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Re: Aluminium Dish Painting?



On Sat, 10 Aug 2002, Roy Schulz wrote:

> You may notice that all the DSS tv dishes are painted a grey colour. I
> imagine that the sun reflections might be part of the reason. I know that my
> downconvertor gets real warm when pointed at the sun; I even went so far as
> to wrap it in aluminum foil.
> Just my opinion...

I think they are grey to make them less asthetically visible.
And wrapping your DC in Aluminum foil is 10 times worse than painting it
black and 30 times worse than painting it white..

Think Baked Potato.
Your thinking is correct about ABSORBTIVITY.  That is Aluminum and white
absorb only 30% or so of what  Black will absorb.  But the difference in
actual temperature is driven by the difference in ABSORBTIVITY and
EMISSIVITY.  There, White gets rid of heat Equally to black, but both get
rid of heat alomst 30 times better than Aluminum.  This is why we wrap
warm foods in Aluminum foil... The heat cant radiate away....

So add Absorbtivity and Emissivity and you get:

White is coolest by a factor of 3 to black
Aluminum gets hottest by a factor of 30 to white.
Never put anythig inside aluminum in the sun uless you plan on baking
it...

Or paint it white and it will be cool...

Of course, these factors apply in free space, the difference is less on
the earth where there is air conduction and convection cooling too...

de WB4APR, Bob


 > regards, > > Roy
VE7BPB > Vancouver
>
>
>
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de WB4APR@amsat.org, Bob

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