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Re: [APD] Plexi lid vs. Glass

First surface reflections are almost the same from glass and plexi 
(about 4% for each surface), but plexi does absorb less, particular of 
the important red light. Glass can absorb up to 10-15% of that in 1/4" 
thick panels. I have measured it. [Notice how green the edges look? 
That's because most red has been absorbed and only blue-green propagates 
the long way through it.]

Unfortunately, the warpage in plexi (acrylic) is not from lamp heat, but 
from absorbing water and swelling. When that happens on one side, it 
results in truly impressive bowing. [It also melts at very low 
temperatures, so lamp heat isn't a minor issue if an end moves up 
against a hot tube.]

There are other good glazing plastics, like Lexan (expen$ive) that do 
not bow, but avoid plexi unless you are willing to cement on ribs to 
resist the warping. Those will, of course block and scatter more light. 
Check out polyester and polycarbonate.

You may find that "water-white" glass, the "Pyrex"-like stuff, is the 
toughest and most economical material for covers. It is nearly as clear 
as plexi and much more durable and cleanable(if unbroken). Ask your 
local glass shop.


>Message: 2
>Date: Tue, 20 Sep 2005 14:43:30 -0400
>From: "David Grim" <grim1214 at bellsouth_net>
>Subject: Re: [APD] Plexi lid vs. Glass
>To: "Aquatic Plants Mailing List" <aquatic-plants at actwin_com>
>Hi, you guys have been discussing how much light is lost through glass. That
>is good to know. Anyone know if more or less is lost through a 0.25 inch
>piece of plexi from Home Depot? I friend of mine added this to his tank and
>I was considering doing
>it also. I have so much evaporation with an open top.
>Kelleen Harris
>The main issue using plexiglass for tank tops is warpage of the plexi, I am
>assuming from the heat of the light. This from previous threads on this

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