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I How do PIR detectors react to plants?

  1. Jun 27, 2017 #1

    sophiecentaur

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    I thought that plants respire so slowly that their surface temperatures must be pretty much the same as ambient. However, on windy nights, I have experienced three different PIR security lights in three different positions that are triggered by nearby leaves when the wind blows them about.
    None of the three PIRs are pointing at the house and there are no houses in the direction they point. What could possibly be the explanation, I wonder? Could the leaves be reflecting IR that originates in the house? Difficult to believe in one case because it is solar operated and 10m away from the house, pointing along a deserted lane.
    The problem goes away when I trim the growth back to a spacing of more than a metre.

    Possible answer:
    I found one paper (this link) that suggests values of heat evolution of respiring plants of the order of a few mW per gram of plant material. A small branch of the size I am cutting off, would perhaps have a leaf mass of 200g, which would imply less than a Watt overall. For just a couple of leaves (mass, say 10g), the heat evolved would be only a few tens of mW. Would that raise the surface temperature appreciably? Any opinions?
     
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  3. Jun 27, 2017 #2

    Drakkith

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  4. Jun 28, 2017 #3
    What is behind those leaves? If it's the sky it will probably be colder. You might get get a mixture of atmospheric IR at various altitudes. It willl be much more significant than the temperature rise from plant respiration on a windy night.
     
  5. Jun 28, 2017 #4

    sophiecentaur

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    Thanks for that list. I have had most of those problems at one time or another. I ended up using a Radar based detector once, in a really difficult situation. That sorted it. Most of the items in the list don't fit the scenario, though. The only thing that triggers the misfire seems to be wind plus nearby leaves.
    I think you could be right about background IR being shuttered by the moving leaves. Another idea I had was reflection from the house behind - but the house is not behind one of the detectors. All three PIRs are tilted significantly downwards so not much sky is seen and there is no artificial lighting closer than 1km. Perhaps it could be down to warm ground but the view is mostly just greenery. The effect is no worse or better in cold weather, although, to be fair, I guess there is no growth up there in winter.
    No opinions about my Hot Leaves possibility? I suppose I could try experimenting with a resistor on a stick. . . . . . . :wink:
     
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