[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

RE: Black & White Question

On Mon, 23 Jul 2001, Tom Clark wrote:

> > It gets very hot here so I'm not sure about using white > paint, ...
> the temp > was 42C two weeks ago on the roof, ... the white may
> reflect > too much into > the preamp, .... then again black paint may
> absorb too much > heat and warp,

In space, (no convection cooling) Here is the steady state equilibrim
temperature of a flat plate in the very hot sun.

   WHITE     50 deg F
   BLACK     90 deg F
   ALUMINUM 788 deg F  (no typo)

In air, of course there is convection cooling, and the dish is surrounded
by the environment at ambient temperaturee with which the dish can
exchange energy by radiation.  So these numbers aren't realistic on the
ground.   But the point is that you just cannot beat WHITE for keeping
things cool..  

As long as the white paint is not GLOSS, there is no focusing effect on
the preamp.  The light diffuses equally in all directions.

FOr what it's worth, aluminum gets so hot because it absorbs solar
radiation about the same as white paint, but aluminum can only radiate
heat back out at 5% the efficiency of white.  Thats why a baked potato
stays nice and warm when wrapped in aluminum foil.  (It's also why thermos
bottles are coated on the outside of the glass jar with aluminum deposit)

Lay a piece of aluminum in the sun on a backing of cardboard or other
insulator so it does not get any air cooling on the back.  Then you will
burn  yourself when you go to pick it up later..  A white piece right
next to it will feel COOL to the touch...

Back in 1993, I built my first APRS GPS roof-top trackers in aluminum
baking pans and left them shiny aluminum to "reflect" the sun and keep em
cool....  duh.. I COOKED them!  Painted them white and they are cooler
than the car roof. And still work just fine...

de WB4APR, Bob

Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org