Possibly an obtuse question, but a PTC heater should have the same temperature regardless of the environment right? Providing the environment's temperature is lower. Any environmental difference should just contribute to getting to temperature quicker.
There's a bit of confusion here. The heater in the bed is controlled by a PTC sensor, most are not a "PTC Heating element" as the temp needs to be adjustable. With the heated bed, the PTC sensor is attached to the underside of the printbed in a calculated voidspace in the middle of the heating element. As designed, this presents a pretty good closed loop for temp control. When you go adding material, like a 3mm mirror tile, on top of that, you add multiple covalent layers between the heat source/sensor and the print surface. This tends to make the control loop a little bit more... pear-shaped, if you will.
If I smeared some thermal grease between the mirror tile and the Tevo-Tak, it would probably be right on. But that would pretty much destroy the "cheap & convenient" part of the mirror tile equation.
A for effort. That’s more impressive than RevK’s ADSL over wet string.
EE prank in future: replace potting compound with vegemite
Heh. I remember getting into an argument with an "armchair engineer" over how important "Arctic Silver" type thermal compounds were. I explained that they just became popular with processors that had a very bad thermal design, like early pentium-xx with the bare core exposed. Later designs that had a proper envelope with sufficient covalent area didn't need it, even under duress.
I explained that with a properly designed component that had adequate covalent area, what was more important was stability of formula and manufacturing process of the paste. I proved my point by replacing the thermal paste on a P4 with peanut butter, and overclocking it to 3GHz. It got a few degrees warmer than with the arctic silver then stabilized and was quite happy. Now if we'd left it that way long enough to cook away all the oil in the PB, things would've gotten hairy... but it made the whole shop smell like Christmas baking.
mnem
*Off to Pokemon Club with my son*