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(Fwd) ALuminum, black and white



as some members of this list might not have been around in 1996, allow me 
the liberty to repost a message from bob bruninga about black and white 
paint. i already sent a copy to paul but it seems it might benefit others here.
73...bruce

------- Forwarded message follows -------
Date sent:      	Fri, 26 Apr 1996 09:50:14 -0400 (EDT)
From:           	Bob Bruninga <bruninga@nadn.navy.mil>
To:             	aprssig@tapr.org
Copies to:      	amsat-bb@amsat.org
Subject:        	ALuminum, black and white

KEEPING ELECTRONICS COOL IN THE SUN.

WHile building a GPS unit for mounting on my dashboard and noting the 
comming summer months, I looked up the difference in absorption and 
emissivity for Aluminum, Black paint, and white paint.  Satellite builders
are well aware of these facts, but many of us landlubbers are not.

ALUMINUM will get 30 TIMES hotter than WHITE paint!   (in a vacuum)

The following table is for a vacuum and accounts for RADIATIVE effects.
It does not account for convective or conductive cooling (air)..

            Absorbtion  Emissivity  Ratio    Temp C

ALUMINUM       .4         .03       11:1      400
STEEL          .6         .4         3:2      150
BLACK PAINT    .9         .9         1:1      110
WHITE PAINT    .25        .85        1:3       72

Most people are aware that Black gets hotter than white, but the fact 
that bright, reflective, shinny Aluminum gets 10 times hotter than BLACK 
is a surprise to most people...

So, if it sits in the sun, paint it white!  If you dont believe this, 
put an aluminum baking sheet in the sun.  I baked my first roof mount GPS 
stand alone tracker thinking that the upside down baking pan would 
reflect the sun...  WRONG!
Painted it white and it is now as cool as a cucumber.

The difference in Aluminum is the POOR EMISSIVITY at infrared.  It can't 
radiate the heat away...


------- End of forwarded message -------


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Bruce Paige, KK5DO                    Internet: kk5do@arrl.net 
Houston, Texas                                  kk5do@amsat.org 
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