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PAR what does it mean?

Discussion in 'Are you new to aquatic plants? Start here' started by crystalview, May 2, 2009.

  1. crystalview

    crystalview Guru Class Expert

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    45g 24" tall No CO2 EI daily excel


    light tested 96w 6700k 4" above water

    water level meter read 140

    middle half way down read 40

    Left side half way down read 33

    Right side half way down read 33

    Gravel middle 24

    Gravel left side 15

    Gravel right side 15

    36" T5 10000k 42w (flat on tank Inch from water)

    water level meter read 114

    middle half way down read 59

    Left side half way down read 58

    Right side half way down read 58

    Gravel middle 30

    Gravel left side 30

    Gravel right side 28

    If reading should be for planted tanks: Low - 50 , Medium - 75-150 , High - above 150

    What do I aim for most of my plants are: stem fast growers, 3 types moss, 2 types of Fissiden several, large anubias and an african water fern.
    For now I have pennywort floating on 1/4 of top with UG floating in it. I have no idea what I am to do with these readings or which light I should use. The first light is 4mo old and the second has new bulbs.

    Since this set up is only about a month old I have Slight GSA and GDA will add oto's soon. When I use the watts per gallon measurement the first light is to many watts and the second light is too low. When I put both readings to these requirements for low light I get very lost. It seems that there is a large variance in the readings which I understand is different with the light spread and the penitration levels of the light. I want this to be manly be a moss, fissiden and few stems tank. What would you do in my case?


    Here is a post Vaughn did on the PAR but it still leaves me confused. because of the above readings

    "Unfortunately, we are just beginning to accumulate the data needed to answer that question well. Tom has posted that the approximate ranges for low, medium and high light for many plants are: Low - 50 , Medium - 75-150 , High - above 150. The effect of the light level depends a lot on whether you have good CO2 or not. With good CO2 low light intensity still gives good growth for most plants, but without CO2, many of those plants may not grow at all. I think what we are doing with the PAR meter can be considered to be just gathering information for now. But, comparing your readings to those above will give you some idea about how intense your lighting is, and whether you might want to lower it or increase it.

    Also, don't forget that the intensity near the top of the tank will be a lot higher than at the bottom of the tank. If you are trying to grow HC, for example, it is the intensity at the substrate that is of most interest, but for taller plants, it is the intensity in the middle area of the tank. Once you get two readings over the same spot on the substrate, and measure the distance from the light for each reading, you can predict pretty accurately what the intensity is at other distances from the light.

    You can also see how uniform your light intensity is from side to side, and from front to back. Ideally, the intensities will all be about the same, where ever you are in the tank. But, you can be sure they won't be.

    If you decide to try for more or for less intensity, having two readings over the same spot on the substrate, at known distances from the light, allows you to accuratedly determine how much higher or lower to locate the light for the intensity you want."
     
  2. Philosophos

    Philosophos Lifetime Charter Member
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    PAR is the range of spectrum that stimulates chlorophyll A, B and carotenoids. PAR is to plants as LUX is to the human eye; it's weighted for the application. Intensity of electromagnetic radiation (ER) is measured in milimoles, light is electromagnetic radiation, plants only use a certain range of nanometers wavelength of electromagnetic radiation. The milimoles of PAR are what you're looking at.

    For your situation, choose low-light stem plants. If you're using a glass hood between the light and water at any point, pull it off and test again; it'll make a huge difference. If your shorter plants are having trouble, move the light forward. If all else fails, get more light.

    -Philosophos
     
  3. VaughnH

    VaughnH Lifetime Charter Member
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    Here are the data for the center of the tank, plotted on log-log paper: With the green data points from the 96 wat PC light and the orange data points from the 42 watt T5 light.
    [​IMG]

    Two things it immediately shows are:
    The 42 watt T5 light gives as much intensity as the 96 watt PC bulb.
    The drop off in intensity isn't as much as it would be if it were an inverse square relationship - intensity is proportional to one over the distance squared. I haven't figured it out yet, but it looks like about half way between a drop off directly proportional to the distance and the inverse square drop. This is very good!

    But, you don't have as much light intensity as I thought you had, with either light alone. With both lights, you have good solid low light intensity, which with CO2, lets you grow most plants very well, just slowly.

    Any way, that's how I interpret the data.
     
  4. crystalview

    crystalview Guru Class Expert

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    I was really surprised at the readings. Even the visual for both lights is about the same. But the intensity should not be the same. The 96w must have lost a lot of its wattage in the six months of use.
    I do not have CO2 so should I use both lights? I use the excel everyday.
    Thanks for the chart it is easier to see the fall off in the deeper part of the tank.
     
  5. VaughnH

    VaughnH Lifetime Charter Member
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    As I remember didn't you raise that 96 watt bulb to reduce the light a bit? If so, I would lower it back to the top of the tank. That should increase the light to about about 30 micromols at the substrate and about 50 micromols half way down into the tank. Without CO2 I think that would be about as much light as I would want to use. Or, better still, just use the 36 inch T5HO light, which I think is 39 watt not 42 watt, right at the top of the tank for about the same intensity, but better uniformity front to back and end to end.

    The reason the light drops off the deeper you go into the tank is the larger area that the light covers, the further you get from the bulb. That gives the same total light, but spreads it out over more area, so the micromols per square meter per second is less. Generally that intensity drops about proportional to one over the square of the distance from the bulb, but for a bulb the same length as the tank, it seems to drop less than that. That may also be because of light reflected off the front and back glass.
     
  6. crystalview

    crystalview Guru Class Expert

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    Wow I am impressed :) I ordered two 21w 10000k bulbs (which were original). I did not noticed till you mentioned and I rechecked the bulbs. You are right they are 39w so I have 78w total. Now I know why the readings were not to off from each other. Oh well I should have known you don't always get what you order.
     
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