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

[Keffka]

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….??

What’s your RH?

 

[Keffka]

Without knowing the ins and outs of your setup well it’s hard to give you a reason why you would be above your ambient temp.. I strongly suspect it is related to your RH or possibly an IR light source. I looked at your grow light spectrum and it looks like it’s not your cause. So that leaves RH or genetics.

If I were you I’d go with what the leaf temperature is measuring as and operate off of that.. if your plants growth isn’t slowed and you’re not seeing odd or irregular growth then you’re in good shape. However I would try to nail down why your leaves are running hot or what’s causing a false measurement.

 

[Makeminefullgrown]

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.

I have a couple of other devices that measure the temp and RH……I don’t try to measure air temps with the infrared gun.

 

[Makeminefullgrown]

Without knowing the ins and outs of your setup well it’s hard to give you a reason why you would be above your ambient temp.. I strongly suspect it is related to your RH or possibly an IR light source. I looked at your grow light spectrum and it looks like it’s not your cause. So that leaves RH or genetics.

If I were you I’d go with what the leaf temperature is measuring as and operate off of that.. if your plants growth isn’t slowed and you’re not seeing odd or irregular growth then you’re in good shape. However I would try to nail down why your leaves are running hot or what’s causing a false measurement.

The only odd growth I’ve gotten are some twisted sugar leaves, mainly on one of the 3 plants.
Today shortly after the lights came on, the air temps were steady in the 74-75F range and the RH was holding steady in the 44-46% range. This was with the heater and humidifier turned off. I did have the dehumidifier on in the lung room.
I watered one of the plants and after I finished and was ready to leave them for the day, I measured the LST and for some reason the plant that was not watered was very close to the air temp….maybe 1 degree warmer. But the plant I watered was a solid 3 degrees above air temps.
I was wondering, could I have too much airflow? Could I be blowing too much air with 3 fans in such a small space?
My 4” exhaust is rated 171cfm…..I have to admit I don’t know the proper amount of air replacement for my 2.5’x6’ x8’ closet space.

 

[Makeminefullgrown]

You can see some of the twisting leaves on this bud….

 

[bluter]

theres so many things to happily not worry about while you get stuff sorted long before leaf temp guys

 

[Keffka]

You can see some of the twisting leaves on this bud….

@bluter nailed it.. Leaf temp accuracy isn’t going to end your grow.. you’ve got a good looking bud there so you’re on the right track.

Your rh is a bit low for your temperature.. with it being lower than recommended, even for late flower, your leaves are working harder than they need to be.. not much harder mind you, but until it’s locked in it’s hard to identify other causes. Hence why the measurements are important, even as little as 1-2 degrees and 1-2 percent rh either way.

I’ll attach a VPD chart to this comment so you can see where your RH should be for your growth stage. Currently your VPD is around 1.7-1.8 which exceeds the recommended VPD for late flower. 1.5-1.6 is the goal in the final 2-4 weeks, and it goes lower the earlier in the grow. Your sugar leaves could be twisting due to the rh, possible nute excesses (looks like nute burn on a fan leaf in the pic), heat, light, or it could just be a genetic mutation referred to as “sugar rails”. Personally I think it looks like a mutation because of the concentration of trichomes on the edges creating a rail effect.

 

[Gee64]

If your using LED and your thermometer for room temp is hanging at the same height as the leaf you are using the IR thermometer on and your LST is higher than your room temp, or more to the point, less than 2 degrees below room temp, chances are that your light is too close to the plant. Raise your light a drastic amount, say a foot. Wait a couple hours and check. See if LST drops. If it does lower it 6 inches, wait an hour and check again, then slowly dial it in. Also if you are checking early in the day just after lights on, your LST will be higher from the dark period. Once the light is dialed in then you can adjust room temp and humidity to the desired values but as you do you will have to adjust the light height. The size of your rootball is also a limiting factor but only once you are in the ballpark and are fine tuning the 3 corners of LST VPD. Don't confuse room VPD with LST VPD when you are researching VPD. Always follow advice on LST VPD for this application. Download a free VPD calculator that has all 3 values, LST, room temp, and RH if you havent already, and stay away from VPD calculators with only 2 inputs. Then once your light is at the proper height you can dry-run a change on the calculator to see its effect before you do it. Once the values of room temp and humidity go outside of your desired zones then its time to adjust the light again. Hope this helps you. Play with your calculator lots. It helps to form a good mental picture of whats going on.

 

[Gee64]

Also all plants are not the same. Even 2 identical clones may need different heights from the light so you will have to check each plant 1 by 1 and add or subtract height underneath your pots to dial each plant in. Dont worry if this makes your canopy uneven, you will see results. Quite often this allows the shorter plant to catch up because its happier now.