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Although this does not involve oxygen collecting in a LN2-containing Dewar, it is a comment on safety - or lack thereof. The exact details have been clouded by time. During 1955-6 as a graduate student at the U of Minnesota I was engaged in tow-temperature x-ray crystallography of boron hydrides. LN2 had to be added frequently to the camera system from a 2 L Dewar carried upstairs during month-long round-the clock data collection run from a storage Dewar. We had no formal safety instruction. One day I noticed that the LN2 was distinctly blue and didn't want to use it. While the bulk storage Dewar was being replaced by the vendor, I took a bus to the near-by plant and returned on the bus with a "smoking" Dewar of LN2 to save the data collection run. Later on, the run was terminated when a student tending the apparatus at night, tripped coming up the stairs.
Ernest Lippert
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From: "Heather Redmon" <redmonh**At_Symbol_Here**GMAIL.COM>
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Sent: Friday, March 12, 2021 11:13 AM
To: DCHAS-L**At_Symbol_Here**Princeton.EDU
Subject: Re: [DCHAS-L] Condensation of liquid oxygen in open Dewar flasks of liquid nitrogen
I am not sure the LOX would displace the LIN to make an oxidizing mix. Unless you are using refrigeration the LIN will evaporate as it warms, so the O2 and other gases in air would not coalesce. Dewars usually work like fancy thermoses, and don’t provide refrigeration.
The bigger hazard would be oxygen displacement in the room if it were not well ventilated I would think.
Thanks!
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On Fri, Mar 12, 2021 at 7:26 AM Daniel Kuespert <0000057d3b6cd9b7-dmarc-request**At_Symbol_Here**lists.princeton.edu> wrote:
An interesting question occurred to me today: When you leave a Dewar flask of liquid nitrogen open to the atmosphere, such as when you’re using it to cool the trap of a Schlenk apparatus, oxygen will condense in it since the boiling point of oxygen is 90 kelvins vs 77 K for LN. How fast this happens will depend on the air-exchange into the flask, so if it’s covered loosely, presumably the dynamics will slow down. Eventually, though, the flask contains liquid air, not liquid nitrogen, which could become excessively exciting for someone who empties the flask by dumping it out somewhere near something combustible.
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I’ve not seen any data, though, on how fast this actually happens. Has anyone ever seen data on this? If you have, please let me know. It would be useful to know how long one can really leave an open Dewar sitting around.
Regards,
Dan Kuespert
Laboratory Safety Advocate
Johns Hopkins University
Daniel Reid Kuespert, PhD, CSP
410-992-9709
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