Re: [APD] Acrylic Tanks - Sunlight Photos

[Wright Huntley <whuntley at verizon_net>]

The pictures are dramatic evidence that one (or both?) surface of the 
acrylic is badly scratched. [It looks more like 200 grit than 2000, 
BTW.] Unfortunately, we cannot see which surface is involved. 
Internally, it is almost a non issue. Externally it would be totally 
unacceptable. Water and acrylic have almost exactly the same index of 
refraction, so there is virtually no scatter from haze or minor 
scratches on the inside surface, when filled. The difference between an 
index of 1.0 for air and 1.4x for acrylic guarantees that external 
scratches will show like mad.

To tell the surface is quite easy. Take an illuminated spot, like the 
first pic, and wipe a wet rag across the area on the inside of the tank. 
Look at it. Dry and do the same to the outside. While wet, before all 
the water beads up, the scratches should disappear if they are on one 
side only. You may want to add a few drops of mild detergent to improve 
wetting ability.

If nearly all that haze is inside, you have only one remaining problem. 
Cast acrylic is a little bit like tempered glass in that the high 
internal stresses between surface and interior can make it haze and 
crack with age or mechanical stress. Each of those scratches can (will 
be?) be the starting point of a crack propagating through the thickness 
of the sheet. It is way slower than shattering tempered glass, but just 
about as inevitable.

It needs to be heated to near the boiling point of water, and slowly 
cooled to relieve those stresses. You can see the stress pattern between 
a couple of sheets of polarizer material, and the rainbows should be 
minimal (crossed polarizers will look pretty black) for a well-annealed 
tank, I think.

HTH

Wright

>Message: 8
>Date: Sun, 25 Sep 2005 17:43:25 -0700
>From: Jerry Baker <jerry at bakerweb_biz>
>Subject: Re: [APD] Acrylic Tanks - Sunlight Photos
>To: aquatic plants digest <aquatic-plants at actwin_com>
>
>http://www.flickr.com/photos/jerrybaker/sets/1013776/
>
>Here are some pictures of the phenomena in the direct sunlight. Make 
>sure to view them full size.
>

-- 
Wright Huntley - Rt. 001 Box K36, Bishop CA 93514 - whuntley at verizon_net - 760 872-3995

"...watching Mayor Nagin and Governor Blanco handle the New Orleans/ Louisiana debacle is like watching Moe and Curly trying to work a Rubik’s cube."
Doug Giles

                 
http://www.self-gov.org/wspq.html

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