Leaf temps under LED/LEC/HPS

What leaf temps are people getting under different light sources (led/cmh/hps) in comparison to ambient?

At what light levels do you get this temp?

How much do you factor this in to calcultating VPD?

The troubled grow is one that just went from HPS (well dialed in) to CMH (having calcium def as soon as reaching higher light levels).

Would this all be down to enviroment? Temps have been about 22-24C with around 80 RH; low vpd?

i also had problems with cmh in cold temps

have never had any problems at high temps really

Here’s a really good article about leaf surface temperature in regards to different lighting.

https://www.blackdogled.com/lst

Rocket Soul said:

The troubled grow is one that just went from HPS (well dialed in) to CMH (having calcium def as soon as reaching higher light levels).

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This sounds like photo-inhibition since it happens when switching from HPS to CMH "at higher light levels". More blue can cause it.

If that's the case, it sounds like you're going to have to dial in your nutes to the new light. (More Fe, more Mg, depending on what's deficient)

Or just go back to HPS.

Rocket Soul said:

What leaf temps are people getting under different light sources (led/cmh/hps) in comparison to ambient?

At what light levels do you get this temp?

How much do you factor this in to calcultating VPD?

The troubled grow is one that just went from HPS (well dialed in) to CMH (having calcium def as soon as reaching higher light levels).

Would this all be down to enviroment? Temps have been about 22-24C with around 80 RH; low vpd?

Click to expand...

Good questions. I think another important factor is the location of temperature and humidity probes in the tent/grow space. I've seen a lot of variation between different areas of a tent.

I use a laser thermometer to check my leaf temps and with high levels of led my leaf temps are usually same as ambient or slightly higher. With hps is always much much higher. This is why I THINK led guys dont get mites as badly as hps guys.

1212ham said:

variation between different areas

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I often wonder if the temps/RH many folks think they have, is anywhere near what they actually have, because of that very notion.

1212ham said:

Good questions. I think another important factor is the location of temperature and humidity probes in the tent/grow space. I've seen a lot of variation between different areas of a tent.

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That's why I keep a temp gauge up top, middle, and at the bottom to monitor the RH and temp, and this is true. It can vary as much as 10% RH and 5+- degrees F

Med68w said:

Here’s a really good article about leaf surface temperature in regards to different lighting.

https://www.blackdogled.com/lst

Click to expand...

Thats really what i was looking for, but no CMH on the list.

churchhaze said:

This sounds like photo-inhibition since it happens when switching from HPS to CMH "at higher light levels". More blue can cause it.

If that's the case, it sounds like you're going to have to dial in your nutes to the new light. (More Fe, more Mg, depending on what's deficient)

Or just go back to HPS.

Click to expand...

Not very likely, they were under cmh for most of the veg. Its hard to up the nutrients as the extra nutes would likely build up salts in the coco. I have a feeling that what my friend is suffering from is "calmag" problems similar to those you get under leds, due to the plant not transpiring enough as his VPD seems low

pirg420 said:

I use a laser thermometer to check my leaf temps and with high levels of led my leaf temps are usually same as ambient or slightly higher. With hps is always much much higher. This is why I THINK led guys dont get mites as badly as hps guys.

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I guess i could have been clearer in what im asking about. Been using this:
http://opennlabs.com/vpd/VPD_calculator.php

to try to calculate vpd. Its a really nifty resource as it allows for setting how much hotter or colder the leaf temp is, compared to ambient.

Weve measured left temp (1-2 below ambient), and got different values with both infrared/laser meters. Also the flir meter in the article above got much higher values on all light sources.

Is there anyway to figure out what would be the right value? Is there a way to figure out if a laser meter is consistently giving you high or low values?

What values would people use for for leaf temp of you were to try to figure out the vpd for HPS / CMH / LED? How much higher/lower would you rank your leaf temp?
And at what light levels, as a 400w hps wouldnt run as hot as 1000w hps?

