I have a 5 kW, 17065 BTU, 480V heater that is positioned about six inches away from a A30 steel structure. How can I determine the surface temperature of the steel structure.
Area
Ambient temperature
Color
Surface finish
Relative angle
Emissivity
Thickness
Mounting contact
Heater duty cycle
Heater surface temperature
and
Any heater reflector aspects
Another words you can't.
Get a thermocouple and tape it to the structure out of direct view of the heater and measure it empirically.