Hi Tszaboo
Outside box still air temp is 112C-125C. Radiative black body temp the box will see is about 90C (there is a heatshield) .
Internal heat - 0.35W (device 1 - electronics good for 105C ambient ). and 10W (device 2 electronics good for 100C jn, 80deg ambient)
Run time - 6 hours
Basically I have to calculate whether sufficient internal thermal mass + insulation might do the job for device # 1. That's easy enough
Device #2 will need the plumbed-in cooling.
Originally, I was going to have a internal cold plate/ cold box, with the thermally conductive potting material coupling the heat to the cold plate (cooled via piped air ) (hence the question about conductivity and potting) , and insulation to the 'outer box'
If the outer surface of the outer box box is shiny, that will reflect the radiant heat, reducing that heat load input, leaving only conduction (from air) .
Piped air might be the best cooling solution, since water will need a loop and the consequences of a leak are fewer.
Air is surprisingly 1J/g/K specific heat, and with air density call is 1kg / cu3 --- a cubic meter of air per second will remove 1kW of heat per K difference.
In this case I want to remove 10W, and if the input air temp is 50deg C and desired internal temp is 80 deg C, I have a 30K difference and need 0.333grams/second of air.
or about 0.34 litres per second (20 lpm) assuming a 1/4" ID pipe , 5m long , pressure drop at 10 m/s airflow will be only 20mBar. easy.
So, plumbing in some low pressure airflow seems the easiest option.
Will do the sensor calcs next...IE thermal mass+ insulation. 6 hours at 0.35W input is total 7.56kJ . Ally is 0.9J/g/K, so if it starts at 50 deg C and terminates at 105C, I have 55K of difference, and I need 152 grams of Ally (57 cu cm) or a plate 5mm thick, 10x10cm... a bit big so I will need to thicken it up (or use copper - approx 2.5x higher specific heat). Copper it will be.
That's without any external heat input. External heat input area perhaps 0.015m2 area. Now i need to calculate the heat transferred to the box total area of 0.015m2due to conduction from the 125C air temperature.
( will assume all sides of a cube are receiving heat equally to simplify the job)