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Re: Lumens per liter
On Wed, 6 Dec 2000, Wayne Jones wrote:
> That seems pretty much in line with what I think. If you have very good
> reflectors 200 lumens per gallon will grow anything I have tried but then
> again I havn't tried everything. The problem with a lumens per liter (or
> gallon) rule is that you have to know some hard to get information about
> your system in order to apply the rule.
Well, all those details are far too involved to constitue a "rule of
thumb". I suspect that the 30-50 lumen per liter rule is based on nominal
lumens -- i.e. on the lumen rating of the light source. If you need to
base something on actual lumens then probably you're better off getting a
meter, measuring lux and converting to lumens.
The point is that I think it would be useful to have a way of predicting if
a given system will provide the necessary amount of light. The watts per
gallon rule of thumb is not very good for this. It only provides a probable
range of adequate lighting. Which is it 2 or 4? The method I am using is
much more accurate than the watts per gallon rule and so far it has proven
to be quite accurate in practice for me. The formula appears to work well
for predicting the lighting adequacy of other setups that I hear about on
the net too. Most people probably don't want to buy a Lux meter but they
would like to know how much lighting to buy and the watts per gallon rule
can't tell them this.
Roger Miller wrote:
Lumens are a measure of light as perceived by the human eye; plants
respond to light in a very different way. Maybe Ivo can comment on this,
but I seem to remember from comparing across a number of commonly used
fluorescent lights that the wattage rating of a lamp was about as accurate
as it's lumen rating when you need to estimate the light's
photosynthetically active light output.
Of course I am perfectly aware of this but it is not a factor in the method
I am describing. I am using the lumen rating to sort of sample the size of
the spectral curve. If two lamps with the same spectrum are compared and one
has a 20% higher lumen output then it is also producing 20% more photons. As
long as you compare apples to apples and use the lumen rating of an 841
phosphor for every system you look at and if the system lumens per gallon
exceeds 200 lumens per gallon then you will have enough light to grow very
light demanding plants. Naturally most peope won't choose a lamp with that
spectrum either but again it doesn't matter. Put in whatever lamp you like
and the same system that produces 200 lumens per gallon with 841 phosphor
lamps will also grow plants just as well with some other lamp. It is
actually pretty simple too. B.F. X 841 lumens per lamp X #of lamps X .8-1.0
(to account for fixture and water surface losses)/gallons>200 then you have
enough light to grow high light plants.
Ivo's very elegant data aside, it truly does not seem to matter what lamp
you put in there. If it is a 750 an 850 or a 950 phosphor lamp it doesn't
seem to make much difference. The quality of growth appears to be higher
with the 950 lamp but not the volume of growth.
Supposedly the 950 lamp is less efficient but I am not really buying this. I
think Ivo's data has some problems because he used the nominal wattage of
lamps to calculate lamp efficiencies. In some cases this makes some lamps
appear to be significantly more efficient than others when it could be more
a result of whether the reference ballast was electronic or conventional.
The efficiency of the lamp varies according to the ballast. To eliminate
this problem he would have to use the actual wattages used by the lamps. The
same lamp producing the same number of lumens with a conventional ballast as
with an electronic ballast will draw many more watts with the conventional
ballast. It is not so much the phosphor in the lamps but the ballast and to
a lesser degree the lamp diameters, gas fill, lamp lengths, shape of the
lamp and coatings inside the lamp that make fluorescent lamps more or less
efficient. All of these things are taken into account with a lumens per
gallon formula and not with a watts per gallon rule of thumb.