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  • 2 Post By Dorsetman
  • 2 Post By raybark

PAR meters. Using. Applying results... questions....

This is a discussion on PAR meters. Using. Applying results... questions.... within the New Growers: Ask the Senior Members forums, part of the New Growers category; I have been using a photometer to read the visible light falling on orchids, in ...

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  1. #1
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    Default PAR meters. Using. Applying results... questions....

    I have been using a photometer to read the visible light falling on orchids, in the wild and in my greenhouse, for many years. It is finally soaking into my thick head that the light we see, and the "light" that plants see - and need - are two different things, and that to read the latter I need a PAR-meter. Maybe I need much more, but hey, I'm an amateur, and with limited resources. A PAR meter I can afford, just about - if the expenditure is worth it to me in improving my cultivation.
    Some of this is started by having accumulated a few hundred pots of dendrobiums, hanging above one bench in my greenhouse. They love it, and grow well, but I am realising that I am not seeing any flowers on the plants beneath them ; not enough light(?) probably.... So I plan to add some lights there, but the space is limited, although the choice seems rapidly expanding. But how to read what light they are putting out , and how to match that with what my plants need ?
    Advice will be appreciated- has anyone here gone there yet ? ( I note that AOS is still giving advice based on mere foot candles ( or in Europe we would use Lux) but both are reading visible light and not the spectrum that plants need.
    And yes, I do know that for PAR, spot readings are not enough, and averages over the growing area are needful, but still - what units do these meters read in ? and what values do different orchids want ?

  2. #2
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    I think many growers need some advice here. Thanks for posting this question.

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    I don't think there's a database - yet - of orchid needs in PAR terminology, but fortunately, there's a fairly straightforward way to estimate it.

    PAR meters tend to give their readings in PPFD - Photosynthetic Photon Flux Density - expressed in terms of moles of photons per area per unit time - with the specific units being micromoles per square meter per second (Ámol/m2/sec).

    Full sun at is about 10200 foot-candles and 2000 Ámol/m2/sec, so the factor is simply 1:5. So...if you have a plant that the AOS culture sheet says needs 3000-3500 FC of peak light intensity, in PAR terms, that's 600-700 Ámol/m2/sec.

    That works great for plants that are naturally lit, but for those of us growing under lights, we have to consider that the sunlight intensity goes from zero at dawn, to the peak at noon, and back to zero at dusk. The sum of the photons received by the plant is called the Daily Light Integral (DLI). Fortunately, we can estimate that sunrise-to-sunset curve as a triangle, whose area (the DLI) is calculated as 1/2 base x height, where the "base" is the hours of light and the "height" is the noontime peak. That simply means that if we take that recommended peak intensity and divide it by two, we have the light intensity for continuous illumination. AND... that means that for artificial lighting, the ratio is 1:10 instead of 1:5, meaning that for the "3000-3500 FC" plants, our lights need an output of 300-350 Ámol/m2/sec.

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    Orching is offline Junior Member
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    The problem with all those measurement tools is calibration. You probably can buy a PAR meter relatively cheap in that famous online auction site, but the chance it is properly calibrated is almost not existent. Thus, with the cheap meter you can be 10%, 20% or more off from the actual reading. A good quality calibrated meter probably will cost orders of magnitude more than a non calibrated.

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    Aquii Finn is offline Member
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    I am new to led's, and just purchased an apogee mq500 par meter to be able to tell what light is falling where. The older apogees have limitations, and the half priced competition is 30% off in accuracy!!! So i went with the most expensive, unfortunately, for accuracy..

  6. #6
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    I havejust got a PAR meter. Now have to find out how to use it to any advantage.
    I have mostly 600 watt “dual spectrum” metal ballast lamps, shunting back and forth across my main bench, plus one 240 watt LED unit doing the same. But my side benches are overhung with my dendrobiums and small vandaceous stuff and the flower count on these side benches is dismal. So I have recently installed a certain amount of extra lower powered lighting hanging above those side benches but below the dendrobes etc. There are three different kinds of these, and I want to discover which is going to be the best before buying more.

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    What did you get, Geoff?

    Some meters have different settings for incandescent, fluorescent, and LED light sources.

  8. #8
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    It is a Hydrafarm Quantum meter. Not much choice over here, unless you have the sort of funds that university a or big research departments can tap.

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