Re: [Phys-l] paint your roof?

[Bernard Cleyet <bernardcleyet@redshift.com>]

> The
> amazing thing was when the sun went down.  The roof got cold.  Not  
> ambient
> temperature... but cold.  That white roof was radiating like crazy.  I
> thought maybe I was imagining it, so I got a digital thermometer  
> and placed
> it in contact with the roof and it was about 60 F when the ambient air
> temperature was about 72 F.  This made me rethink my thoughts about
> blackbody radiators.

I suspect not "radiating like crazy" -- if it were "black", it'd be  
much colder.  The point is it's well insulated so it's not warmed  
from below.

"Carlisle’s ENERGY STAR compliant Sure-Weld membranes, with their high
Sure-Weld Fully-Adhered Systems begin with insulation [my emphasis]  
mechanically
attached to the roof deck. The membrane and substrate are then coated
with Carlisle’s Sure-Weld Bonding Adhesive. Sure-Weld membrane is
rolled into place, and seams are hot air welded to form a monolithic
assembly."

Regarding dew shields --  better than a heater as it'll distort the  
lens.   Mine, black foam from building materials store, works  
reasonably well.  when It fails I point the tele at the ground for a  
few min. and the plate clears.   I've also, when at home, used a hair  
dryer.

bc hasn't been out in the cold for a while tho.





On 2009, May 27, , at 12:44, Edmiston, Mike wrote:

> > If it's not coupling (radiating) to the air, and not coupling to your
> hand, what *is* it coupling to?
>
> I reckon that would be "outer space."  I suppose the membrane has high
> radiance in infrared as well as visible.
>
> Astronomer hobbyists know this also happens to telescope objectives  
> and
> that's why we need "dew shields," especially in a humid area like  
> Ohio.
> If I am out at night in a field or lawn, high-humidity air is rising
> from the field.  The objective lens is radiating heat to space and is
> colder than the ambient air.  As the humid air comes into contact with
> the lens, it can condense and fog the lens.  A dew shield helps  
> channel
> the air past the lens (which is recessed by the shield) and helps
> prevent the condensation.
>
> On numerous cold nights when the ambient air temperature was in the
> forties, I had the corrector plate on my Schmidt-Cassegrain telescope
> frost over, even with a dew shield.  I finally made a heater with  
> series
> carbon-film resistors sewn into elastic fabric and stretched around  
> the
> perimeter of the corrector end of the scope.  It only requires a  
> couple
> watts or so to drive it.  Just need it warm enough to prevent
> condensation.
>
>
> Michael D. Edmiston, Ph.D.
> Professor of Chemistry and Physics
> Bluffton University
> Bluffton, OH 45817
> (419)-358-3270
> edmiston@bluffton.edu
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