Yeah, i know, lots of questions just hoping for some input. I used -1°C for leaf temp on the CMH (par 250) , got very low vpd on 22-24°C, 80RH.
Using +3°C for HPS 600w with same temp/rh gives pretty much perfect vpd.

What does everyone think?

Rocket Soul said:

I have a feeling that what my friend is suffering from is "calmag" problems similar to those you get under led

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There's no such thing as "calmag" unless you're talking about marketing..

Rocket Soul said:

Thats really what i was looking for, but no CMH on the list.

Not very likely, they were under cmh for most of the veg. Its hard to up the nutrients as the extra nutes would likely build up salts in the coco. I have a feeling that what my friend is suffering from is "calmag" problems similar to those you get under leds, due to the plant not transpiring enough as his VPD seems low


I guess i could have been clearer in what im asking about. Been using this:
http://opennlabs.com/vpd/VPD_calculator.php

to try to calculate vpd. Its a really nifty resource as it allows for setting how much hotter or colder the leaf temp is, compared to ambient.

Weve measured left temp (1-2 below ambient), and got different values with both infrared/laser meters. Also the flir meter in the article above got much higher values on all light sources.

Is there anyway to figure out what would be the right value? Is there a way to figure out if a laser meter is consistently giving you high or low values?

What values would people use for for leaf temp of you were to try to figure out the vpd for HPS / CMH / LED? How much higher/lower would you rank your leaf temp?
And at what light levels, as a 400w hps wouldnt run as hot as 1000w hps?

Yeah, i know, lots of questions just hoping for some input. I used -1°C for leaf temp on the CMH (par 250) , got very low vpd on 22-24°C, 80RH.
Using +3°C for HPS 600w with same temp/rh gives pretty much perfect vpd.

What does everyone think?

Click to expand...

the best value.... is the one that make your plants grow the best in your environment. that calculator is just a tool, and it will work only as good as you make it work. you can use tools right and wrong. try this, slowly inch up your temps and watch the plants. you'll know better than anyone else here when they start looking worse or better. i don't think there is one correct answer, just what works best for you. observe and report. i think people are making growing a plant out to be harder than it really is. and the greenest thumbs i've seen let the plants tell them what they need, not the other way around.

churchhaze said:

There's no such thing as "calmag" unless you're talking about marketing..

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i think he means cal and mag deficiencies are common to be seen together because they are both uptake related to transpiration

CobKits said:

i think he means cal and mag deficiencies are common to be seen together because they are both uptake related to transpiration

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If you have a Mg deficiency, adding Ca will usually make it worse. The term "calmag" should be considered harmful. There really is no correlation between the 2 other than they antagonize each other. What's common isn't them being deficient in tandem, but people calling their problem a "calmag" problem (code for having no clue what the problem is).

right- they compete. i guess my point is they both point to the same cause/cure - low temps and low transpiration.

Rocket Soul said:

Is there anyway to figure out what would be the right value? Is there a way to figure out if a laser meter is consistently giving you high or low values?

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You should be measuring the average temperature over a reasonably large area. Usually a square meter or something. like that and preferably under a reasonably shallow angle (not straight down). of course without walls and such in view of the IR sensor.

Unfortunately this is often not possible in a small grow, but still a spot measurement can vary a lot. Some leaves/tops will be hotter than others. You'll need some averaging to get a useful reading

Those handheld IR temp sensors usually have quite an IR sensor with a small beam angle. So then you wood need to keep some distance to get a decent sized area covered.

You could try measuring the floor or wall or something and then compare it to a measurement with a regular contact thermometer.

I calibrated my DIY plant temperature camera by hanging a piece of paper in front of it in a closed grow tent (with the lights out) and then comparing the IR measurement of that piece of paper to the other temp sensors in the tent. They all gave me the same temperature (well within the accuracy of the sensors).

Rocket Soul said:

Would this all be down to enviroment? Temps have been about 22-24C with around 80 RH; low vpd?

