[time-nuts] temperature stability basics

[Neville Michie]

Dont get the idea that radiation is only significant for large  
temperature differences.
For two parallel surfaces at any distance apart the black body  
radiation between them (around room temperature 300K) is near to 6  
watts per square metre per degree (C*) of temperature difference.  
That is an R rating of 0.14 in parallel  with the conduction through  
the insulation that is IR transparent.
The rate is proportional to the temperature difference. The fourth  
power law only becomes significant when the temperature difference is  
quite large.
If the surface is clean and polished the emmissivity of the surface  
can be significantly reduced, hence the silver lining in a dewar.  
With sheet materials, an oil film, corrosion and dust can rapidly  
increase the emmisivity. Gold is good because of its freedom from  
oxidation and discolouring and ease of "flashing" onto any surface.
If you make a sandwich with N layers of metal foil, the radiation  
transmission is reduced by 1/N.

Cheers, Neville Michie