Can leaf temps be higher than air temps under LEDs??

Makeminefullgrown

Makeminefullgrown

Well-Known Member
I know the answer but I can screw things up pretty good if given the chance…..so I’ll ask you folks.
I have 3 plants in a 2.5’x6’ closet with 8’ ceiling.
My light is an HLG 320 xl r-spec LED, hanging about 22” above the canopy. I bought one of those laser thermometers to check my leaves. I also have a couple other temp/humidity gauges in the closet. No matter what I do the laser always reads about 2F degrees above the ambient temps.
Right now for instance, I have 3 different thermometers in the closet that all read between 75.2 and 75.7F……so I feel like all 3 cannot be wrong. When I shoot the red dot onto one of the top leaves it’s reading 77.0-77.6F.
My understanding was w/ an LED the leaf surface temps will always be a couple degrees cooler than the air…..but mine says otherwise.
I’m guessing and hoping that the laser thermometer is off and needs to be calibrated, which cannot be done with this model.
Am I safe to assume that a 75F air temp will result in leaf temps of approximately 73F?
What’s the deal?
I’ve tried for the ideal VPD throughout the grow and just went off the air temp and subtracted 2 degrees but I figured I would ask about the chance of that reasoning being flawed.
It seems to be working I guess, as I’m in week 12+ of my light flip to 12/12 and the plants seem fine and should be ready in a week or two…..one actually has already been harvested so I’m down to 2….but I digress.
thanks for chiming in.

L
 
LST (leaf surface temperature) needs to be about 88f. So with LEDs you should aim for an ambient temperature of about 84f—LST is always a few degrees above ambient. HPS lighting is fine with ambient temperatures at 75f because they produce an LST about 10 degrees hotter.
 
Makeminefullgrown said:
My understanding was w/ an LED the leaf surface temps will always be a couple degrees cooler than the air
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How did you come to that understanding? Leaf temps will be warmer than the air during the light cycle as they are absorbing energy. Have you ever felt something black in the sun and realized it's hotter than the air? Same concept. Also, you're over complicating a simple concept. Most experienced growers use the back of their hand held at the plant tops to sense for proper temps to discern light source distance. The best part about that simple method its....it's free! In my experience over thinking and over complicating grows adds more potential issues than it solves. It sounds like you're having though and really that's what it's all about. If you're plants are healthy and you're having fun keep doing what you're doing growmie.
 
Hi - yes, any surface will be slightly warmer than the air temp as air dissipates heat more quickly and evenly
What he said ^
 
Well I’ll be damned….I’ve been doing things completely in reverse the entire time…..everything totally flipping backwards. It’s a wonder my plants have grown at all.
I don’t know where I started confusing things…but thank everyone for clearing that up.
 
Makeminefullgrown said:
I know the answer but I can screw things up pretty good if given the chance…..so I’ll ask you folks.
I have 3 plants in a 2.5’x6’ closet with 8’ ceiling.
My light is an HLG 320 xl r-spec LED, hanging about 22” above the canopy. I bought one of those laser thermometers to check my leaves. I also have a couple other temp/humidity gauges in the closet. No matter what I do the laser always reads about 2F degrees above the ambient temps.
Right now for instance, I have 3 different thermometers in the closet that all read between 75.2 and 75.7F……so I feel like all 3 cannot be wrong. When I shoot the red dot onto one of the top leaves it’s reading 77.0-77.6F.
My understanding was w/ an LED the leaf surface temps will always be a couple degrees cooler than the air…..but mine says otherwise.
I’m guessing and hoping that the laser thermometer is off and needs to be calibrated, which cannot be done with this model.
Am I safe to assume that a 75F air temp will result in leaf temps of approximately 73F?
What’s the deal?
I’ve tried for the ideal VPD throughout the grow and just went off the air temp and subtracted 2 degrees but I figured I would ask about the chance of that reasoning being flawed.
It seems to be working I guess, as I’m in week 12+ of my light flip to 12/12 and the plants seem fine and should be ready in a week or two…..one actually has already been harvested so I’m down to 2….but I digress.
thanks for chiming in.

L
Click to expand...

If LST is 2°F < than ambient you're in good shape because that means that your plant is transpiring and the leaf surface is being cooled as a result of the latent heat of evaporation. That's how humans cool off as well - sweat on the surface of the skin evaporates, removing heat from the body.

