Hello everybody,
I have an update on my own table. I just made various measurements on a C245-906 and a C470-015 tip.
First, I measured the thermocouple in a molten wax bath (two to be precise).
For this, I melted roughly 250 ccm of wax in a pot with a diameter of 8 cm and a height of 5 cm. The idea was to have a big mass that cools down slowly and uniformly. Unfortunately, it became increasingly difficult to heat the big bath beyond 170°C, and the fuming became excessive; hence, I stopped at that temperature with the big bath.
To get above 170°C, I created a much smaller bath of 2 ccm, which only fit the C245 tip. However, the two tips behaved very similarly in the big bath; hence, I think the thermocouples in these are of the same materials, and we can transfer the findings to the C470 tip.


The second run with the smaller tip definitely produced some interesting changes in the µV/K values at higher temperatures. Thermocouples are not linear; however, the big jump of the µV/K and the fact that the second run ends up with lower µV values at the same temperature suggest that the cold junction heated up.
I had another thermocouple in the banana jack of the DMM that measured the voltages, which showed no significant temperature change. Hence, the cold junction must have been somewhere else. Can anybody elaborate on where the cold junction is, when you have multiple metals and hence thermocouples connected in series?
Furthermore the data hints to the thermocouple's non-linearity coming into effect above 170°C and the µV/K rises to somewhere in the 25 µV/K region. Future research in this region is needed (or just a multipoint calibration of the soldering station).
Lastly, I made several measurements that confirm the equivalent circuit from the first post. If I should elaborate further, please let me know.
If anybody is willing to donate a C105, C210 or C420 tip for the same torture - measurements I mean - please feel free to contact me. However I can't guarantee, that the tips survive. I haven't powered mine yet, so I don't know if the wax got into the tip and it will explode when it heats up.