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That humidity seems high, what stage are the plants at, early veg., late flower etc.?
Having said that, with very little grow experience, I see zero deficiency and rapid growth at 27-30C (80-85F) and 65-80 RH. 25,000 lux with 3500K strips in late veg. Hempy bucket fed with Mega Crop at 4g/gallon or 550 ppm. I'll get leaf temps when my infrared thermometer stops hiding from me.

Thank you everyone for your advice, i i think i got this figured out now.

Med68w said:

Here’s a really good article about leaf surface temperature in regards to different lighting.

https://www.blackdogled.com/lst

Click to expand...

Very nice article. Funny how it doesnt give the same numbers as Laser temps but i guess Flir and laser temps come out different. The relative difference is helpfull though, comes out about the same as expected between led/hps

wietefras said:

You should be measuring the average temperature over a reasonably large area. Usually a square meter or something. like that and preferably under a reasonably shallow angle (not straight down). of course without walls and such in view of the IR sensor.

Unfortunately this is often not possible in a small grow, but still a spot measurement can vary a lot. Some leaves/tops will be hotter than others. You'll need some averaging to get a useful reading

Those handheld IR temp sensors usually have quite an IR sensor with a small beam angle. So then you wood need to keep some distance to get a decent sized area covered.

You could try measuring the floor or wall or something and then compare it to a measurement with a regular contact thermometer.

I calibrated my DIY plant temperature camera by hanging a piece of paper in front of it in a closed grow tent (with the lights out) and then comparing the IR measurement of that piece of paper to the other temp sensors in the tent. They all gave me the same temperature (well within the accuracy of the sensors).

Click to expand...

Thanks Wietefras, the temp calibration is prob the missing piece of the puzzle.
Re: ir temp sensors: you suggest a fair bir of distance from cannopy. Directly from above for example? The sensor we got is one of thoose that dont have a single dot, is more like a "shotgun" pattern.

I heard back from my friends grow, higher temps and slightly lower RH has got the plants praying and no purple/calcium deficiency.

@wietefras may I ask you what IR sensor module are you using ? or temperature camera, whatever you like it calling

1212ham said:

That humidity seems high, what stage are the plants at, early veg., late flower etc.?
Having said that, with very little grow experience, I see zero deficiency and rapid growth at 27-30C (80-85F) and 65-80 RH. 25,000 lux with 3500K strips in late veg. Hempy bucket fed with Mega Crop at 4g/gallon or 550 ppm. I'll get leaf temps when my infrared thermometer stops hiding from me.

Click to expand...

Flower dosage on MC ?

Rocket Soul said:

Thanks Wietefras, the temp calibration is prob the missing piece of the puzzle.

Click to expand...

Yes temps are very important since a small change in leaf temp can give you big swings in VPD. If you look at thos blackdoge FLIR images you see how much leaf temperature van vary. Calibration and averaging out is very important.

Re: ir temp sensors: you suggest a fair bir of distance from cannopy. Directly from above for example? The sensor we got is one of thoose that dont have a single dot, is more like a "shotgun" pattern.

Click to expand...

Not directly from above. It should be at a "shallow" angle. Otherwise you risk looking through the plants to the (much colder) floor, Usually it says the spread of the sensor beam somewhere on the side of those IR temp sensors. Or otherwise in the specs. Try to adjust the distance so you get a sized area covered.

In greenhouses they measure quite large area's:


They call it a 79 degree angle, but I would call that an 11 degree angle. It's a very shallow angle to the canopy.

I'm assuming this won't fit in your grow. It sure does not in mine. So I use a 45 degree angle. Not perfect, but I don't have the space to get it any more shallow. Otherwise I measure the walls.

I heard back from my friends grow, higher temps and slightly lower RH has got the plants praying and no purple/calcium deficiency.

Click to expand...

Lower RH gives a higher VPD, more transpiration and therefore more Ca uptake. So yes, that could be the difference.