If LST>= ambient that means that the leaf surface is warmer than the environment and that means that your leaves are warming up. That's no bueno. Having a positive LST offset is not that common situation with LED's since they generate very little if any IR. If you run into a situation where LST > ambient, check your airflow and RH.

This site has good info on an lot of cannabis growing topics.
 
My LST is 2-3 degrees Fahrenheit lower than ambient temperatures under LEDs.
 
Delps8 said:
If LST is 2°F < than ambient you're in good shape because that means that your plant is transpiring and the leaf surface is being cooled as a result of the latent heat of evaporation. That's how humans cool off as well - sweat on the surface of the skin evaporates, removing heat from the body.

If LST>= ambient that means that the leaf surface is warmer than the environment and that means that your leaves are warming up. That's no bueno. Having a positive LST offset is not that common situation with LED's since they generate very little if any IR. If you run into a situation where LST > ambient, check your airflow and RH.

This site has good info on an lot of cannabis growing topics.
Click to expand...
Ok I understand your comments and they make a lot of sense. Are the folks who say that LST is always greater than air temps referring to different lights….not LEDs?
If my LST is always a bit higher than air temps, how on earth do you follow and maintain a proper VPD?
I’m in the final week or 2 (max) of flower and certainly don’t want any sort of mold or rot to occur. Therefore, I really try to keep my RH down around 40%. According to the VPD chart I would have to be running my grow space in the 67F range just to be marginally acceptable.
I’m running a 4” Hurricane exhaust and it’s cfm (I think that’s the right term) is well above the suggested rates for my size space.
What gives?
I don’t wanna overcomplicate a fun hobby but controlling a couple of variables like temps and humidity shouldn’t be all that tough to handle.
Thanks for your time and I’m anxious to hear your thoughts.
 
LKABudMan said:
My LST is 2-3 degrees Fahrenheit lower than ambient temperatures under LEDs.
Click to expand...
So am I doing something wrong? Is my infrared thermometer wrong?
How is it even possible for my LST’s to be higher than the air temps? I’m not crowding my plants up against the light…heck right now I’ve lowered the intensity and raised them a few inches….but it doesn’t matter….and it’s driving me nuts.
 
Maybe it's a matter of how much transpiration is carried away by airflow, as that would have a cooling effect, and there's a shed load of variables there
 
Makeminefullgrown said:
Ok I understand your comments and they make a lot of sense. Are the folks who say that LST is always greater than air temps referring to different lights….not LEDs?
If my LST is always a bit higher than air temps, how on earth do you follow and maintain a proper VPD?
I’m in the final week or 2 (max) of flower and certainly don’t want any sort of mold or rot to occur. Therefore, I really try to keep my RH down around 40%. According to the VPD chart I would have to be running my grow space in the 67F range just to be marginally acceptable.
I’m running a 4” Hurricane exhaust and it’s cfm (I think that’s the right term) is well above the suggested rates for my size space.
What gives?
I don’t wanna overcomplicate a fun hobby but controlling a couple of variables like temps and humidity shouldn’t be all that tough to handle.
Thanks for your time and I’m anxious to hear your thoughts.
Click to expand...
"Ok I understand your comments and they make a lot of sense. Are the folks who say that LST is always greater than air temps referring to different lights….not LEDs?"
I couldn't say.

"If my LST is always a bit higher than air temps, how on earth do you follow and maintain a proper VPD? "
Hmm…very carefully?

I just checked the VPD whartmaker that the folks at pulsegrow.com publish at, in veg, with an ambient temp of 79°, you need 78% RH is LST offset is 2. If it's -2, RH would be in the low 60's.

I hit a positive offset on my last grow and turned up the tent fan a bit to resolve it.

"I’m in the final week or 2 (max) of flower and certainly don’t want any sort of mold or rot to occur. Therefore, I really try to keep my RH down around 40%. According to the VPD chart I would have to be running my grow space in the 67F range just to be marginally acceptable. "
Plant health is maximized when VPD is in range. If you're not in range, the plant will not grow to its maximum genetic potential.

