Wpg Rule Ticks Me Off

[Spiceweasel69]

I must say i have a bit of an issue with the WPG rule.

I have just added reflectors to my tank therefore can assume im getting 99% of the light omitted from the tube into my tank. I have 2 38Watt tubes in my 250litre tank. Basically i come out with a wpg of around 1.2.

However....if i drop my water levels by half (none of my plants have grown to half the height of the tank yet), then it will come in at 2.4, which is desirable. But hey, it looks exactly as bright!! The light coming through the top half of the tank is the same intensity, no matter wether it is filled with air or water, ergo, the water does not filter out the light (assuming its clean and clear)......okay, ill give you it may filter out a small portion of it, about 3%-5%. The point is that Gallons is a unit of volume, and what we need here is a unit of surface measure. The amount of light that reaches the earths surface is measured in watts per sq meter, not watts per meter cubed (the volume of space between that square meter on the earths surface and the sun). We need a reccomendation of light that shows us watts per square feet. Im still looking into all the variables, distances and inverse square light distriution and suchlike.

I read the wpg rule is more lenient for tanks over 200 litres, but what equation do we use in this case?

Post your comments. Ill upddate soon

 

[TwoTankAmin]

Firstly, it is important to keep the tank filled to above when the top frame starts. When filled the inside glass is like a mirror which reflects back light that hits it, Lower the level and light escapes.

Secondly, it is not possiblt to return all the "wasted" light back into the tank with any reflector. Some of the very best designed and highest quality I have seen are from AH Supply. That link will explain it with diagrams (click on the 35-55w link and read).

Thirdly, the wpg rule is a general rule of thumb based on an average depth tank. Deeper tanks need fewer wpg and shallower ones need more to provide the same light to plants (of course much deeper requires more light due to absorbtion). The reason for this is dispersion- the farther from the light source light gets, the greater area it will cover and the less intense it will be. In tanks the depth differences are no real great so they will effect dispersion more than intensity. Basically in a shallow tank the light hits bottom before it can spread out very much, so you need more wattage to cover the area.
The most effective way to light a tank is to have bulbs over both the back and the front of the tank (you can also use bright lighs such as metal halides hunf over the middle).

Fourthly, re absorbtion:

This is the same effect that causes underwater photos taken below three feet to be so blue. Just as the atmosphere absorbs non-blue light so does water, except water absorbs non-blue light at a much greater rate. Almost all non-blue light below three feet of water is absorbed.

This is why underwater pics look so blue.

 

[Themuleous]

Hi there

You make a good point, however there are few things that you seem to have been lost along the way. If I can explain;

Please do not be offended by this, it is only meant as advice, I hope that’s why you posted the thread after all!

Firstly, water, any water no matter how clear 'absorbs' light as it passes through it, this heats the water and is why when you place a clear bowl of water on a window sill it warms up (there is also a certain element of the light being reflected back out of the water, but I'll stick with absorption for simplicity sake). This process reduces the amount of light the deeper you get. This is way everything living on the bottom of the sea is an animal and not a plant, because the light cant get that deep so plants cant photosynthesis that far down.

If we take sea water for example (yes I know its salty but it’s the best I could find on the net):

0m Water surface 100% light
1m by 1 m about 60% of the light is absorbed.
10m by 10 m about 85% of the light has been absorbed.
150m by 150 m about 99% of light has been absorbed.

See http/www.punaridge.org/doc/factoids/Light/Default.htm for more info.

The WPG rule is therefore applied in order to try and provide an easy way of assessing how much light you need for a tank of a given volume as the deaper the tank the more light will get absorbed. By using a measurement of volume it provides a crude (but effective) way of allowing for the fact that in general the bigger the tank (and therefore the bigger the volume of water) the deeper it will be and therefore the more light will get absorbed before it reaches the plants. This is also why really big tanks may actually need relatively less WGP, as after a point they stop getting taller, but get just longer, hence the light absorption has reached its maximum in the aquarium.

On this basis, if you empty half your tank of water, whilst the light over it will be the same, the amount getting to your plants will be a great deal more, as there is less adsorption. You say that none of your plants are over half way up you tank, clearly then much of the light you pump in will be lost as it passes through. I guess you could lower the water level until your plants grow taller or just have all your plants at the surface?

The WPG rule should only ever be used as a guide. You could get into light intensity metres and measuring it at given depths in your tank but that'll probably cost millions! The WPG rule is free!

Hope this helps? I guess at the end of the day the WPG rule must work somehow or people would have stopped using it by now!

Sam

EDIT - TwoTankAmin also gives some good advice, which I would mostly agree with

 

[George Farmer]

Both well said.

Spiceweasel - The WPG is merely a guide to simplify the otherwise complex subject of light measurement in planted aquaria.

Please feel free to come up with a better "rule" that can be appiled to all circumstances. I fear the inevitable complexity of such a rule will go over most of our heads though.

Have you read this? - http/rexgrigg.com/./mlt.htm

 

[Spiceweasel69]

Whilst i can agree with some of the points raised, i find others quite hard to believe.

The fact that water at a depth of 1m has "absorbed" 60% of the light is simply not true. The fact is that its surface has reflected around 55% of the light, and the water has only absorbed the last 5%. Ever flown over the sea on a sunny day. An incredible amount of light is reflected back up.

