Estimating leaf surface temperature w/ LED quantum boards?

Hey all,

Just trying to further dial in my grow-tent-environment. I don't currently have a IR thermometer for leaf-surface temperature measurements...I do know that it differs according to types of light used. Makes sense, given the different IR output of different types of light. I'm currently using 3x HLG 225's (both have deep red and near red diodes).

Is there a good general rule of thumb for making a decent estimate of leaf surface temperature given temp readings of sensors that are hanging at canopy level? For instance, I'm averaging lights on temps of 83f today with a swing between 80.2f and 84.7f. I've been assuming something like ~3-4f degrees lower. Is that correct in your experience?

Now that I have reasonably strong lighting, I'm finding that it's easier to adjust my airflow intake/exhaust and change the temps/humidity in the room, so I'd like to tailor it for the best environment (duh, like everyone would like to). For instance yesterday my average was a bit lower, right around 79f; so I reduced my fan setup downwards and have ended up at 83f.

Of course best answer is 'get a IR thermometer' but I'd want to get a decent one if I'm doing it and I don't have the budget for it at the moment.

My rule of thumb for plant/canopy temp is to put the back of my hand under the light & move it closer till it gets hot, then I slowly move my hand away until it's a comfortable warm....that is minimum distance.

Goodluck

Hugo Phurst said:

My rule of thumb for plant/canopy temp is to put the back of my hand under the light & move it closer till it gets hot, then I slowly move my hand away until it's a comfortable warm....that is minimum distance.

Goodluck

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Thanks for your input.

My lights are fine...I actually have effectively used Photone to dial in my DLI for all the plants (and double checked with PPFD into a DLI calculator), and all my lights are literally raised to their peaks, so they're not creating the kind of heat that can burn the plants or buds. It's more about knowing there's a different temperature at the leaf than the ambient temperature at the canopy level and trying to figure out the best tent ambient temperature for the plants.

In other words, when my sensors read 83F degrees is that really 79f degrees for the plants and I should think about bumping it up a bit, or is 83f degrees really 83f to the plants? I know that some of the bigger home growers here aim a bit higher on their temps and I'm just trying to find a comfy place for mine.

I can drop my temps in the tent down into the low to mid 70's by just turning up my fan, or I can pull it back further by slowing my fan down even more...but what's the target for the room so that the LST is where it should be?

Yeah they're usually like 2-3 degree cooler

Gemtree said:

Yeah they're usually like 2-3 degree cooler

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Sounds like running in the 80-86f range is the place to be.

This is what my temp/rh look like today. The swings are mostly due to circulation fans going on/off at timed intervals within the tent.

During the summer I was growing under a 300w 3-5k +ir and a 240w 4k +ir 18" from the canopy.
The leaf temps were consistently 26-27c the same as the ambient temperature.

Star Dog said:

During the summer I was growing under a 300w 3-5k +ir and a 240w 4k +ir 18" from the canopy.
The leaf temps were consistently 26-27c the same as the ambient temperature.
View attachment 5008885

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Huh. No variation, but also IR which does heat leaf surface more than non-IR sources if what I've read is correct.

Does that graph have some kind of smoothing involved? I've seen several other growers' environmental data graphs and they look more similar to mine, though Mine also relates to three fans in various combinations as mentioned before.

LeastExpectedGrower said:

Huh. No variation, but also IR which does heat leaf surface more than non-IR sources if what I've read is correct.

Does that graph have some kind of smoothing involved? I've seen several other growers' environmental data graphs and they look more similar to mine, though Mine also relates to three fans in various combinations as mentioned before.

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I made my own hybrid fan controller it lets me fine tune the environment to within 0.3c.
I've read its typically 2c under ambient but not ime testing with 2x ir thermometer (name escapes atm) costing £100 each.

Star Dog said:

I made my own hybrid fan controller it lets me fine tune the environment to within 0.3c.
I've read its typically 2c under ambient but not ime testing with 2x ir thermometer (name escapes atm) costing £100 each.

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Yeah, I figured if I get a therm it'll be one of the plant specific ones for accuracy and I've got other things to spend money on besides $180 for a decent one.

My tent will always have some variation. It's in the room with our house's boiler (modern 5 zone water heat) but that's shown only to be about ~5 degrees when run hard and the door to the room stays open. During the winter our lower level remains in the mid to upper 60's no matter what, so the tent should be in good shape. I'm planning on switching back to a climate triggered exhaust setup momentarily as well, right now it's just set to run at a low-mid speed.

My lights-out shift is about 10F degrees and 8-12 points humidity, which has been fine. Lights on brings it up to upper 70's and a round of showers and breakfast prep/dishes ends up raising the room to 85 before ratcheting downwards for the rest of the day.