"I don’t wanna overcomplicate a fun hobby but controlling a couple of variables like temps and humidity shouldn’t be all that tough to handle."
"overcomplicate" - that's out there with "don't overthink". The only time I've really heeded that admonition is when I was in the Army because we were trained that, in the event of incoming artillery fire, we had about 8 seconds to make some very important decisions. We also had 4 seconds to deploy our reserve parachute. Other than those two situations, I tend to disregard people's comments about "overcomplicate".

For my grow, controlling temperature is a usually a non-issue. If it's too hot, I turn on a 14000 BTU portable AC unit and watch the electric meter spin. If it's too cold, I turn on a small oil heater.

RH is PITA for me. One issue is even determining the current value of RH. All of my sensors are calibrated to 0.1% and they read differently even though they're within a 2" radius in the tent*.

What's the answer - accept that all the values are "wrong", accept that it doesn't matter that they're all "wrong", try to keep RH within a couple of %, and move on to something else.

I've attached a screenshot of a video by Dr Bruce Bugbee. It's a great guide for getting your grow environment nailed down. If you work through each of the nine parameters and get them even pretty close to good, you will end up with a lot of weed.

BTW, the biggest issue that I see is growers not giving their plants enough light (cue Dennis Miller "I don't want to get off on a rant here…").


Parameters of Growth.png
 
Roy Growin said:
Maybe it's a matter of how much transpiration is carried away by airflow, as that would have a cooling effect, and there's a shed load of variables there
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Windspeed is a factor but also RH and temperature differential. IIRC, it's due to the "latent heat of evaporation".
 
Makeminefullgrown said:
So am I doing something wrong? Is my infrared thermometer wrong?
How is it even possible for my LST’s to be higher than the air temps? I’m not crowding my plants up against the light…heck right now I’ve lowered the intensity and raised them a few inches….but it doesn’t matter….and it’s driving me nuts.
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"How is it even possible for my LST’s to be higher than the air temps?"
Off the top of my head, LST is a function of RH, ambient temperature, light type, hang height, and spectrum. It wouldn't surprise me if there were more factors involved.

"heck right now I’ve lowered the intensity and raised them a few inches"
Every action you take in your grow should have a rationale and a clearly defined goal.

When I doubt, and if there's the luxury of time, change only one thing at a time.

"it’s driving me nuts."
That's what Xanax is for. :)

It could be that your thermometer is out. They're not very high tech but they can still get out of whack. Even if you can't calibrate it, you can determine its accuracy. That's a quick Google away. If it's really an issue, get a new one. They're cheap.

BTW, your light might have some IR output so that will tend to raise the temperature in the tent.

Biggest contributors are the light (LED's do give off heat), wind or lack thereof, RH. It's probably one of those three. If you're so inclined, join the forum at pulsegrow.com and ask questions there.
 
Makeminefullgrown said:
Well I’ll be damned….I’ve been doing things completely in reverse the entire time…..everything totally flipping backwards. It’s a wonder my plants have grown at all.
I don’t know where I started confusing things…but thank everyone for clearing that up.
Click to expand...
You've got conflicting answers so far in this thread. Have you just googled it?
 
@Delps8 already answered in detail your questions I’ll add in my experience..

LST is always 2 degrees lower than whatever my ambient temperature is, if it wasn’t, I’d be concerned something was off. A healthy, transpiring plant shouldn’t be hotter than the ambient temperature, that means transpiration isn’t working properly and you’ve got something throwing you off. It would be like if I started sweating, but that somehow made me even hotter than the air outside.. something would be wrong. My body’s transpiration keeps my surface temperature lower than the ambient temperature, by design, so I don’t overheat and cook my innards.

Reading about your setup.. do you only have one place for airflow? Meaning.. you’re in a closet.. do you have a spot for air to come in and a spot for it to exit? Or are you trying to exchange air from a single point?

Also.. I’ve never listened to people who say don’t overcomplicate or overthink stuff.. that doesn’t help if you’re a chronic over thinker like I am lol.. if you’re an over thinker you just need to refocus your thoughts into things that are useful.. the key part is to not let those thoughts become actions. Think out and game out every thing you can, but any actions you take need to be deliberate and controlled.. meaning.. don’t change 10 different things about your setup or grow at once.. introduce changes incrementally with a clear purpose and plan for the change, and see what effects it has on your grow, and move forward from there.
 
