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Re: In defense of Watts-per-gallon -- or - Plugging an old adage

Jeffery Ludwig expressed dissatisfaction with the use of
watts per gallon as a measure of the amount of light.

The best would be to measure the actual output of actual
lamps (reflectors and bulbs) in place in a given
circumstance and compare that to the specific spectral
needs of the plants in that tank.

Short of that, one could use the spectral output ratings
for a lamp using particular bulbs.  But you won't find this
for many (any?) lamps (reflector and bulb) -- you'd even be
hard pressed to find this info for a given bulb.  You could
substitute the lumen or candlepower rating but you will be
hard pressed to find that for many bulbs.  And in any
event, if you have a rating for a bulb, you will have to
adjust that rating for the particular ballast you are going
to use because the ballast can have a terrific impact on
the bulb and not all ballasts have the same effect.  Not to
mention, adjustments for ambient temperature, which also
effect the output of fluorescent lamps  ;-)  .

That's more than many folks have time to do, more than many
would enjoy doing.

WPG actually works rather well as an ersatz rating for
light output.  Not only is it easily obtained, but it's a
reasonably practical and reliable measure, so long as you
don't try to press it too far.  It is especially useful
when comparing similar lamps and similar bulbs.  Do
different reflectors make a difference?  Sure and if you're
aiming for about 3.5-4 wpg, you can feel comfortable
heading for the low side if you're going to have good
reflectors.  White plastic reflectors -- not very good.  AH
Supply reflectors -- excellent.

The wpg rules of thumb are usually presented as just that,
guidelines that help folks decide what to buy. 1.5-2 wpg is
a whole different neighborhood than 3.5-4 wpg.  Is 4 better
than 3.5?  That's trying to cut a little too finely for
most folks' purposes.  I think most folks have to grow with
their tanks; learn to adapt to the tank and to adapt the
tank as things progress.  You can set up two tanks the same
and before too long, they can be two very different tanks. 
But you can decide what neighborhood you want to be in when
you set up.

And as George pointed out, you can do these days with a
couple of wpg what used to take many more.  Personally, I
worry less about the wpg of a setup than how often the tank
owner is going to do maintenance.  Unless I know for sure
that the tank owner will do weekly maintenance, I aim for
the low-side neighborhood and put on what physically fits. 
Hmmm, two 55watt PCs fit nicely but that gives me only 2
wpg and I was aiming for 2.5 wpg -- I don't worry about

I'd rather worry about CO2 and O2 levels -- no there's
something one can get really accurate about  ;-)  

Scott H.

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