Cables and Currents

I don't get that. Vacuum conducts heat better than air? 0.5 mm of plastic conducts heat as well as 5 mm of air?

But it is complex. More insulation increases the radiation surface, which works against the T^4 radiation curve. And more insulation conducts heat out to the surface of a given radius, better than a vacuum gap would... depending on the thermal conductivity of the insulation. All that math is way past my pay grade.

I think Phil is calculating the thermal conductivity of air, then equating that to some thermal loss from radiation in vac. But he has to pick some temperature rise... 1 deg K or something.

And the air number has to be for low temperature differences.. or convection kicks in...

Grin... right do the measurement and use that to check/adjust your math. :^)

George H.

Yup. In vacuo, the heat just radiates away into space, so any thickness of vacuum has the same thermal resistance, namely the derivative of the Stefan-Boltzmann law, i.e.

L = epsilon sigma T**4,

where epsilon is the thermal emissivity. Two parallel surfaces with different temperatures will exhibit a power transfer per unit area of

Delta L = epsilon_1 epsilon_2 sigma (T_1**4 - T_2**4),

which for small delta-T is

alpha = dL/dT = 4 epsilon_1 epsilon_2 sigma T**3.

It's modified some by the thermal emissivity of the emitter and surroundings.

That's how superinsulation works--you have many layers of metallized Kapton, spaced out so that they don't touch. Works great, but it's an absolute bear to bake out--all that surface area, the constricted space for gas to diffuse out, and the superior insulation making it hard to get it all hot.

Cheers

Phil Hobbs