Makeminefullgrown said:
So am I doing something wrong? Is my infrared thermometer wrong?
How is it even possible for my LST’s to be higher than the air temps? I’m not crowding my plants up against the light…heck right now I’ve lowered the intensity and raised them a few inches….but it doesn’t matter….and it’s driving me nuts.
Click to expand...
How are you measuring the temperature of the air?

An infrared thermometer needs a surface to get the reflected infrared temp from which makes it easy to use them to measure the canopy temperatures.

Infrared thermometers cannot measure the temperature of air based on the bit of reading I have done.
 
LST is impacted by Ambient air temperature, relative humidity, leaf physiology and pigmentation, genetic/metabolic differences, and light spectrum.

Air temp sets your baseline, it will cool warmer leaves and warm cooler leaves. Humidity can affect LST by interfering with the evaporation portion of transpiration.. you can feel it as a human.. when there’s too much humidity it becomes more difficult for water to evaporate (sweating becomes ineffective and you overheat), this in turn drives your internal temperature up as well LST.

Then there’s light spectrum, and plant characteristics.. These 2 portions could be entire courses themselves.. lights made specifically for plants are more efficient for photosynthesis causing less of a temperature rise in LST.. Inefficient lights, or those not designed with plant photosynthesis in mind drive LST higher.

So yes.. it is very possible to drive your LST way higher than ambient temp.

This is an extremely basic and rudimentary breakdown of it, but the gist is, if you’re running LED grow lights and your LST is high, you need to figure out what’s going on with one of the previously mentioned factors. As @SmokingWings mentioned, I’d check your thermometers.
 
Makeminefullgrown said:
Well I’ll be damned….I’ve been doing things completely in reverse the entire time…..everything totally flipping backwards. It’s a wonder my plants have grown at all.
I don’t know where I started confusing things…but thank everyone for clearing that up.
Click to expand...
Haha! That is funny. Just to share one of my newb fails to show we all make mistakes and learn from them. On one of my early grows, I over watered my pants. Any time the surface of the soil looked dry I'd water them. I was also very lazy and hated dealing with the drainage in my closet so I would only water until a few drops would drain out and soak it up with a paper towel. Well, I developed the worst case of root rot on all of the plants in 5 gallon plastic pots. The roots were thick black sludge and it smelled of decomposition and death. I've since learned about proper irrigation intervals, occasional droughting and the importance of the amount of drainage to cleanse the soil and roots of buildup. I've made many other mistakes that cost my entire grow to be trashed. At least you haven't killed your plants.
 
Keffka said:
@Delps8 already answered in detail your questions I’ll add in my experience..

LST is always 2 degrees lower than whatever my ambient temperature is, if it wasn’t, I’d be concerned something was off. A healthy, transpiring plant shouldn’t be hotter than the ambient temperature, that means transpiration isn’t working properly and you’ve got something throwing you off. It would be like if I started sweating, but that somehow made me even hotter than the air outside.. something would be wrong. My body’s transpiration keeps my surface temperature lower than the ambient temperature, by design, so I don’t overheat and cook my innards.

Reading about your setup.. do you only have one place for airflow? Meaning.. you’re in a closet.. do you have a spot for air to come in and a spot for it to exit? Or are you trying to exchange air from a single point?

Also.. I’ve never listened to people who say don’t overcomplicate or overthink stuff.. that doesn’t help if you’re a chronic over thinker like I am lol.. if you’re an over thinker you just need to refocus your thoughts into things that are useful.. the key part is to not let those thoughts become actions. Think out and game out every thing you can, but any actions you take need to be deliberate and controlled.. meaning.. don’t change 10 different things about your setup or grow at once.. introduce changes incrementally with a clear purpose and plan for the change, and see what effects it has on your grow, and move forward from there.
Click to expand...
Thanks for your input. I have my passive intake holes (3 holes, 2” diameter each) about 6” from the floor on the left hand side. My exhaust, a 4” hurricane, is at the opposite side roughly 7’ in the air on a small wire shelf, with the exhaust tube exiting about a foot above the fan through the ceiling.
The exhaust fan is actually sitting above the light. I believe it’s cfm is plenty high enough for my 2.5’ X 6’ (w/8ft ceiling) space.
Maybe my positioning is flawed….??
 
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