You are quite correct. TC is merely a calculation based upon resistance change and the known TCR of the metal. We do not directly measure the temp of the coil. About the only way I can imagine doing that would be to attach a teeny tiny thermocouple to the wire. Or the wick. Or the chamber. Or 3 teeny tiny thermocouples to all of them for monitoring. Not very practical, and not known which one of those thermocouples will provide the data required to determine the probability if producing nasty stuff. But if you vape on a silica wick in a SS chamber like that performed in the Wang PLOS paper, I guess you could use a direct measurement as they did to extrapolate the risk at a given temperature.
I think concluding that higher temperatures are more likely to result in breakdown products of possible toxicity. And the Wang study (or Evolv study because they're the ones who dragged that in to begin with) appears to demonstrate that, under the laboratory conditions as spelled out in the material and methods, aldehyde production rises considerably about 470F, and primarily is derived from the glycerin/gylcerol stuff, and little from the PG (let's not go anywhere near flavorings as that's a whole separate 50 page thread). So, how does that correlate to real world experience?
The real world experience is shown in this other slide (posted before) by separate investigators measuring aldehyde production with actual tanks and coils we vape on.
Walking through this again, measurements of actual levels of formaldehyde and aectaldehyde with a Subtank coil (maybe Kanthal maybe SS they don't say) produces very small amounts with wattage (not temp control) settings up to 26W. The absolute amounts measured are lower than those reported by Wang/Evolv but it wasn't being done by temp so there's that issue. In addition, it appears from that slide that the same holds up for a Nauti, a Cubis, and a KF clone.
So in a real world setting, with hardware in actual use in the wild and not a SS chamber with a glass wool/silica wick, there isn't anywhere near the levels we would be overly concerned about. What is a bit weird is comparing the emissions above with the emissions reported in the Evolv presentation.
Now, the scales are different, and Evolv seems to be reporting Acetaldehyde (not formaldehyde too, and not sure why not) in amounts per 25 puffs. They then attempt to compare (at least it's the way I see it) the 25 puff exposure to 1 cigarette. It appears when correct to per puff, at the 480F mark which crosses the red cigarette line, it would be 1000 µg/25=500 µg per puff. Those other guys above who did the test with actual vape gear is lower by several orders of magnitude of about 0.5 µg/puff at 25W (total aldehyde 0.34+0.16=0.5 µg).
So the interpretation I take away from that back of the envelope statistical analysis is that there remains considerable variation in the measurement of bad stuff with vaping which will hopefully be clarified with further research. Whether you decide to run with the Evolv curve created out of Wang's published study or the info from Kistler's presentation about actual tests done with a Subtank, it's apparent that running a hot coil is probably a bad idea for your juice (come on, formaldehyde just does not taste good) and maybe your health.
