I know it will hurt, but this is a rubbish design, and not because of the effort you went to in building it, but because the design demonstrates that you never understood how heat is flowing, through the walls and (closed) door, out of your furnace. You do need a more than nodding understanding of
Fick's laws to understand the spread of heat inside furnace walls and thus the resultant temperature distribution inside the furnace. I am not here to belittle your effort, I am writing to provide an explanation for your furnace's behaviour. Once you understand why you get this behaviour, you should be able to think up possible means to improve on it.
First, temperature measurement. Forget about using a pyrometer (i.e. infrared radiation thermometer), because this will produce a meaningful reading only if you measure the radiation coming out of a black hole. With "black hole" I am not referring to the astronomical object, but to a box with a central, large cavity that is accessible only via a small hole. Only this small hole has an emissivity of 1, all other surfaces will have emissivities less then 1, emissivities that depend not only on the material, but also on surface roughness and time. Thus, forget about looking up any emissivity, it does not produce a trustworthy temperature reading without the black hole emitter.
You could also estimate your temperature using
Seger cones.
Or you can attach some weight to the tip of a thermocouple and place that at different positions inside your furnace, to measure a temperature distribution chart. If you achieve a temparature variation of less than 50
oC, you have build well. From the pictures you provide on the placing of your heating element, I would expect a spread of 200
oC or more, just what you have observed.
Once you have measured that and understand this is not simple bluster, we can talk again, this time about where to place the control thermocouple, and where to place the heating wires.
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