PAR Reduction do to 1/4" glass?
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Old 11-30-2012, 02:40 PM   #1
jmhart
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PAR Reduction do to 1/4" glass?


Anybody have a thought on reduction in PAR due to 1/4" of standard glass? I have a sheet of glass on top of my 60-P to reduce evaporation. I'm debating between two light fixtures and a guess at how much PAR I'll lose to the glass would be helpful.

Thanks!
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Old 11-30-2012, 03:04 PM   #2
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Originally Posted by jmhart View Post
Anybody have a thought on reduction in PAR due to 1/4" of standard glass? I have a sheet of glass on top of my 60-P to reduce evaporation. I'm debating between two light fixtures and a guess at how much PAR I'll lose to the glass would be helpful.

Thanks!
Hi jmhart,

I responded to a similar question on another forum about this subject and the results I got from a PAR meter. The results were interesting. It was a standard 30 gallon (36" length) with an 2X36 watt (6700K) AH Supply kit over a Aqueon Versa-Top (3/16" clear glass). With new bulbs and no glass top the PAR = 110; with cleaned Versa-Top in place PAR = 101; with "dirty"* Versa-Top in place PAR = 96. All readings were taken with new bulbs @ substrate level about 13 inches below the fixture. If the glass is clear, I don't believe the difference in thickness will impact the results substantially.

*dirty = lots of dried water spots on top; some "etching"; condensation; dust
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Last edited by Seattle_Aquarist; 11-30-2012 at 09:58 PM.. Reason: corrected glass thickness
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Old 11-30-2012, 03:07 PM   #3
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Hi jmhart,

I responded to a similar question on another forum about this subject and the results I got from a PAR meter. The results were interesting. It was a standard 30 gallon (36" length) with an 2X36 watt (6700K) AH Supply kit over a Aqueon Versa-Top (3/8" clear glass). With new bulbs and no glass top the PAR = 110; with cleaned Versa-Top in place PAR = 101; with "dirty"* Versa-Top in place PAR = 96. All readings were taken with new bulbs @ substrate level about 13 inches below the fixture. If the glass is clear, I don't believe the difference in thickness will impact the results substantially.

*dirty = lots of dried water spots on top; some "etching"; condensation; dust

Very helpful. Thank you. Great contribution to the community!

I really appreciate the variables you accounted for. Normal state for me would likely be dirty glass.
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Old 11-30-2012, 03:15 PM   #4
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Very helpful. Thank you. Great contribution to the community!

I really appreciate the variables you accounted for. Normal state for me would likely be dirty glass.
lol, how do you think I got the 'dirty' reading??
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Old 11-30-2012, 04:29 PM   #5
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Hi Seattle. Your results are exactly in line with our experience. When light passes through glass, you are going to have two losses from Fresnel reflections. We typically use a 4% Fresnel loss estimate per side of glass (8% total for clean glass), which is exactly what you calculated. Nice :-)

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Old 11-30-2012, 05:00 PM   #6
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Originally Posted by Seattle_Aquarist View Post
Hi jmhart,

I responded to a similar question on another forum about this subject and the results I got from a PAR meter. The results were interesting. It was a standard 30 gallon (36" length) with an 2X36 watt (6700K) AH Supply kit over a Aqueon Versa-Top (3/8" clear glass). With new bulbs and no glass top the PAR = 110; with cleaned Versa-Top in place PAR = 101; with "dirty"* Versa-Top in place PAR = 96. All readings were taken with new bulbs @ substrate level about 13 inches below the fixture. If the glass is clear, I don't believe the difference in thickness will impact the results substantially.

*dirty = lots of dried water spots on top; some "etching"; condensation; dust
Your lid is 3/8"?
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Old 11-30-2012, 09:57 PM   #7
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Your lid is 3/8"?
Hi xmas_one,

Correction, thank you xmas-one....my 36" Versa-Top is 3/16" thick; my 20" Versa-Top is 1/4" thick. Hadn't had my coffee this morning when I measured.
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