So i grow in an open smallish room. So ive been controlling my room (in flower) vpd to 76f and 55% humidity and its spot on 1.4vpd. But i never knew you had to calculate the leaf temp! So i measured the leaves with my ir laser thermometer and they are between 3 to 4 degrees cooler than the room. So i added the leaf temp to a vpd chart and my previous 76f and 55%h is about 1vpd. I don’t want to go above 55%h so my room temp has to be 85f to hit the 1.4vpd. Is this correct?
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Sorry not familiar with this issue but I’m going to follow along for the knowledge. What do you think @Spiney_norman ?
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With a -4 leaf temp offset at 55% RH, approximately 83f to 86f would put your leaf VPD in that 1.4 kPa neighborhood. Checking two different calculators that I trust gives two slightly different numbers, but the resolution is not as fine on one of them.
However, your RH in the grow space will probably start to drop a little as your air temp rises in there, so you might be able to close in on your leaf vpd target number at a lower air temp if the RH drops enough simply from the air temp rising. Walk up the air temps a degree or two at a time & watch what happens with your leaf temps & RH once everything stabilizes with each adjustment, & go from there.
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Absolutely follow for the info. Knowledge will never be a bad thing.
For me this arena is just something I cannot play in. My tent stays open to the lung room that has an HVAC vent in it. It has one door to the house that if not just open it is walked through all the time. Winter to summer temps can be extreme and the heat and air run and change the room all the time. It would be a major thing for me to try to dial in and maintain a specific vpd for the sake of a small difference in growing a plant that I already throw away a good 1/4 to 1/3 of. But I totally understand those who wish to take their growing to another level. There was a time in a totally different hobby that I chased perfection and so I know the fun of it. We all have differing needs, circumstances and abilities within our current circumstances.
To illustrate, I could cook a totally awesome pizza in a 900 degree wood fired pizza oven, but I don’t have one nor a place to put one. However I do cook completely acceptable pizzas in my 500 degree electric oven and am ok with it.
I don’t find any fault with those who want to take their growing further. I often read threads about other techniques to see the results. Sometimes I will take away something that I can use or at least mimic to a degree.
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Practically speaking I think the best thing to do may be to get your hottest leaf temperatures up to around 75f, & see where everything ends up, & then go from there with measuring & adjusting leaf vpd. If you start with that, your leaves should be in a productive temperature range with a few degrees on either side to play with. And from there you can still adjust RH or air temp to try & move the leaf VPD. You can try to adjust leaf temp also, but toughest to do out of the three. You’ll probably go with the one or both of the other two, & just note any leaf temp & leaf temp offset changes that follow.
Have you noticed your leaf temperature offset changing ever? Eventually you’ll see how your offset runs & if there’s a range there. Then maybe use the middle offset number or whatever it is most of the time, when adjusting leaf vpd. The offset can be interesting to watch in terms of how the plant is running in terms of transpiration. If you’re watering to dry-back then you might see a range in your leaf temp offset. I’m in DWC so mine usually stays around the same once they warm up, unless they start seeing some sort of stress. Then the offset starts to tighten up & move from -xx towards zero. I’ve never seen mine go into positive territory, but that could happen too.
Should i be taking the temp and humidity from a probe in the canopy or the room temp and humidity. I grow in a open room without a tent and i notice the humidity is always higher when taken within the canopy vs the room humidity. Example, canopy 60% and room 55%.
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For your leaf vpd calculations, you’d want to locate your hottest leaves & then try to measure the air temp & RH from right around there. If you have a lot of leaves that are transpiring, you might notice higher RH if you measure right over the leaves, & you could measure an inch at a time higher to get the RH of the air that is above that layer of higher RH from the transpiring leaves. I noticed about 1% RH lower for each inch I measured higher above the canopy, until I got to about 5" above, where it seemed to level out. I thought about if I should use the RH number from directly over the canopy or from further above it, & decided that I should use the reading from the less-affected air from a little higher above the canopy, since my oscillating fans should be blowing that air around regularly. I noticed after a big defoliation that the RH layer had more or less vanished, & the readings were similar from right on top of the top of the canopy compared to readings as I moved the sensor up an inch at a time. Air temps were about the same before & after the defoliation. So I guess if you have a lot of leaves you can get your air temp readings from right over the hottest leaves, but you might want to test for a humidity layer & grab the RH number from where you break out of any RH layer. If you do a big defol, then you can probably grab both readings from right above the hottest leaves.
I hear you brother. It seems that I am always striving for that ideal VPD, yet never achieving it.
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Hey @canucanoe2 , Welcome to the forum.
Pull up a chair and enjoy the company of a bunch of decent people.
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