I worked with SEM/EDS using an LN2 Dewar to cool the EDS detector for about 10yrs. Never seen it happen. However, this older article does anecdotally report it happening and resulting in ignition.
"…We heard of an accident where ignition was due to the condensation of oxygen from room air in liquid nitrogen and subsequent elaboration of an oxygen-rich gas as the nitrogen boiled away. Because of the fire hazard, one should neither allow liquid N2 to evaporate to dryness…"
J. Bastacky, T. L. Hayes, Safety in the Scanning Electron Microscope Laboratory, SCANNING Vol. 7, 255-272 (1985)
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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.
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.
Laboratory Safety Advocate
Johns Hopkins University
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