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Old 05-14-2019, 09:09 AM   #1 (permalink)
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Default Converting open air PAR to water

I just purchased the 36" Finnex Planted Plus 24/7 CRV series which is being delivered tomorrow. The website states it's fully customizable (color and intensity) at all time changes. They give a nice PAR table (see pic)for the different size lights and depths but it's in open air values! Is there a reasonable way of estimating the actual PAR in water? I'm clueless as to how much to dim it. Tank is 13.5 in from top to substrate level and is *heavily* planted. No CO2 of course. Thanks for any advise!

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Old 05-14-2019, 09:13 AM   #2 (permalink)
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Default Re: Converting open air PAR to water

For 13.5 in, would a 50% decrease in the intensity work? It states approx 70ish open air PAR at that depth. I figure up to 30 for low tech.

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Old 05-14-2019, 10:30 AM   #3 (permalink)
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Default Re: Converting open air PAR to water

Hoppy knows the answer specifically.
From what I can remember, water in our aquariums, a few feet don't diminish PAR much. The walls of the aquarium reflects the light back into the tank so increases PAR a little.
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Old 05-14-2019, 10:49 AM   #4 (permalink)
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Default Re: Converting open air PAR to water

Quote:
Originally Posted by mistergreen View Post
Hoppy knows the answer specifically.
From what I can remember, water in our aquariums, a few feet don't diminish PAR much. The walls of the aquarium reflects the light back into the tank so increases PAR a little.
I didn't know that! I just assumed water would decrease it. But maybe that's at great depths like lakes and rivers.

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Old 05-14-2019, 11:03 AM   #5 (permalink)
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Default Re: Converting open air PAR to water

Lots of particulates in rivers too so that'll block out light.
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Old 05-14-2019, 11:10 AM   #6 (permalink)
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Default Re: Converting open air PAR to water

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Lots of particulates in rivers too so that'll block out light.
True!

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Old 05-14-2019, 02:55 PM   #7 (permalink)
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Default Re: Converting open air PAR to water

Summoning @hoppycalif
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Old 05-14-2019, 04:50 PM   #8 (permalink)
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Default Re: Converting open air PAR to water

I just saw my BatMan light flashing in the sky!!

Clear water doesn't significantly reduce light intensity for distances up to at least 30 inches. But, if you keep the glass walls of your aquarium clean on the inside, and clean and not painted on the outside, you can get a big boost in intensity from light reflected off the glass to air interface. My LED light gives about a 50% increase at the substrate! Lights with reflectors will give a much smaller increase. Also, when a strip light is over the center of the tank, the refraction of the light as it goes from the air to the water also increases the intensity by a small amount.

I suspect most of us do not wipe the inside of our glass walls every day, so it is probably the best idea to just assume that the PAR in air is the same as the PAR in water. It is also much easier to measure PAR in the air than in our aquarium filled with water, let alone in a fully planted aquarium. To further complicate this, Apogee went for years insisting that their PAR meters read the same in air and water. A few years ago, when they improved their PAR sensor, they found that there is a big correction that needs to be applied when using their meters in water - about a 15% correction, making their PAR readings 15% too low when used in water. This correction is a function of the refraction as the light goes through the water to diffuser interface, and that is also a function of the curvature of the diffuser. When I was making PAR meters, I ran into this and ended up by giving up on making them, because I couldn't meet Apogee's "same reading in air and water" standard. (I also got too much arthritis in my hands to continue.)
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Old 05-14-2019, 05:16 PM   #9 (permalink)
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Default Re: Converting open air PAR to water

Thanks Hoppy! Interesting! That must be what I recall, that it is a limitation of the meter itself not that the actual PAR value was different. I try to keep the interior clean but I do have a bit of green dust algae. And I have light colored play sand cap.
Right now I have the original version Planted Plus 24/7 on 4 in risers. So, do you think decreasing intensity by 50% would do it? I love that this newer version is so customizable! Their chart shows about 70ish PAR at approx 13 in. I'm assuming that's at the full intensity.

Edit for typos.
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Old 05-14-2019, 08:05 PM   #10 (permalink)
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Default Re: Converting open air PAR to water

I found that a low light, non-CO2 tank, using Excel for carbon, will do pretty well at 35-40 PAR at the substrate. That table of intensity for each length of light fixture, and at various distances suggests that you will get about 60 PAR at about 13.5 inches. And, you would get about 35-40 PAR at about 22 inches from the 36 inch long light. All of those PAR numbers could easily be off by +/-5 PAR.
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