Temperature problem with CXB Cobs

It is really interesting (and possibly more relevant) to measure the leaf surface temperature rather than the tent ambient temp. I thought my cab was running way too hot until I tested leaf surface temps and found they were often lower than the tent ambient temp.

LED grows can afford to be (and should be) a little warmer than HID grows because of the lack of Infrared light in the spectrum heating up the surface of the leaves. From what I have read you can have very happy plants at 85f leaf surface temps.

...but yeah, 16xCOBs in this space is defs pushing it. cheers

MrTwist1 said:

It is really interesting (and possibly more relevant) to measure the leaf surface temperature rather than the tent ambient temp. I thought my cab was running way too hot until I tested leaf surface temps and found they were often lower than the tent ambient temp.

LED grows can afford to be (and should be) a little warmer than HID grows because of the lack of Infrared light in the spectrum heating up the surface of the leaves. From what I have read you can have very happy plants at 85f leaf surface temps.

...but yeah, 16xCOBs in this space is defs pushing it. cheers

Click to expand...

MrTwist1, You have made some excellent points. What is your method for measuring leaf surface temps? Infrared thermometer?

Cheers,
Will

MrTwist1 said:

It is really interesting (and possibly more relevant) to measure the leaf surface temperature rather than the tent ambient temp. I thought my cab was running way too hot until I tested leaf surface temps and found they were often lower than the tent ambient temp.

LED grows can afford to be (and should be) a little warmer than HID grows because of the lack of Infrared light in the spectrum heating up the surface of the leaves. From what I have read you can have very happy plants at 85f leaf surface temps.

...but yeah, 16xCOBs in this space is defs pushing it. cheers

Click to expand...

thanks for the tips
if the plant leaves can be happy at 85 does that mean the ambient temp can be higher than 85°F without causing stress ?

OGHulk said:

thanks for the tips
if the plant leaves can be happy at 85 does that mean the ambient temp can be higher than 85°F without causing stress ?

Click to expand...

Yes I have noticed that I can be getting around 87f with an ambient temp reading around the height of the canopy, whilst the leaves appear to be right around 85f. I bought a cheap IR thermometer to do this and what I learned was really interesting. It cost me like 15 bucks or so, so it might not be mega accurate, but I trust it enough and my girls seem happy enough. Obviously a lot will depend upon how close your lights are to the canopy.

Edit - ...with the caveat that ideally the rootzone should be kept cooler than this. I battle very high temps so I have to do what I have to do, and at the moment there are some things that I cannot change. such is life

Will Thayer said:

MrTwist1, You have made some excellent points. What is your method for measuring leaf surface temps? Infrared thermometer?

Cheers,
Will

Click to expand...

Yeah I bought a cheapy one on amazon and it was a great investment - I find myself measuring the temperature of all sorts of stuff round the house now lol.

But yeah I'd say it was one of the better 15 bucks I've spent in my time.

This is an interesting topic.

I have a FLIR camera and have used it to check my canopy temps. You are right in saying canopy temps under LEDs may well be below what you'd expect. I'm only using cheap chinese COBs so the effects might well be more pronounced with top spec lights. Anyway I've taken some pics to demonstrate:

Canopy:


Ambient:


Also interesting is that you see the tops glowing. They're a little hotter than the surrounding leaves:


You can even make out where the lower bud sites are below the leaves from where you see hot spots.

I've also attached a PDF of a paper which looks at ideal PPFD, temps and CO2 levels for cannabis. The whole thing is interesting but most relevant is:

Maximum rate of photosynthesis (PN max) was 24.60 μmolm-2s-1 at 30C and under 1500 μmol m-2s-1 PPFD.

Click to expand...

loftygoals said:

This is an interesting topic.

I have a FLIR camera and have used it to check my canopy temps. You are right in saying canopy temps under LEDs may well be below what you'd expect. I'm only using cheap chinese COBs so the effects might well be more pronounced with top spec lights. Anyway I've taken some pics to demonstrate:

Canopy:
View attachment 3724644

Ambient:
View attachment 3724645

Also interesting is that you see the tops glowing. They're a little hotter than the surrounding leaves:
View attachment 3724646

You can even make out where the lower bud sites are below the leaves from where you see hot spots.

I've also attached a PDF of a paper which looks at ideal PPFD, temps and CO2 levels for cannabis. The whole thing is interesting but most relevant is:

Click to expand...

I have noticed also that the tops are a little hotter than the leaves. That flir camera really shows it well!

But yeah my reading seemed to suggest exactly the same - 85/86f leaf temp recommended.

MrTwist1 said:

I have noticed also that the tops are a little hotter than the leaves. That flir camera really shows it well!

But yeah my reading seemed to suggest exactly the same - 85/86f leaf temp recommended.

Click to expand...

Yup... and this would suggest you get almost a 25% increase in photosynthesis going from 25C/77F to 30C/86F under the same lights.


Although there's more to growing than just photosynthesis rates. Mold/Budrot/Rootrot/Pests could easily wipe out any advantage higher temps offer on photosynthesis rates. Still it's some food for thought.

OGHulk said:

it's an inline duct fan ( RVK sileo 125E2-L) it can move up to 323 m3/hour but i decreased the speedto minimum because of the noise

Click to expand...

That's only 190 CFM and you're dropping it to some unknown level so that's likely the heat issue.

200w of 3590 at .7 amps will give you nice results. Running twice that in a 2x4 is way more than you need as mentioned.

