What is the flash point of acetylene?

On episode s10e11 of the show Criminal Minds, a confrontation occurs in a freezer full of acetylene gas. In this confrontation, the main character confidently fires her gun because the freezer was "18 below" which is below the flashpoint of acetylene.


This fact seemed ludicrous to me because if the temperature really was below the flashpoint of acetylene, then my knowledge of how flashpoints work tells me that the acetylene should be condensing out of gas phase into either solid or liquid, and acetylene sublimes at -84°C.




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So what really is the flash point of acetylene?


[PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/Acetylene#section=Flash-Point) says it's -18°C citing an [AirGas MSDS](https://www.airgas.com/msds/001001.pdf). I've tried to search for a primary source, but I feel like all the MSDSs are just citing each other.


The flash point must be below the sublimation/boiling point right? (Which is -84°C for acetylene)




The UK National Fire Chief Council reports the flash point of acetylene with $\pu{-17 ^\circ{}C}$ ([reference](https://www.ukfrs.com/guidance/search/acetylene)); while the US American (Californian?) [firefightinsider](https://firefighterinsider.com/acetylene-flammable/) states $\pu{-0.7 ^\circ{}F}$ or $\pu{-18.15 ^\circ{}C}$. It didn't take long to find these values (open access) on the internet. I speculate average Jane and Joe (regardless if they are [gas] welding, or not) don't mind the small difference but treat the gas with the respect it deserves.


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My comment is getting cited by Google, so here's my best attempt at calculating the flash point. I'd still love for someone to give a primary source for the -18°C number though.


In 1915, Burrell and Robertson did quite a bit of research on acetylene. They published one paper1 showing that acetylene has a range of flammability behaviors between 2.4 and 3%, but concluded that the Lower Explosive Limit for acetylene is 2.53%. In another paper2, they calculated the vapor pressure of acetylene all the way down to -143°C, but they flipped a sign in their equation and so I don't think modern sites like [NIST](https://webbook.nist.gov/cgi/cbook.cgi?ID=C74862&Mask=4&Type=ANTOINE&Plot=on) picked it up. (It should be + 0.001511).


[![Acetylene vapor pressure graph and flash point calculation](https://i.stack.imgur.com/X7p4e.png)](https://i.stack.imgur.com/X7p4e.png)


Using their corrected equation along with the 2.53% Lower Explosive Limit, I calculate **-124°C (149 K) as the flash point of acetylene**.


1Burrell, G. A., & Oberfell, G. G. (1915). The Explosibility of Acetylene (Vol. 112). US Government Printing Office.


2Burrell, G. A., & Robertson, I. W. (1915). THE VAPOR PRESSURES OF ACETYLENE, AMMONIA AND ISOBUTANE AT TEMPERATURES BELOW THEIR NORMAL BOILING POINTS. Journal of the American Chemical Society, 37(11), 2482-2486.


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