This is a quick-and-dirty test to see how hot the neighborhood around a cartridge heater equipped MK5 might get, with the intent of determining where to put an overtemperature cutout switch.
I stuck one thermocouple inside the Kapton tape wrap, outside the ceramic wool insulation above the left-hand heater block, to get an idea of the actual surface temperature. Another thermocouple rests against the small heatsink at the top of the Thermal Riser, where it’s probably measuring a bit of the heatsink and some air temperature; it should be inside the small brass tube epoxied to the heatsink, but I was not going to tear the head apart for that.
As before, a pair of 25 W cartridge heaters raised the internal Thermal Core temperature to 225 °C in a bit under 15 minutes, according to the TOM’s usual thermocouple. I think baking the water out of the insulation wrap had a lot to do with the decrease, but the difference wasn’t more than a minute or two.
After cooking at 225 °C for 15 minutes, the outside of the insulation stabilized at 133 °C. Opening the front window of the build area let enough of a draft inside to affect the temperature, even under the tape wrap, so that’s definitely not a solid temperature.
The thermocouple at the top of the Riser Tube reached 83 °C and was also affected by drafts.
The thermal cutout must be solidly mounted to either the Core itself or the Riser Tube, in order to prevent irrelevant temperature readings. I’m beginning to favor the Riser: it’s out of the way, shouldn’t get too hot in normal operation (because it’ll melt the filament), and has a solid thermal connection to the Core. A pain to get access in there, but you only need that occasionally.
The only question now is how to determine the actual temperature seen by a thermal switch in there. I think a clamp around the tube with a tab sticking out beyond the support structure is in order.