OGHulk said:

thanks guys
i think i will setup a second grow tent after i finish this grow
the good news is that i fixed the temp issue by powering only 8 Cobs which according to you is enough and putting a stronger fan in the tent !
i will try powering all the 16 at half the power and see if the temp problem reappears

Click to expand...

This should help to take out some of the guesswork:

(Not my data. Thanks to NOGOD here: https://www.rollitup.org/t/cob-efficiency-spreadsheets.865238/ )

You just have way too many cobs for a 2x4. Plain and simple. You have 8 sq ft and 16 cobs. Rule of thumb is 1 cxb per sq foot but I only use 8 in a 4x4 but at 1750ma (64watts) and I even suffer bleaching sometimes. :/

ya i have 4*3590 and 4* vero 29 they run between 300 and 400 watts and its pretty easy to get white tops lol

So where is the ideal spot in a tent to place the thermometer for the most accurate reading?

ThaMagnificent said:

So where is the ideal spot in a tent to place the thermometer for the most accurate reading?

Click to expand...

Anywhere really, but close to your canopy.

BM9AGS said:

We here prefer to go off PPFD. lumens are in the color range we don't so much care about. He's pulling about 1300 PPFD and the max a plant can take is 1500 PPFD with 1500ppm of co2 and 86deg F.
.

Click to expand...

@BM(AGS - I've been thinking about what you said earlier.
Thanks for the details re PPFD and on how we do things in here and what we don't care about..
If I offer advice or comments on forums, I always try to help the enquirer without introducing ambiguity, so:
How should the OP (or anyone else reading this thread) find out how many PPFD a Cree CXB3590 running at 25W will produce?

Mullumbimby said:

@BM(AGS - I've been thinking about what you said earlier.
Thanks for the details re PPFD and on how we do things in here and what we don't care about..
If I offer advice or comments on forums, I always try to help the enquirer without introducing ambiguity, so:
How should the OP (or anyone else reading this thread) find out how many PPFD a Cree CXB3590 running at 25W will produce?

Click to expand...

Say your COB produces 2 umol/watt. And running 10 COBs at 50w each (numbers plucked out of my head). You have 500w in total. And 500x2=1000umol/s. That is your total PAR output.

If you want to know ppfd you need to figure out your total area in square metres. So 1.2x1.2m=1.44m. Then divide your total PAR by the area... 1000/1.44=694umol/m2/s

loftygoals said:

Say your COB produces 2 umol/watt. And running 10 COBs at 50w each (numbers plucked out of my head). You have 500w in total. And 500x2=1000umol/s. That is your total PAR output.

If you want to know ppfd you need to figure out your total area in square metres. So 1.2x1.2m=1.44m. Then divide your total PAR by the area... 1000/1.44=694umol/m2/s

Click to expand...

1.2m x 1.2m = 1.44m??
In other words,
3.5'x3.5'= 12.25'.
1.44m isn't 12.25'. What am I missing? I feel like I'm retarded right now?

Mullumbimby said:

@BM(AGS - I've been thinking about what you said earlier.
Thanks for the details re PPFD and on how we do things in here and what we don't care about..
If I offer advice or comments on forums, I always try to help the enquirer without introducing ambiguity, so:
How should the OP (or anyone else reading this thread) find out how many PPFD a Cree CXB3590 running at 25W will produce?

Click to expand...

You could put the figures into.;...

https://dl.dropboxusercontent.com/content_link/HpdMHJdr764eTeu448eHyl1wjgtWECgEZdX8ZduVgWIUQO8xT4xMNpOx5ra1fvoP/file?dl=1


It's the COB calculator created by one of the members... sorry forgotten the name...

Hope it helps.

I got...

CXB3590CD36V3500K 1 COBS @0.7A ON 1.813 PROFILE HEATSINK
1 SQ.FT. CANOPY 94% EFFICIENT DRIVER @30 CENTS PER KWH
Total power watts at the wall: 24.47
Cobs power watts: 23
Total voltage forward: 33
Total lumens: 4770
Total PAR watts assuming 10% loss: 13
Total PPF: 60.45
PPFD based on canopy area: 646.81
PAR watts per sq. ft.: 13.0
Cob efficiency: 64.01%
Power watts per sq. ft.: 23.0
Voltage forward per cob: 32.72
Lumens per watt: 207.39
Heatsink riser thickness / number of fins / fin's length: 0.3in/6/0.95in
Heatsink area per inch: 100.94 cm^2
Total heat watts: 8
umol/s/W / CRI: 4.65 / Estimated

Mullumbimby said:

@BM(AGS - I've been thinking about what you said earlier.
Thanks for the details re PPFD and on how we do things in here and what we don't care about..
If I offer advice or comments on forums, I always try to help the enquirer without introducing ambiguity, so:
How should the OP (or anyone else reading this thread) find out how many PPFD a Cree CXB3590 running at 25W will produce?

Click to expand...

Download the COB LED calculator on this forum made by @bggrass . Input all of your perimeters and you get all kinds of awesome info!

https://www.rollitup.org/t/diy-cob-led-calculator.890214/

Airwalker16 said:

1.2m x 1.2m = 1.44m??
In other words,
3.5'x3.5'= 12.25'.
1.44m isn't 12.25'. What am I missing? I feel like I'm retarded right now?

Click to expand...

Your conversion from m to ft is off.

1.2m = 4ft (not quite but close enough)

Then 1.2x1.2=1.44 sqm
4x4=16 sqft
1.44 sqm = 15.5 sqft (close enough)

If you converted m to ft properly then you'll get everything to match up perfectly.