Why Doesn't The Wpg Rule Work For Small And Large Tanks?

[aaronnorth]

I have a 180l tank with 1.2WPG, so this would work out higher than it is? 2 x 24w HO T5's

and a 14l with 3WPG, so this would work out lower than it is? 1 x 9w flourescent tube

But Why, i'm so confused

 

[dave_gray2077]

It doesn't realy work because WPG is a very arbitrary measurement at best. And in larger tanks the surface area does not drastically increase compared to the lighting.

Take a look at http/www.fitchfamily.com/lighting.html in this article the anther proposes that you use surface area to calculate the amount of light.

There is also a calculator at the bottom of the page which is useful.

 

[frozenbarb]

How much watts do you have on those two tanks?, Not the WPG

Wpg doesnt work on smaller tanks because it just doesnt.

Having a one gallon and a 3 watt bulb will give it 3wpgs. But that isn't enough light to grow anything

 

[fish-aholic]

Couldn't say it better myself. Clicky

 

[SuperColey1]

Your tank may have 1.2WPG but if I remember rightly they are HO T5 lights.

WPG rule was calculated on T12 lights.

IMO HO T5 means you can almost assume that 1.2Watts of HOT5 is equal to 2+ on the T12 WPG scale.

I recently 'downgraded' from 2.5WPG to 1.8WPG (the 1.8WPG being HOT5) and the plants zoomed upwards like crazy and they pearled like mad.

I now use 0.9WPG for most of my 10 hour photoperiod (with the other bulb on for the central 2 hours) and the plants are still pearling like crazy!!!

Its not just small tanks and surface areas. Its the huge improvements in lighting since the T12 WPG rule was calculated.

Andy

 

[Luketendo]

Something to do with light threshold or something.

 

[plantbrain]

Also, as the distance from the light source increases, so does the scatter and the various angles and bulbs used.

So the spread of light is different.

Few small tanks have several bulbs over a wide area.
It's more point source.
This can affect plants a lot.

I think this is more likely the issue.
Also, wpg is really applied to 10 gallons to 300 gallons or so.
And most folks have tanks that size and wpg works just fine there for NO FL's.
If you use T5's, PC, HQI's, you can get away with less.

The WPG rule was never meant to be APPLIED SO RIGIDLY.
So apply with common sense.

If you want to use more accurate methods, well, there really are not any other than using practical experience and using as PAR meter.

Regards,
Tom Barr

 

[garuf]

Forgive my ignorance tom, when you say point source are you suggesting that multiple points of light I.e. more tubes is better?
I'm looking into replacing my single 36w PC for something else, most likely 2x 24w on my 10 gallon would the fact more light is over the tank mean better plant growth or is it more a case of better reflectors and a single 36w?

 

[SuperColey1]

What Tom means is that if you have 1 light then the output is diagonal like this:

If you have 2 then the light overlaps like this:

More lights produce more overlapping.

What it can mean is that if you are trying to grow Glosso right at the front of the tank with a single lamp which is rated at 2WPG you think you have enough light, but the intensity is in the centre of the tank where the shortest route is whereas at the front it is not so intense becuase the diagonal is a long distance etc. Overlapping means there is more light etc.

Something like that. I think I have basically got it right but Tom can confirm..

Andy

 

[lljdma06]

The WPG rule was never meant to be APPLIED SO RIGIDLY.
So apply with common sense.

If you want to use more accurate methods, well, there really are not any other than using practical experience and using as PAR meter.

Regards,
Tom Barr

I agree, but at nearly $250, PAR meters can be expensive. I've seen some as high as $1000. My orchid-growing friends use them a lot, but they are trying to make some money with propagation, and I am not.

WPG serves as a guideline. It was never meant to be 100% accurate. I see it as more of a ballpark figure that hobbiests can use to gage what plant species would be best for their tanks, based on an estimated lighting level. You have to ask yourself whether or not you are satisfied with its accuracy? Personally, I am. Using it has worked well for me in the past and I am happy with my current plant growth. I tend to use either PC or T5s now, so I know I can get away with less wattage in an average tank, then the original T12s WPG was designed for. When I used T8s, I had to use more than I currently do. For larger tanks, I need even less. For smaller tanks, I need more. When you have less experience, it is often a process of trial and error. Once you have more experience, you can often gage how much you'll need for the specific setup.

What Tom means is that if you have 1 light then the output is diagonal like this:

If you have 2 then the light overlaps like this:

More lights produce more overlapping.

What it can mean is that if you are trying to grow Glosso right at the front of the tank with a single lamp which is rated at 2WPG you think you have enough light, but the intensity is in the centre of the tank where the shortest route is whereas at the front it is not so intense becuase the diagonal is a long distance etc. Overlapping means there is more light etc.

Something like that. I think I have basically got it right but Tom can confirm..

Andy

It sounds very logical to me, Andy, and your diagrams have made me notice that all my tanks have at least 2 or more "points of light". But again, I take a very laissez faire approach to this hobby. Did you do the diagrams yourself? Nice!

llj

 

[SuperColey1]

I just did them quickly on MS Paint. They just show how positioning the lights can affect your choices.

The overlap will be the brightest spot.

Andy

 

[aaronnorth]

Thanks for all replys, nice drawing!