PFK

Discussion in 'General Planted Tank Discussions' started by Joecoral, 19 May 2008.

  1. Joecoral

    Joecoral Member

    Messages:
    694
    Location:
    Neath, South Wales
    According to the latest PFK, you need at least 2 WPG to grow crypts and anubias, and even more to grow hygrophila and vallis...
    :?
     
  2. Which bit is that in?

    Tom
     
  3. Joecoral

    Joecoral Member

    Messages:
    694
    Location:
    Neath, South Wales
    In the "new to fishkeeping" article under the heading "why don't my plants grow?", p46

    Don't get me wrong, I'm not out to attack PFK, it just baffles me that they have someone like George on the staff, then publish info like this. Some newbie is probably gonna read it, go buy >2WPG for their lightly planted community tank, then wonder why there is an algae explosion
     
  4. George Farmer

    George Farmer Founder Staff Member

    Messages:
    7,089
    Location:
    Cambridgeshire
    I'm not on the editorial team, so have no control of magazine content that I've not written myself.

    By the way, Neale states 2 watts per 4.5 litre (UK gal.), which is considerably less light than the 'regular' 2wpg, that assumes US gal. So for some set ups, this isn't really as far out as it may appear.

    Cheers.
     
  5. Joecoral

    Joecoral Member

    Messages:
    694
    Location:
    Neath, South Wales
    fair enough, shows who the newbie is then (me!). just seemed a bit much when i read it, i assumed WPG was in UK gallons
     
  6. Dan Crawford

    Dan Crawford Founder Staff Member

    Messages:
    3,266
    Location:
    Daventry, Northants
    I've addressed this matter before but the general opinion is that the whole WPG thing is so inaccurate that it is all much of a muchness.
     
  7. beeky

    beeky Member

    Messages:
    879
    Location:
    Chippenham, Wiltshire
    From what I know about physics (which isn't alot!!) watts in electrical items is the power consumed. So watts when used in terms of lighting is the amount of power is takes to light the tube, not the amount of light that is emitted. This is why it's so confusing with watts per gallon because it all depends on the light that is being used. What if you're using the new LED lights from TMC?

    If you know physics, feel free to rip all that apart! :lol:
     
  8. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

    Messages:
    8,952
    Location:
    Chicago, USA
    beeky, watts isn't limited to electrical power only. Power is defined as the ability to do work (to be more technically accurate it is the rate at which work is done). Lifting buckets of water during a water change is also an version of power consumption and can be expressed in watts for example. :lol:

    Apart from all that photosynthesis does not depend on wattage per se but instead on the total amount of photons striking the leaf surface. Each photon that strikes a chloroplast causes that chloroplast to emit exactly one electron. the continual bombardment of photons results in something called The Electron Transport Chain, which in effect is an electrical current. It is this electrochemical energy that powers the engine of food production within the plant.

    The true measure of the lighting therefore depends on the number of photons per second striking the chloroplast. This measurement is called the Photon Flux Density. High wattage of light results in a higher density of photons, however the density is reduced as the distance from the light bulb increases. This photon density reduction follows the "Inverse Square Law" so that at twice the distance from the bulb there is four times less photon density. Additionally, more photons reach the leaf due to reflections from neighboring surfaces, but there is no way to account for this in a general rule. As the plant gains altitude it enters zones closer to the bulb so there are flux density changes here as well.

    One of the reasons that the wpg rule breaks down for large and small tanks is because of this inverse square law. In small tanks the distances are much smaller so the photon flux densities are enormous. In large tanks the wpg calculations become so high that they become unrealistic because the height of the tanks are not that much greater than on medium size tanks.

    Photons are photons so it doesn't matter what the source is. The new LED lighting needs to be compared with T5 in order to determine if similar wattages result in similar Photon Flux Densities. Then there will be no mystery. We already know that T5 produces high densities than T8 for a given wattage so it should not be difficult using a PAR meter to compare the LED system.

    Now, try explaining all this to a newbie. It's a lot easier to just use a wpg rule of thumb for average sized tanks to avoid confusion, although I would have clarified that 2wpg T8 is different than 2wpg T5. The Imperial versus US gallon doesn't change the flux densities all that much because the distances from the bulb isn't affected.

