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Thread: Help me understand VPD

  1. #1

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    Default Help me understand VPD

    Hi guys n gals,

    Been trying to get my head round vpd for a while now but its just not clicking and hoping someone can give me some clarity.

    Ive been reading in depth online about the calculations and the effects on your garden, but when calculating something doesn't add up.

    Im using a vpd calculator app from android market place which has got good reviews, but when I punch in my air (room) temp, RH, and leaf temp I get a kPa well out of range.

    Im in late veg/early flower (flipped today), so that a range of 0.8 - 1.2 kpa

    Air temp around 26°
    Leaf temp 28°
    RH 52%

    Based on this calculator and the reading above that gives me vpd of 0.27.

    In order to get into the lower end of the ideal range of 0.81 kPa for this stage of growth, my leaf temps need to be 46°c. Surely that cant be right can it?

    I have tried tweeking the room temp and RH number to achieve a similar 0.81 kpa result but its just not doable, even decreasing these to single digits gets me nowhere near.

    Can someone shed some light please

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  3. #2

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    If I punch in numbers:

    Air temp 18°
    Leaf temp 40°
    RH 12%

    Gets me to 0.80 kPa

    Basic grow info would normally tell you that for this stage of growth number should be more like;

    Air temp 22-28°
    RH 40-50%

    So surely 18° and 12% cant be serious... can it??

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  5. #3

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    You want higher VPD when you enter flower in the 1-1.5 range. 0.8 is far to weak a deficit.

    Most of the charts and calculators suggest having RH a little too high for us percy growers in smaller tents so you will see a lot of folk in the 1.5-2.0 range or possibly higher. Its not ideal but it does remove budrot anxiety.

    Ideal is around 25°C at 55-65RH

    Type of lamp is important as leaf temperature is different based on radiant heat.

    If you run lower VPD, eg a cooler/ damp room, you should feed higher EC but still can get deficiency (calcium usually)

    High VPD, hotter / dryer, you should feed weaker EC unless you want burns

    Fair to say you need a decent environment controller as the ideal range is quite tight.

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  7. #4

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    Quote Originally Posted by gardro View Post
    You want higher VPD when you enter flower in the 1-1.5 range. 0.8 is far to weak a deficit.

    Most of the charts and calculators suggest having RH a little too high for us percy growers in smaller tents so you will see a lot of folk in the 1.5-2.0 range or possibly higher. Its not ideal but it does remove budrot anxiety.

    Ideal is around 25°C at 55-65RH

    Type of lamp is important as leaf temperature is different based on radiant heat.

    If you run lower VPD, eg a cooler/ damp room, you should feed higher EC but still can get deficiency (calcium usually)

    High VPD, hotter / dryer, you should feed weaker EC unless you want burns

    Fair to say you need a decent environment controller as the ideal range is quite tight.

    Thanks Gardro, this is interesting. So I need to ideally be thinking about this from the start of the job dont I.

    I would say my environment is on the dryer hotter side being between 20-29° (avg 26° most of the time during lights on) and RH 37-52% over a 24hr period. But my ec is already at 1.6 after seeing deficiencies at 1.2 and increasing until they looked happy. Based on what you've said above might getting my vpd into the required range have a negative effect now my ec is so high?

    Unless I could cool the room down and introduce the humidifier, although its not adequate for the room but would help.

    I am in coco using Dutch Pro nutes, and under 600W hps. I have try bring the lights a touch closer but obviously dont want to cause too much damage, especially now Ive just flipped them.

    I dont know if I can share a link to the vpd app on here but its on the marketplace, one by cannafused.life
    I am struggling to get up to 0.8... 1.5 seems impossible, even when making the numbers up. For example; if I use your numbers above of to punch into the calculator;

    Temp 25°
    RH 55%
    This says my leaf temps need to be 60° for a vpd of 1.52 kpa. 60° is surely plant murder isn't it?

    Im new to vpd so this is all assumptions.

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    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.

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  11. #6

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    What is vpd ?


    Sent from my iPhone using Tapatalk

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  13. #7

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    Quote Originally Posted by Uv123_4 View Post
    What is vpd ?


    Sent from my iPhone using Tapatalk
    Good question lol probably could have explained that. Its vapour pressure defecit.

    Its the defecit / difference between the vapour pressure in the plant and that of the ambient air.

    How easily the plant can sweat or breathe out.

    Bit like if your hot, and stand in a hot room, you cannot lose heat. If you stand in a freezer, that heat will fly off you. If you stand in a room a few degrees cooler, it will cool you down but will take a lot longer. But for vapour.

    You can have highest vapour pressure with low temps and high humidity.

    Lowest vapour pressure with low humidity and high temps.

    Thats after Sour Livers, sorry

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    I just follow this and alter my humidity to required range

    Sent from my SM-G975F using Tapatalk

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  17. #9

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    Quote Originally Posted by Done420 View Post
    I just follow this and alter my humidity to required range

    Sent from my SM-G975F using Tapatalk
    Watch the leaf temps with LED though bud

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  19. #10

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    Is it like evaporation cools us, so that's what the plant is trying to do with vpd?

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