The Watts per gallon / liter saga

[phoenix44]

My take on the Watts per gallon saga

‘Back in the day’ when people used incandescent bulbs to light up fish tanks and grow plants (however inefficiently) everyone had the same type of bulb and it was easy to measure how much light came out of a 60W bulb because everyone’s 60W bulb was the same.

However with technological advances such as metal halides, CFL tubes, T8s, T5s, T5HOs, T5VHOs and LED’s it was possible for more light to be produced for each Watt of power used.

The efficiency of the lights also increased with less and less energy being converted into other forms of energy like heat.

The Watt is thus a derived unit of power (named after Henry Watt) and not a unit that represents how much light a tube/bulb can produce.

Light output is measured by Lumens. Physicists will be accustomed to the term ‘candela’ (the two terms are closely related).

Therefore now-a-days I feel it is incorrect / inaccurate to use the term Watts per gallon / L (WPG/WPL) as the very scale that forms the basis for comparison is not constant. This is amplified by the fact that an 18W CFL tube produces the equivalent amount of light as a 100W incandescent bulb (the old type).

So practically speaking if someone had 2, 100L tanks; one lit with a 100W incandescent bulb and one with an 18W CFL according the the WPL calculation the person with the 100W bulb would have more light (1WPL) than the person with the 18W CFL (0.18 WPL) – and we know that this in practise is not true.

Thus it is possible for someone’s 108W of T5VHO light to be far brighter than 108W of ‘normal’ light from a tube, and so the WPG / WPL scale no longer holds as we strive to achieve higher levels of efficiency.

[SamH]

it.

[kiwiplymouth]

Agreed. But I feel that you are simplifying things somewhat or should that be somewatt

Without getting anal about stuff I think that there are two other simple variables that you should have been mentioned.

1) The depth of water that the light needs to penetrate.

2) the kelvin (k) rating of the bulb that is being used.

[phoenix44]

Didn't mention Kelvin ratings and height of water to penetrate as they are independent of the WPG theory.

Under the WPG theory - A 100W bulb on top of a tank that holds 10G of water at a column height of 10cm is the same as a 100W bulb on top of 10G of water with a column height of 1cm, because the amount of water is the variable not the height of the water column. In both cases the 'amount of light' (if I dare say that) is 10WPG.

The above was only intended to deal with WPG. I think suphew and Warren have done a blurb about Kelvins.

[douwe]

In other words we should work in Lumen per Litre

So if you know the lumen of both lights mentioned you would be able to compare them and then now how much energy (light) you actually put in

[phoenix44]

In other words we should work in Lumen per Litre

well that's where it gets tricky and nitty gritty. it would be more accurate, but I'm not convinced on its practical uses.

A lumen is the power of light as is perceived by the human eye and is measured at the light source, and so does not account for the height of the water column.

I'd be more inclined to use candela - but the use of candela is neither practical nor feasible as it accounts for the distance between the light and the point at which you measure it - which is a good thing, but its hard to measure. so in a tank, glosso (that lives at the bottom) gets less light than the top of a tiger lotus plant that gets more light.

secondly to measure candela you need a meter to do so, but in the olden days 100W = 120 Candela or there abouts.

[douwe]

A lumen is the power of light as is perceived by the human eye and is measured at the light source, and so does not account for the height of the water column.

But that is the same for watt

I'd be more inclined to use candela - but the use of candela is neither practical nor feasible as it accounts for the distance between the light and the point at which you measure it - which is a good thing, but its hard to measure. so in a tank, glosso (that lives at the bottom) gets less light than the top of a tiger lotus plant that gets more light.

secondly to measure candela you need a meter to do so, but in the olden days 100W = 120 Candela or there abouts.

So we should go for Radiant flux (or power) which is the measure of all light?

[phoenix44]

But that is the same for watt

Not really, a watt does not measure light; it measures power consumption. so a 60W bulb that is terribly inefficient and produces more heat than anything else would produce less light. but it still consumes 60W of power.

It's just that most of us knows how much light is produced by an incandescent 60W bulb so have a basis for comparison. That basis for comparison gets thrown out the window if we compare 60W of incandescent light to 60W of LED lighting or any other different type of lighting.

So we should go for Radiant flux (or power) which is the measure of all light?

Perhaps, but I wouldn't as it is neither practical nor feasible. It would certainly be more correct to measure it in that way.

If I had a say in the matter I would say one could use WPG/L as long as the people in discussion were talking about the same type of light.

eg.- I have 216W of T5HO on my 200L tank. thus i can compare that to someone else's tank with 108W of T5HO light, and say I have twice as much light on my tank than they do on their 200L tank.

However I can't compare it to 216W of MH lighting or 216W of LED lighting as the amount of light emitted for the same amount of power consumption (W) is different for each type of lighting.

Does that sort of help to understand what I am saying?

[douwe]

phoenix44 wrote:

and so does not account for the height of the water column.

But that is the same for watt

I actually referred to that

Does that sort of help to understand what I am saying?

Yes it does

But it will make discussions quite a bit harder before you are down to apples with apples

[phoenix44]

I actually referred to that

Ah LOL. :lol: It has been a long day. I thought you were comparing a lumen to a watt. Apologies.

