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**To**:**"apd" <aquatic-plants at actwin_com>****Subject**:**Re: watts per gal****From**:**"Wayne Jones" <waj at mnsi_net>**- Date: Sun, 9 Jun 2002 09:12:43 -0400

Gordon wrote: questions about the whole "watts per gal" idea in tank lighting. I'm not an electrical engineer, a mathematician, or a botanist, so what I'm thinking is most likely wrong, or something like that :) Numerous websites have formulas to calculate how many "watts per gallon" of light you need to grow low-mid-high light plants... The questions I have are: Isn't watts a rather unreliable measure of light output? I mean, a 75 watt standard incandescent puts off as much light as a standard 20 watt cool white fluorescent, or a 13 watt compact fluorescent twist-bulb. Which wattage do the formula's speak of... I assume they mean watts fluorescent (since that appears to be the most popular)? Are there any formula that use something more related to actual light output, like lumens or candlepower? Or formula that are calibrated to the wattage to light output ratio for standard fluorescent bulbs (can't afford the power compacts yet). ------------------------------------------------------------------------ -------- The watts per gallon rule is fairly accurate in a lot of cases with fluorescent and MH lamps. It can however be grossly misleading as well. Lumens per gallon or candle power would be even worse. There are at least 5 things that screw up the watts per gallon rule. Ballast efficiency, lamp efficiency, fixture efficiency, PAR efficiency and how well your lamp spectrum matches the photopic response of your plants. To apply all these factors is quite difficult if not nearly impossible so the watts per gallon rule is what people use. Suprisingly often the different efficiencies in each setup cancel each other out and the watts per gallon rule works great. There are other ways of predicting the light output of a fixture that work pretty good. If you ignore the lamp spectrum you can pretty much get a good prediction just by calculating the system lumens per gallon of your light fixture. Then you pick whatever spectrum you think will be best. This works great for fluorescent lamp systems. You must always use the same lamp spectrum to make this calculation. I pick the 841 spectrum as it is pretty much the same for every type of lamp. You then find out the lumen output for your particular lamp style multiply this by the the ballast factor and a guesstimated fixture efficiency. If you have 200 lumens per gallon you have a high light tank. If you wish to compare MH lamps to fluorescent lamps or as in other cases where you cannot find an equivilent spectrum you have to convert the different lamp spectrums by using a PAR/lumen conversion factor. Those conversion factors are available on Ivo Busko's lamp comparison page. To make it easy to calculate I consider an AH Supply type fixture to be of maximum possible efficiency. All other fixtures need to be derated according to a guesstimate of how good they are. The worst case scenario would be T12 lamps with close lamp spacing and a semi-reflective background. Many home made fixture suffer from this sort of problem. I think you have to consider this fixture efficiency to be at least 1/2 that of the best fixtures available. If lamp spacing is reasonable say with 2 lamp diameters between the lamps and you have an excellent difuse reflector then the fixture efficiency might be 85% of the best fixtures. An example calculation might be to compare a 2 55 watt PCs to 3 32 watt T8s and 6 20 watt T12s on a 55 gallon tank. If you have a Phillips ballast and PC combination the lamp lumens is 4800, the Phillips ballast factor is .88 and if you have an AH Supply reflector you would have the best fixture efficiency. 4800 lumens/lamp* 2 lamps * .94 * 1.0/ 55 gallons = 164 lumens per gallon. The second setup using Phillips lamps and a 1.2 ballast factor is also pretty good. 3100 lumens per lamp * 3 lamps * 1.2 * .85 fixture efficiency = 172 lumens per gallon. The 20 watt lamps are only about 1000 lumens per lamp and the ballast factors for the ballasts that run them are often quite low. I picked a .77 B.F You cannot quite get 2 lamp diameters between the lamps so you fixture will function not quite as good as the T8 fixture. I am calling the efficiency .80 So, 6 lamps * 1000 lumens/lamp * .77 * .80 equals 67 lumens per gallon. Now you can see if everything is stacked against you the watts per gallon rule means nothing. Nobody would try to light a 55 gallon tank with 6 20 watt lamps or would they? I see lots of people trying to use compact fluorescents to light some pretty large tanks. It is a hopeless endeavor. Naturally since I basically made up the fixture efficiencies based on luminaire data I found on the net, this method is not as accurate as it appears but it is far better than the watts per gallon rule. Someone really needs to do a study on lamp spacing and how it relates to fixture efficiency in an aquarium. Likewise for specular reflectors verses difuse reflectors. Wayne

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