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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.

So what really is the flash point of acetylene?

PubChem says it's -18°C citing an AirGas MSDS. 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)

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  • $\begingroup$ I believe your reasoning is correct. Perhaps the Airgas MSDS refers to acetylene dissolved in acetone? We can't know for sure until we find the original source! $\endgroup$
    – Anger Density
    Commented Sep 20, 2022 at 18:50
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    $\begingroup$ Considering acetylene dissolved in acetone (liquid pressurised acetylene would be powerful unpredictable explosive), they would rather measure the flashpoint of acetone, which is -20 Deg C. $\endgroup$
    – Poutnik
    Commented Jul 22, 2023 at 5:49
  • $\begingroup$ The SDS on this gas states -18C or -.29F as flashpoint. In the show, that would be referencing -18F which would tranlaste to almost -30C, in which case she is right, and they are well below the flashpoint stated on the official Safety Data Sheet. $\endgroup$
    – Jill
    Commented Oct 13, 2023 at 18:13

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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 picked it up. (It should be + 0.001511).

Acetylene vapor pressure graph and flash point calculation

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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The UK National Fire Chief Council reports the flash point of acetylene with $\pu{-17 ^\circ{}C}$ (reference); while the US American (Californian?) firefightinsider 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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    $\begingroup$ Well so, you've identified the crux of the issue again here. Everyone reports values of -18C, but where did they get that number from? Acetylene boils at -84C, so presumably it's flash point should be around -100C or something. Already at -84C, acetylene is capable of reaching 100% air concentrations. $\endgroup$
    – thepenguin77
    Commented Jul 21, 2023 at 13:20
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    $\begingroup$ There still is the possibility to get in touch with the suppliers in the field (e.g. AirLiquide) for a literature reference to cite. Though, for safety reasons (e.g., to prevent decomposition when closing the valves) their gas cylinders typically contain it as solution in acetone on solid support … $\endgroup$
    – Buttonwood
    Commented Jul 21, 2023 at 20:38

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