Dear Steve:
I read exactly as what you said in an control publication, it was about an aquarium, but it recommended to place the sensor near heater. But please consider that the metallic walls of the chamber actually warms the load so the temp of wall should be closer(temp only read when chamber is not moving).

Dear Michael:
Thank you for reply, fortunately I have enough resources for that, I use a multi channel 10 bit ADC with still free terminals. If I couldn't reach good results with one sensor I might try more sensors and use all of them(if they actually differ).

thank you very much