At the moment when I hear the term WPG I immediately have to ask what sort of light is being used so I can then judge the relative lighting on the tank.

[kiwiplymouth]

At the moment when I hear the term WPG I immediately have to ask what sort of light is being used so I can then judge the relative lighting on the tank.

Which brings me back to my original point.

To have any idea if lighting is adequate you also need to ask.

"How deep is your tank" and

"what is the colour temperature (kelvin) of your lights"

I.E.

I may be able to grow glosso in a 10cm deep tank with a 30w incandescent bulb but not in a 100cm deep tank with 200w MH bulb.

and

If I am using 30w of 4000K light in one tank and 30w of 10000k light in an identical tank my choice of plants that will thrive in each tank will be completely different.

[alanmin4304]

The other point that people forget sometimes is that the light reaching the bottom of a tank 600mm deep is only a fraction of the light reaching the bottom of a 300mm tank---not half. Therefore if you double the depth you need to more than double the light output. This leads to a couple of problems--you can end up with light at the top causing problems with being too intense in order to get the required light at the bottom, and different light frequencies penetrate water differently so some types of light may be completely filtered out at the bottom. This is known by the salties because red light is quickly filtered out in seawater so they use blue lights.

[Warren]

You also need to consider surface area as well as the depth. Then you are looking at the lumens/m² at the surface and can consider if it's high enough for the depth of the tank.

Efficiency of reflectors must also be considered - is all the light making it to the surface of the tank or only 50-90%. Also, is the light evenly distibuted or significantly more intense in some areas (MH).

I think all factors mentioned so far really need to be considered and yes, watts is a very poor value to go by given the huge range of lights, colour spectrums and efficiencies available now.

[Ice222]

I agree with what's been said, but another factor I have problems with is Kevin rating. From what I know Kevin rating is simply the colour temperature of lights, but it does not really represent where the peaks of the spectrum are. I've seen some tubes, usually the expensive ones branded for aquarium use, with pictorial representations of their colour peaks on them, but most lights don't have these. So correct me if I'm wrong, but it sounds as though it's possible to have two lights with the same wattage, same type of lighting(eg T8,T5, etc.), same kevin rating, but still differ on performance for plants based on their colour peaks.

[phoenix44]

I agree with what's been said, but another factor I have problems with is Kevin rating. From what I know Kevin rating is simply the colour temperature of lights, but it does not really represent where the peaks of the spectrum are. I've seen some tubes, usually the expensive ones branded for aquarium use, with pictorial representations of their colour peaks on them, but most lights don't have these. So correct me if I'm wrong, but it sounds as though it's possible to have two lights with the same wattage, same type of lighting(eg T8,T5, etc.), same kevin rating, but still differ on performance for plants based on their colour peaks.

That is more or less how a powerglo works. It is not really 18000K, which is why it's more correct to say '18000K powerglo' and not an 18000K tube which strictly speaking it isn't.

[Warren]

And what makes it even more difficult is there doesn't seem to be any data available as to which frequencies of light are needed most and in what relative levels and which plants need what. It means finding the right tube is guesswork to some degree. The best way is likely to see which types of tubes other people find useful and use those. The same could be applied for lumens/m² and depth - follow the success of others.

Please, correct me if I'm wrong about the data for which plants need what light spectrums...

[Jennifer]

Kelvin ratings for designer lights are a marketing ploy although wavelength does matter when you have salt water since the ionic properties of the salt absorbs the radiation of low energy spectrums (toward the red end of the spectrum). That is why blue wavelengths are used since they are high energy and thus have better photon flux/light penetration of the saltwater.

Freshwater does not absorb the wavelength radiation as much, so both high and low energy light spectrums can be used to grow plants. It doesn't matter what wavelength you choose, the plants will adapt either with clorophyl or any of the axillary pigments. Light spectrum is a minor factor compared to all the other factors contributing to growth and there is plenty of evidence out there to prove that. Your light spectrum will not make your plants go red nor will yellow and green spectrum prevent your plants from growing. The plain and simple truth is that you can pick the colour of lights that you like, it will not make a darn bit of difference either way once that plants have adjusted.

What will make a difference is the photon flux/lux/PAR/light penetration. Smaller tubes have a smaller light emitting surface and will always be less effective than larger tubes. The 'watts per gallon' was derived for T12 tubes as a way to estimate the penetration of light and there have been some recent investigations as to how those factors compare with T8 and T5 lights and lighting calculators are being discussed as a way to help take away some of the guess work. The best way to be certain is to use a PAR meter to measure the penetration of photons in your tank but if you don't have one available, you can only guess.

In short, there are a few good rules for plant growth:

1. Buy whatever tube spectrum you like

2. Buy the longest light fittings you can for your tank

3. Use the lowest amount of light that will grow your plants

4. Measure the light penetration if you can, and if you can't, start with watts per gallon and work up slowly

[Bishop]

Handy thread. :thup:

The plants in my tank are not dying so I am happy with my lights but I want to look into getting a PAR meter, just because I like to know.