What is finally the correct TC? TC 120, 40 W, 390F? TC 180, 40 W, 370F? TC 220 (I read here somewhere), 35 W, 380F?
One reason there's so much variation in recommended TCR values is that people also tweak their cold ohms settings, and both of these are factors in the mod's calculation of the coil temp in TC mode.
You can see this in the formula that relates resistance, temperature, and TCR. Rearranging the terms from the formula
here, you get:
T2 = T1 + [(R2 - R1) / (TCR x R1)]
Where R1 and T1 are baseline resistance (in ohms) and temperature (in Celsius), R2 is the resistance at the higher temperature, and T2 is the temperature readout of the mod (again in Celsius). In other words, the mod uses this formula to "guess" at the current coil temperature.
Both TCR and R1 are in the denominator of the second right-hand term, so
increasing the value of either one will decrease the calculated value of the current temperature, which will make the mod keep feeding current to the coil until it reaches your designated temperature. In other words, increasing either your TCR or your base resistance will make your coil hotter for any given temp readout. (Also note that increasing R1 not only increases the denominator of the right-hand term, it decreases the numerator, so it has a kind of double whammy effect on the calculated temp.)
Of course the accuracy of this formula depends on using accurate values: in other words, your TCR really is your temperature coefficient of resistance, and R1 really is your measured cold ohms. If you use another value for cold ohms, all bets are off about whether the TCR value that works well for you is anywhere close to your actual temperature coefficient of resistance.
Practically speaking, it doesn't really matter if you change the TCR value, or the cold ohms setting, or both, in order to tweak the relationship between actual and calculated temp. And of course you can also just reset your target temp higher or lower to accommodate any error in the calculated temp. But if people aren't using their measured cold ohms value, then comparing TCR values across different setups is apples to oranges.
No less an authority than
@HerbieVonVapster counsels people
here not to deviate from their measured cold ohms setting for this very reason! So ideally I guess we'd all just be tinkering with our TCR values, and maybe that would eventually narrow the range of suggested TCRs. On the other hand, there are so many other sources of error in calculating the temp -- the "cold" temperature setting, the assumption of a linear relationship between TCR and temp (which can be relaxed with TFR files), variations in the splinters themselves, and then just figuring out whether the coil's behavior suggests it's actually at a certain temp (since we have no way of independently measuring temp unless we get crazy with thermocouples) -- that there's probably no getting around each of us having to tweak our own setup, using others' settings as a starting point. But if people are going to share their TCR values, it might also be useful to know whether they tweaked their cold ohms value and by how much.
I do not complain. Maybe I got used to my herbs.