I just like to know that my '83f' on my meter is 80f to the plants, etc. for peace of mind. They're doing OK minus all the fuckups I've done along the way though. I've put 'em through the ringer but they've still been gracious enough to provide what looks like a few ounces until my next more knowledgeable round.

Sorry to quote you but you can only know by dialing in with an ir handheld gun. You can get a very good one cheap nowadays. Guessing is just that I guess
You never are gonna truely know. And with my controller sensor, it's hanging at plant top level but the top can be 10f difference on the top of the actual sensor (checking with temp gum) vs the bottom. So it's not all that accurate.

I'm 5/6 weeks into another led grow (720W) the lower canopy is the same temperature as the ambient, the flowers closer to the light 17-18" measures 28c, 18-22" away its 26-27c and the ambient is 26.5/27c.

Star Dog said:

I'm 5/6 weeks into another led grow (720W) the lower canopy is the same temperature as the ambient, the flowers closer to the light 17-18" measures 28c, 18-22" away its 26-27c and the ambient is 26.5/27c.

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Interesting. You're running 1c above. I wonder how much light distance plays into all of this. We know that IR content very well may.

I'm running my lights HIGH but they're at reasonable if not high PPFD at the bud-tips and the DLI is on the highish side.

Also, I've mostly decided that the sweet spot is 83f (~28c). If my leaf temp rides above a degree I'm good and if it's a little lower that's fine too.

LeastExpectedGrower said:

Interesting. You're running 1c above. I wonder how much light distance plays into all of this. We know that IR content very well may.

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Canopy temperature in my tent is relative to distance from the lights, the tallest buds are the hottest and they cool to ambient as I measure lower down, the hottest is deffo relative to height, outside the lights radiated heat it's ambient temps for me.


I already had quality thermometers but if I was buying one purely for the grow I'd buy a cheap one.

Everything I have seen including a bunch of posts about it is 2f to 4f is typical, but as mentioned above some are close to the same.

With my lights about 2 feet from main canopy my leaf temps are between 82-83 right now, room between 86-87 (high due to supp CO2). I have far reds and IR in full spec 301H diodes and even when I’m running normals temps around 80 by leaf surface temp is always about 3-4 degrees less. When I turn off the IR and reds it’s closer to 4.

i have a few thermometers around the room and at different heights, this is in addition to my fan and controller sensors. I like to hand at basic thermometer at canopy height to give me a good read of ambient temps just above the canopy

Star Dog said:

Canopy temperature in my tent is relative to distance from the lights, the tallest buds are the hottest and they cool to ambient as I measure lower down, the hottest is deffo relative to height, outside the lights radiated heat it's ambient temps for me.


I already had quality thermometers but if I was buying one purely for the grow I'd buy a cheap one.

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My lights are relatively high. I haven't measured recently but they'd been at about 22" and then I cranked them when I realized my PPFD/DLI was too much and I'm closer to ~580PPFD & something like 30-35DLI currently at the bud tips.

No can do on a buying a 'cheap one'...I'm too much of a gear nerd and across the years every time I buy cheap anything I end up going back and buying nicer later. One of my soon to be upgrades will be a nicer pH meter and get out from under the 12 buck yellow generic I'm using that was the 'cheap but decent' suggestion.

LeastExpectedGrower said:

My lights are relatively high. I haven't measured recently but they'd been at about 22" and then I cranked them when I realized my PPFD/DLI was too much and I'm closer to ~580PPFD & something like 30-35DLI currently at the bud tips.

No can do on a buying a 'cheap one'...I'm too much of a gear nerd and across the years every time I buy cheap anything I end up going back and buying nicer later. One of my soon to be upgrades will be a nicer pH meter and get out from under the 12 buck yellow generic I'm using that was the 'cheap but decent' suggestion.

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I’m coveting a bluelab soil pH probe

lakesidegrower said:

With my lights about 2 feet from main canopy my leaf temps are between 82-83 right now, room between 86-87 (high due to supp CO2). I have far reds and IR in full spec 301H diodes and even when I’m running normals temps around 80 by leaf surface temp is always about 3-4 degrees less. When I turn off the IR and reds it’s closer to 4.

i have a few thermometers around the room and at different heights, this is in addition to my fan and controller sensors. I like to hand at basic thermometer at canopy height to give me a good read of ambient temps just above the canopy

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This helps. I'm running quantums with the usual 301B, 660nm Deep Red and 630nm Red. I have two sensors hanging center-tent at the average canopy-top height right now. They're roughly the same in their readings (within less than a degree and within a percentage point or two). I'll consider a 2-4f degree difference weighted to the lower side.