View Full Version : PAR and WPG
12-27-2007, 05:23 AM
It seems to me there is a problem with using watts-per-gallon (WPG) as a particularly effective way to determine how much light one has. I have been reading and thinking about this, and I am unclear on this aspect.
For example, if I have a 10 gallon All-Glass aquarium (20x10x12 in) and an AH Supply 36W Bright Kit with their wood hood. The hood places the light approximately 2 inches over the water, or perhaps glass tank cover. Using WPG, that gives ~3.6 wpg. Pretty high light. I think, however, it is not very even light coverage, however (meaning a PAR meter would not show the same value front-to-back, or side-to-side).
Would not raising the height of the light over the tank eventually get to a point where the PAR values would be much more even and better(?) for the tank?
Obviously, as physics goes, the PAR value would lower. I am not thinking this is a bad thing, per se. As other articles and postings on here have proposed and shown, that lower light is not a bad thing, that 1.5 WPG can be used to grow nice, healthy planted tanks. (Just slowly, in a more controllable manner.)
That said, what are “good” PAR values? What would 1.5 WPG be roughly equivalent to in PAR? And 2, 3, ... WPG?
I realize this message is a little but jumbled, logically, but then so is my thinking and understanding of the material. I could use a little help in sorting out the details.
12-27-2007, 07:42 AM
You cant directly correlate between PAR and WPG. WPG was probably created as a rule of thumb that everyone can figure out easily to get in a ball park... PAR is a property of the spectrum of the bulb - impossibly to figure out without having a specific meter or at least accurate manufacturer supplied information (which most dont). For example two identical bulbs (48" T8 lets say) one is warm white and the other is daylight and both are the same 32W. So either would yield the same WPG. But PAR might be drastically different. Even the same bulbs (both 6500K for example) could be different PAR values if they are from different manufacturers.
12-27-2007, 10:31 PM
I realize that PAR and WPG cannot be directly correlated. One of the things I would like to know is that if a tank is, for example, a low(er) light tank at “2 WPG” what does that mean as far as a range of PAR values in µmol m^-2 s^-1? Or 1.5, 2.5, 3, ... WPG?
Also, what are reasonable PAR values for growing aquatic plants? (I realize that this is quite likely dependent upon the species of plant, but I would expect there is a median value ± a range).
12-27-2007, 11:46 PM
I found a spreadsheet that calculates lumens per square inch and claims to be a better measure of the amount of light you have on your tank, and converts that to a weighted wpg based on both your tank dimensions and type of bulbs. This obviously doesn't take into account the height of the bulbs above the water though.
12-28-2007, 12:02 AM
Do lumens relate at all to PAR? (Yes, obviously not directly!) I mean, is there any correlation such as higher lumens mean higher PAR?
12-28-2007, 01:07 AM
If I am not mistaken:
PAR is unweighted energy usually between 400nm and 700nm
Lumens / LUX is wieghted against the human eye sensativity to light spectrum (more light in the green wevelenghts = higher lux), and
PUR is weighted against the photosynthetic spectrum (more light in the red and blue wavelengths = higher PUR)
Sure there is some correlation. If I took a standard 32W bulb and overdrove it 4X ODNO and roughly doubled the lumens I could also assume that the PAR roughly doubled as well, although I dont know what it was to start out with.
12-28-2007, 03:00 AM
After posting I started to research lumen (via Wikipedia) which led me to CIE XYZ color (human color sensitivity), color temperature, then back to PAR, etc etc. Now I have far too many facts about light. :-)
(Side question: Does color temperature have any practical meaning to PAR when we are buying light bulbs for our aquariums?)
Okay, we know that PAR and WPG are not directly comparable. We know that WPG was created as an easy, shorthand, rule-of-thumb way to help aquarists determine how much light they have over their tank. We know that different light technologies (PCF, T8, T5HO, MH) offer tradeoffs of electrical efficiency, lumen output, color temperature, PAR output and heat generated.
So, I have an Apogee PAR meter. I want to be able to figure in my head that if someone here posts about a tank with 3 WPG, that would likely mean they would have PAR readings at the water surface of X µmol m^-2 s^-1. Then, for example, if I take my PAR meter to a tank that has the same light, but different bulb, and take a reading and see Y µmol m^-2 s^-1, I could generally (roughly) compare X to Y and get an idea how our tanks “metabolisms” are similar or differ. Or perhaps I could equate their 3 WPG tank to something I might be trying out with a suspended light.
12-28-2007, 06:36 PM
I just measure a range over a number of lighting types from the distance from the bulb.
That way I know that a current 130W light at 10, 15, 20, 24" away will have say 300, 150, 50, 30 micromols no matter what gallon range I use.
T5's HQI and other light types are measured this way, and then I have a good idea.
Plants do pretty well down to about 20-30micromoles, 100 is pretty good for most.
12-28-2007, 07:13 PM
Tom, thanks for the input...
That would be 100 µmol m^-2 s^-1 at the surface, or at some level in the water?
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