    Cheers,
     
  9. Another great explanation there :D

    Tom
     
  10. beeky

    beeky Member

    Messages:
    879
    Location:
    Chippenham, Wiltshire
    I'm assuming comparing the same type of bulb? A common household 100w bulb gives the same amount of light as an energy saving bulb of 30w. The number of photons emitted are the same but the wattage is different.
     
  11. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

    Messages:
    8,952
    Location:
    Chicago, USA
    Yes, when comparing wattage it's better to compare within the same bulb types. Remember also that the shape of the bulb determines the shape of the photon emission pattern. A household light bulb emits photons in a spherical pattern whereas our bulb types emit in a cylindrical pattern. A common torch emits it's photons in the shape of a cone. Therefore the total amount of photons is a different measurement than the pattern of how those photons are distributed. We would need a PAR meter to map the photon emission across every square centimeter of each bulb to really say for certain that the 30w energy saving bulb produces the same amount of light as a common 100w bulb.

    Remember also that the frequency of the light has an effect on what we perceive as bright or dim. A 24w Grolux bulb may produce nearly as many photons as a 24w Daylight bulb but looks to us much less bright. But that's partly because we are more sensitive to green light and not so much red or blue. Red light is a lower frequency so that it delivers fewer photons per second than green or blue light which are at higher frequencies. Again, Only a PAR meter can measure the actual photon emission. Along with being more efficient, the energy saving bulb could also be producing wavelengths closer to the green so that it "appears" brighter. I'm not say that is the case but without the PAR measurement data we have to be at least a little suspicious. ;)

    Cheers,
     
  12. So what PAR readings would you look for for a given tank size? Is there a way to calculate it or is it like the wpg rule in that you can't give a definite light requirement per size of tank?

    Tom
     
  13. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

    Messages:
    8,952
    Location:
    Chicago, USA
    Hi Tom,
    That's exactly the problem. There is no way to calculate what is required because it depends on the plant's requirement. Also, PAR measurement is not a single measurement. It is a profile across the length width and height of a tank. The PAR value at each point in the tank would have to be plotted on a three dimensional graph or maybe a two dimensional plot to make life easier (length versus height). Each tank is different. A low light tank may have a PAR measurement of say, 100 micro moles at the waters surface and this might fall off to 30 micro moles at the substrate where the carpet plants are. This might be a 1.5 wpg tank in a dark room. A high light tank may have 200 micro moles at the surface which drops to 50 at the carpet plants. But how can you hand those measurements over to someone who doesn't have a light meter?

    The amount of reflected light depends on what things are in the tank. The type of canopy reflectors also makes a difference. Each day the plant grows so it's energy absorption changes daily. There is also ambient lighting that enters the tank from the outside. My tank sits in the conservatory for example and I have white floor tiles. Someone else has a their tank in a darkened bedroom with brown carpet. Which type arrangement is to be used as a reference? The other thing is that you would then have to match your PAR profile to a CO2 profile, so you would need an equally accurate CO2 reading at each three dimensional point in the tank. :wideyed:

    There are so many variables it's just not feasible to make a rule based on these data. It is important however to understand a little bit about what is going on and to be aware of these concepts. If we are more cognizant of the photons, electron flow within the chloroplast, nutrient and CO2 relationships with respect to light energy input and inverse square law then we have already accomplished a great deal. We can still use the wpg rule and at the same time understand it's limitations.

    The PAR readings are more important when we compare light sources because that gives us an unequivocal reference i.e, how do MH compare with T5's, LED and so forth. So when someone talks about light penetration we can look at a PAR profile of a lamp and determine the validity of their argument.

    Cheers,
     
  14. Egmel

    Egmel Member

    Messages:
    724
    Location:
    Guildford, Surrey, UK
    Welcome to the world of engineering, we spend years learning about all the highly complex systems only to trade them all in for rules of thumb when you actually want to use them ;)
     
  15. beeky

    beeky Member

    Messages:
    879
    Location:
    Chippenham, Wiltshire
    Glad we got that sorted then.


    I'm off for a lie down....
     

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