Very interesting and impressive guy… knows a lot about weed that’s for sure
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Excellent topic, thank you for sharing this @Mr.Sparkle !!
I too have been making adjustments to my environment to maintain better VPD in my flower room recently, partially as a result of upgrading my Bridgelux Array but it all was once again started with a conversation we had on my thread the last cycle about leaf temps. Thank you for all that you share and do here friend 
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Great read! GML has a great video on this:
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Couple questions for you @Mr.Sparkle:
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When measuring leaf temperature and you open your cabinet to use the IR meter, does the now lower ambient temps (cause opened cabinet) effect what’s really going on with the leaf temperature differential or do you think both the ambient and leaf temperature lower equally maintinaing that same differential?
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At what point from seedlings to veg to you think there starts to become a differential? Kind of difficult to get readings on small leaves.
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Got to take measurements quick as the ambient air does cool things quickly though saying that objects within your space retain that heat a bit longer than say leaves of your plant so if I’m measuring its the leaves first right when its opened.
As for when there is a differential that depends and is or can be always there it’s just when plants are small and you have nothing else in your space heat just builds up if your blasting light which has no where to go, though once you start to have enough vegetation then the plants work at cooling themselves and the space by transpiring, I guess if I had to put it in a metric id say for me it’s when I have vegetation filling 35-50% of my area, before that it’s more a low humidity/high temp issue for my space
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I got high temps as well but I can raise the humidity no problem… I’m just a bit confused as to where I should be VPD wise for youngings… all those differential charts have a propagation section so it makes it a lil confusing… I would suspect that seedlings only transpire a lil, so it’s hard to imagine how a 5F degree differential chart would apply for instance yet it still has that propagation section… I’m personally shooting for 1-0.8 VPD but that is pretty much a guess for these seedlings as I can’t really get a reading in them much altho saying that, I might be seeing -3F 
could just be the soil tho.
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I use an app called VPD Calculator. It was recommended by someone her on OG.
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@GnomeyByNature , The pulse grow monitors have set profiles for different stages of the cycle. They show seedlings and clones require 0.8 VPD. Then veg is stated as 1.2 VPD. I have also read elsewhere that in veg you want between 0.8 and 1.2. So for the clones and seedlings i would aim around 0.7 to 0.8.
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Is that all off the 0F degree differential I take it? I believe all of the charts I have seen basically have the same VPD range suggestions but in the end they are different because of the leaf/ambient temperature differential.
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Well the VPD should be the same for a particular stage of the grow. You should be doing the Leaf Surface Temperature offset calculation before applying it to the charts.
As an example: My leaf temp is 4C less than room temp. Someone with a HPS light would have a Leaf temp about the same as the room temp. So we can both have different room temps but the same VPD of 1.2 because we have always calculated the offset of the leaf temp prior to applying it to the graphs.
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Do you ever measure seedling leaf temperature?
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Nah. To hard to measure on such tiny leaves. Most plants will be close to the same so I take the LST offset of the previous grows early veg stage. My LST offset is usually about -4C most of the grow. The thing that changes the LST offset is usually changes in equipment and settings in the grow space as far as I understand it. So as long as I have not changed anything major I assume my LST offset is the same as the previous grows early veg offset.
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So you just assume -4C on the seedlings that are too small to measure, am I understanding that correct? You think they transpire that much at that stage to get that LST?
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I do not know. I am only making the best estimation I can.
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I hear ya, that’s why I’m asking everyone, I going gut feeling of 1 or 2 degrees F for now
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I have a second spot ( different area from my own.) which i use 600W & 400W hps lights not in a tent but a portion of a room about 4 by 4 ft and i think the hps buds are probably more bulky/quicker to finish than the plants i grow under some SolStrips, and they stretch a bit more under the hps than my leds , but it’s probably gonna be different for everyone else as the environment and growing styles are all different too ~ probably know/find out more about this after the BIG MONEY goes into cannabis in the US ( Federally legal.). When research will be done etc.
Great subject @Mr.Sparkle …!
Happy growing peeps…
Gaz
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