always been. The regulator is labelled, and as usual, one venting isn't actually enough to fully release vacuum on our system, we always have to vent twice. The event happened after a single venting so there was still vacuum in the system (we loosened a spectrometer door to see if it moved, but it was still stuck in place). It's been fine this way for 15 years... but hey, 15 years of rock-solid use ain't bad.

The good news is we have a replacement in hand, which has a visually in-tact window. I'll know more about what the old window looks like after we swap units. The EDS unit with the broken still works properly (I didn't test count rates as I don't have any historical data to compare with) as long as the vacuum remains sealed. Within a minute or two of starting venting, however, the high temperature light will come on and the LN will start boiling. As long as we re-pump fast enough, the LN will stop boiling, but if it warms up too much the vacuum wont re-seal past ~3E-2Pa until the entire dewar finishes boiling off (which is why I suspect the dewar was the problem initially).

Our plug didn't get blown out, but that's probably because I found it and moved it before it could seal itself and pressurize. It did make a lot of ice though, which I had to sit around and carefully clean off before it dripped all over the front end electronics.