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Re: [APD] Dreaded Lumens/PAR/PPFD/Watt per Gallon Topic
I do. I think wpg is a rough but handy general reference.
Lumens is certainly no better.
The old 4 wpg rule was based onthe old T12s and back int he
days when many folks were having a hard time getting enough
light, CO2, nutrients. Turns out, with the better, newer
fluorescents, 2wpg works very well for most situations,
even without CO2.
If there was an easy way for folks to get better info, that
would be fine. But it's not like having absolutely stunning
aquascapes depends on 2 versus 3wpg. There's a lot of
latitude with aquatic gardening.
Have plants, have fun,
--- Jerry Baker <jerry at bakerweb_biz> wrote:
> Just a quick question for the group: I have heard it said
> that the old
> "watts per gallon" rule was formulated based on standard
> output T12
> fluorescent lights. Old T12's output about 50 lumens per
> watt or so.
> Would it be a fair extrapolation to say that 3W per
> gallon (for
> high-light plants) is really just a synonym for 150
> lumens per gallon?
> Watts per gallon is an extremely poor generalization even
> generalizations go. Lumens are terrible in the context of
> plants, but
> what else are bulb manufacturers going to supply us with?
> If there is
> someone who thinks watts per gallon can even give you a
> remote idea of
> how much light you need, compare 4 T12 bulbs at 2000
> lumens to a
> color-corrected 150W high-pressure sodium bulb at 16,000
> lumens. Same
> wattages roughly, but the difference in brightness is a
> factor of two.
> Which watts per gallon is correct to get 4W per gallon in
> a 40-gallon tank?
> Anyway, back to my original question. If I assume that
> high-light plants
> need around 150 - 200 lumens per gallon (I know about PAR
> and PPFD, but
> have no data for most bulbs), that means my 135G will
> need about 3 AH
> Supply 96W kits, 6 55W kits, or 3 175W MH fixtures.
> Somehow this doesn't
> seem right to me. I am still teetering back and forth
> between 2x175W MH
> fixtures or 4x96W PCs. The tank is 24" deep, but with
> substrate and
> everything, we'll be looking at 22" - 20" more likely. I
> want to be able
> to grow chain swords well at the bottom of the tank.
> Forgive all this fussing. I assure you it's not
> "measure-bating" as some
> call it. Electricity is very expensive here in CA, and my
> apartment has
> a very low capacity per circuit (10A), so I need to
> maximize my
> efficiency as much as possible.
> PS - Why hasn't someone come up with a metric that takes
> tank depth into
> account? It's not hard. Instead of saying "X watts per
> gallon" we could
> say something like "X lumens per size/depth factor."
> After reading some stuff about the attenuation of PAR in
> fresh water, it
> seems to me like using an approximation of 0.46% of
> attenuation per inch
> of depth in an aquarium is reasonable (ignoring
> reflectivity of
> glass/acrylic walls). This is based upon interpolating a
> linear falloff
> from the equation % transmittance/m = 100(e^k) where k is
> the extinction
> coefficient (I used an extinction coefficient of 0.2
> because aquaria
> tend to be very clear).
> OK, so now we've got a rough estimate of how much light
> is lost due to
> depth. Now we need to know how much light is needed by
> medium-light, and high-light plants in those terms. I
> will assume that
> the rule of thumb (watts per gallon) was developed for
> tanks with a
> depth like a 55G (20"). Using those figures, and a lot of
> math I will be
> happy to show anyone who cares, it seems like the
> original rule of thumb
> gives us a net of 4 lumens per square inch for each watt
> per gallon. In
> other words, one watt per gallon in a standard 55G tank
> using 40W 2000
> lumen tubes gives us about 4 lumens per square inch at
> the tank bottom.
> The following equation should be a good approximation of
> light needed:
> Total Gross Lumens = Total Net Lumens/1-(0.0046 * depth
> in inches)
> So, let's make a real-world example. Say I am preparing
> to get a
> 240-gallon tank. The dimensions are 96x24x24. I want to
> have a tank on
> the brighter end of medium-light plant levels ... say 3W
> per gallon by
> the rule of thumb. Using that as a reference, that would
> mean I wanted
> about 28,000 net lumens on the tank's bottom (4
> lumens/square inch per
> watt). Plug that into my equation:
> 28,000/1-(0.0046*24) -> 28,000/0.8895 -> 31,478 gross
> Now I just need to spread about 32,000 lumens out over my
> tank and the
> light levels at the bottom of this tank will be the same
> as 165W of
> light from standard fluorescents over a 55G.
> It's not perfect, but hey, right now we're working with
> watts per gallon
> which do not take depth into account.
> Jerry Baker
> Aquatic-Plants mailing list
> Aquatic-Plants at actwin_com
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Coming Soon in November, the winners and all the other beautiful entries in the 6th Annual International Aquascaping Contest. Every continent is represented -- except Antarctica. Maybe next year Antarctica, too ;-)
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