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Thread: Help me understand VPD

  1. #11

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    Quote Originally Posted by Blue61 View Post
    Is it like evaporation cools us, so that's what the plant is trying to do with vpd?
    I would say so yes but where as we do it to lose heat, the plant does it to transpire in addition to heat. Both involve vapour

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  3. #12

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    Quote Originally Posted by gardro View Post
    Cannot really comment on your app calculator but it does not seem right at all, almost reversed.

    29C will give a big VPD with all humidity until you get to 70-80% plus.

    Its low VPD where you get deficiency as the transportation of nutrients can stop. Its all driven by transpiration. The plant cannot breathe out. With high VPD, the pot dries quickly and the salts can concentrate in plant as it transpiring very quickly and runs out of water. Too much VPD and the plant has to close its stomata as otherwise it will just dry right out - stops growing.

    You need a VPD where the plant can freely get rid of its moisture in an optimum way. During flower, the higher VPD kind pulls the moisture away quicker.

    Dont over worry about it. I only looked into as I always got bad mold, even with VPD over 2.0. My old groom was just too small in volume for the plants, and in a out building!

    Folk running 30-40% humidity have a very high VPD unless temps drop stupidly low.
    Thanks, maybe I should try another calculator as it seems this one might be sending me in the wrong direction.

    I just started looking into vpd to try and optimise growth to increase yields.

    Theres quite a lot to wrap my head around lol.

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  5. #13

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    There isn't another calculator on the marketplace... might have to do it manually!

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  7. #14

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    [ NO LINKS ]

    Dont know if I'm allowed to post links here..... but i found this online calculator that seems to be working better, giving me vpd of 1.49.

    But why is there room vpd and leaf vpd (LVPD), should they both read the same vpd measurement?
    Last edited by gardro; 08-01-21 at 03:06 AM.

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  9. #15

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    No links please.

    Measure your leaf temps with an infrared thermometer.

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  11. #16

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    Quote Originally Posted by gardro View Post
    Watch the leaf temps with LED though bud
    Aye I've got my ir monitor ready for when the little un goes under it

    Sent from my SM-G975F using Tapatalk

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  13. #17

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    Quote Originally Posted by THCBrova View Post
    [ NO LINKS ]

    Dont know if I'm allowed to post links here..... but i found this online calculator that seems to be working better, giving me vpd of 1.49.

    But why is there room vpd and leaf vpd (LVPD), should they both read the same vpd measurement?
    if you really want to nail it then just get a VPD controller, you can enter leaf temp and it does it all for you

    Sent from my SM-G975F using Tapatalk

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  15. #18

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    I got an IR thermometer, my leaf temps were 24° first time, I dropped the lights a smidge and checked later and they were 27-28°.

    VPD controller??? Thanks, I'll check that out.

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  17. #19

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    Quote Originally Posted by gardro View Post
    No links please.

    Measure your leaf temps with an infrared thermometer.
    On this calculator there is room vpd and leaf vpd. Am i just concerned with room vpd?

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  19. #20

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    Which reading do I reference on the vpd chart?

    Room 25°
    RH 50%
    Room vpd 1.59 kpa

    Leaf temp 24°

    Then its got:

    Leaf vpd 1.40 kpa

    But can obviously only match one to the vpd chart, unless I match the 2 numbers of course.

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