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Re: [APD] Dreaded Lumens/PAR/PPFD/Watt per Gallon Topic
I never understood why the lumens and lux things never worked as well?
in fact everything i've read about lux and lumens seems to be based on
some archaic text i never saw when i was learning about them.
lux is a unit of measurment of the intensity of a light source reaching
any surface. in fact lux is the better deinfed as the measurement of
lumens per square meter. period.
and lumens is the measurement of light output.
and in bulbs is best defined by the efficiency, or lumens produced per watt.
which is saying very simply that not all watts are equal.
obviously, light is electricity converted to heat and light. the hotter
the bulb the less effcient. more heat less light. its a waste of energy
basically. your using twice the electricity to make more light at less
light per watt.
but sometimes this is needed, it's obviously easier to use one hotter
brighter light than alot more cooler more efficient bulbs.
so that seems to me to be very beneficial as to plants. maybe it's just me?
Jerry Baker 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 as
>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 low-light,
>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 lumens
>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.
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