# Re: [APD] light and the w/gal rule

```Well, if you want to go whole hog, use Pearson's R to do a
partial regression on all the relevent factors, and don't
forget to take into account the spectral shift that occurs
with age and variations that occur coming off the
production line, etc. When you're all done, most folks will
find that they wpg still works well, is close at hand and
convenient, and works as anything else. It's not like there
is some critical value of light that has to be obtained or
else plants won't grow well.

sh

--- Jerry Baker <jerry at bakerweb_biz> wrote:

> Jerry Baker wrote:
> > True that. The issue, then, is whether lumens are more
> closely
> > correlated with PAR than watts. Ironically, it is
> nearly impossible to
> > find lumen ratings on bulbs that appear in Advanced
> Aquarist bulb PAR
> > studies.
>
> Well, I used the data at Aquabotanic and it seems that
> lumen ratings are
> much more highly correlated with PAR than wattage. When
> you perform a
> bivariate linear regression on lumens/PAR you get an
> r-squared value of
> 0.96 with a standard error of 14.76, but when you analyze
> watts/PAR the
> r-squared is only 0.86 with a standard error of 22.6.
>
> In other words, if you know the lumens of a bulb you can
> calculate it's
> PAR rating with the following equation*:
>
> PAR = 5.557 + (0.015 * Lumens)
>
> This equation will get you to within about +/- 3.5 PAR
> units.
>
> However, if you try to calculate PAR from wattage, you
> would have to use
> the following equation*:
>
> PAR = -9.889 + (1.206 * watts)
>
> This equation only gets you to within +/- 8 PAR units.
>
> *Based upon the bulb data at
> http://www.aquabotanic.com/lightcompare.htm
>
> --
> Jerry Baker
> _______________________________________________
> Aquatic-Plants mailing list
> Aquatic-Plants at actwin_com
> http://www.actwin.com/mailman/listinfo/aquatic-plants
>

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```