Going on this, as i dont think many of us have tanks deeper than a meter, the difference of light ABSORBTION from just under surface to the bottom of the tank is minimal, as i say, about 3-5%.
If i half empty my tank as you claim, i do not get a brighter floor of my tank, and its certainly not twice as bright as the WPG rule would state. ( i have reflectors fitted so i can assume the vast majority of light gets to where it should be and is not dissepated before).
So i would assume that 99% of forum users have tanks of a depth of between 12 and 25 inches. The light lost in that amount of depth is so minimal that it would never be noticed.
You have stated that it takes a further 9 meters of water for a further 25% of light to be absorbed by sea water, which backs up my claim.

Whats needed is a rule that we can use that gives us watts per unit area, and a compensation for tank depth.
eg
20watts per sq ft minus 2% per foot depth tank.

Ta geroge - thats exactly what i was after. Im gonna get crackin on it.

 

[George Farmer]

Ta geroge - thats exactly what i was after. Im gonna get crackin on it.

All the best.

For your interest I have an even simpler guide for all tank sizes - if you can carpet glosso then it is high light, if you can't then it's low light.

Not as silly as you may first think .

 

[Jen]

I know nothing about physics, but I just wanted to add something.

You stated that if you lower the water level, halving the amount of water in the tank, the wpg should double. Have you taken into account the light that would be lost through the sides of the tank above water? Even though the reflectors are pointed downward, the angle it creates wouldnt limit the light just to the top of the water, unless you altered it further.

Did that make sence?

 

[kenneth_kpe]

The thread is loong ! As much as I would like to read everything Im just too lazy tonight to I just breezed thru it. Anyway the WPG is just a lighting for dummies rule... Halving the volume of water does not mean you get more bang for your wattage of lighting. Anyway here is a link to a better way of explaining things ... the minimum light threshold thingie.. I find it much more accurate and HARDER to understand hehe
have fun
http/rexgrigg.com/./mlt.htm

 

[George Farmer]

That link rings a bell?

 

[kenneth_kpe]

yuuup hehehe I hope its ok to post that link

 

[George Farmer]

yuuup hehehe I hope its ok to post that link

So do I mate, I've already posted it in this thread .

 

[kenneth_kpe]

Both well said.

Spiceweasel - The WPG is merely a guide to simplify the otherwise complex subject of light measurement in planted aquaria.

Please feel free to come up with a better "rule" that can be appiled to all circumstances. I fear the inevitable complexity of such a rule will go over most of our heads though.

Have you read this? - http/rexgrigg.com/./mlt.htm

GOOD GOD IM GETTING BLIIIND!!! clap clap for me hahaha

 

[lljdma06]

Anyway the WPG is just a lighting for dummies rule...

You calling me a dummy, Kenneth_kpe? Why don't we take this outside?

Ok, now you have me curious, gf. I read the article and understand it's principles, very interesting. Now, I'm going to ask a stupid question. How do you calculate your lux rating on your bulbs? I'm probably missing something really obvious, but I now want to know the lighting of my tank based on the article you posted.

Your simple rule, gf, is effective too. My glosso doesn't carpet in my 10g, it grows upwards. The jury's still out on my 15g, the glosso doesn't grow upwards, but I just added it a week ago. The glosso there, however, looks more compact and bushy.

Sorry, to hijack the original post with my question, Spiceweasel, I mean no offense, but I always like to learn something new.

 

[Themuleous]

If anyone fancies yet more reading you could read this thread which I’m sure will add yet more fuel to this debate!

Spiceweasel - if you think of a better rule please let me know, I'm fairly sure I know nothing about this subject. Don't forget to allow for the fact that red light gets absorbed first, then green then blue. Seeing as plants use red and blue more than green light you'll need to devise a logarithm that is able to account for these different wavelengths.

For all who actually give a dam (not many I reckon! ) I got this from the University of Southampton’s Website. I think you'll agree it explains things really rather well (http/www.soes.soton.ac.uk/research/groups/oil_monitor/contrast.htm)

Light from the sun and sky passes through the surface into the water. Not all the radiance is transmitted through the surface, a small percentage is reflected back up. The magnitude of the diffuse transmission coefficient for incident light is dependent on the sun zenith angle, atmospheric conditions, the refractive index of the water and the sea state. Within the water some of the downwelling light is scattered back up by the water and suspended particulates and passes back through the air-water interface where a proportion is reflected back down (again depending on the incidence angle and the seawater refractive index). This water-leaving radiance is dependent on the absorption and scattering properties of the seawater and its dissolved and particulate constituents, and forms part of the signal recorded by and airborne remote sensing instrument above the surface. The recorded signal also contains a contribution from reflected sky radiance. The magnitude of this specular reflectance component is dependent on the viewing angle, the magnitude and polarisation of the reflected sky radiance, the seawater refractive index of seawater and the sea state. Finally there is a path-radiance contribution from atmospheric scattering, which depends on atmospheric conditions, the sun-angle, the viewing angle and the height of the instrument above the sea surface.

Sam

EDIT - I guess what we really need is underwater lights, that would solve all our problems

 

[George Farmer]

I'm all for learning as much as possible on this subject but personally I draw the line where I start to get confused. Light measurement is one thing that does confuse me.

I know as much as I need to - that 4 x T8 tubes in my tank gives me sufficient light to grow pretty much anything.

Good luck with finding out any new "rules" but I think it may have been done before now if it was feasable. Even Tom Barr uses WPG - and that's good enough for me.