How Much Cooler is a Cob than HID? Pics

Well, I'm using a camera from work to see how hot my DIY COB is compared to the HPS.

Here's the drill. The HPS is air cooled, and set at 450 watts, the cob is a DIY 4 x CXB3070, about 220 watts..

I noticed that the canopy is a little cooler under the COB, and almost looks like more activity/transpiration may be taking place. As this is for my personal use, the change to cobs to reduce electricity is my goal. If i said I was small time, that's an exaggeration

The hottest part of the DIY setup is the driver, after running all day (interior temp about 78 F), the temp is about 100 F, which IMO is pretty cool (compared to stuff I see in the electrical world)

The HPS is 146 at the glass, which is a *somewhat flawed* measurement, as glass has weird emissivity, but averages are still higher than the cob, BUT not *that* much hotter. What I did find interesting, was the canopy temps were much different. Without getting into a lengthy radiation discussion, there *is* IMO opinion something going on with these lights.

Whadya think?

chipper68 said:

Well, I'm using a camera from work to see how hot my DIY COB is compared to the HPS.

Here's the drill. The HPS is air cooled, and set at 450 watts, the cob is a DIY 4 x CXB3070, about 220 watts..

I noticed that the canopy is a little cooler under the COB, and almost looks like more activity/transpiration may be taking place. As this is for my personal use, the change to cobs to reduce electricity is my goal. If i said I was small time, that's an exaggeration

The hottest part of the DIY setup is the driver, after running all day (interior temp about 78 F), the temp is about 100 F, which IMO is pretty cool (compared to stuff I see in the electrical world)

The HPS is 146 at the glass, which is a *somewhat flawed* measurement, as glass has weird emissivity, but averages are still higher than the cob, BUT not *that* much hotter. What I did find interesting, was the canopy temps were much different. Without getting into a lengthy radiation discussion, there *is* IMO opinion something going on with these lights.

Whadya think?

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I think that 140f+ would leave a visible but not really painfull burn on the back of your hand if you held it on the glass for 10 seconds. Have you tried it? The bulb is HOT AF though lol

SPLFreak808 said:

I think that 140f+ would leave a visible but not really painfull burn on the back of your hand if you held it on the glass for 10 seconds. Have you tried it? The bulb is HOT AF though lol

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Only by accident I'd take the glass off to measure temps, but it's a hassle and I don't have much headroom... might burn em up.

It would definitely seem (and the camera proves it) that cobs are cooler than HID.. I wonder if there's any information about different transpiration rates. I think the previous thoughts were that LEDs had less transpiration due to heat, but I'm wondering if there is something in regards to the radiation or some reaction with the cob vs HID.

To put it simple, do plants under COB's use more or less water than HID?

chipper68 said:

Without getting into a lengthy radiation discussion, there *is* IMO opinion something going on with these lights.

Whadya think?

Click to expand...

Well if you want a simple answer, I'll give it a whack how about:

It's a combination of the higher efficacy of the COB plus the COB is putting out significant less IR heat. The calories created are the same but the method to create the light though is quite different. Also the COBs cooling helps pull some heat away from the canopy, probably more so with active cooling. Funny thing is the lens is probably creating a bit more heat from transmission losses.

chipper68 said:

To put it simple, do plants under COB's use more or less water than HID?

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Hmmm, if your COB grow is perfectly setup it will use more water for most of the grow due to COBs better spectrum leading to periods of very aggressive growth however for the most part HPS and MH will probably use more water from beginning to end with their attendant extended IR and usually a bit of an IR spike radiating everything. And I'm also of the opinion HID will use more water than Red and Blue blurple based LED grow lights, especially the budget stuff. But again I don't consider that written in stone since environment, grow style and genetics are gonna have their say in the water equation too.

Cool photos Chipper, do you have any idea why one side of the LED cooler than the other? Is there a fan or exhaust on one side?

FranJan, thanks for the replies! That's some great insight as to what may be happening.

As far as the IR portion, is that something that is considered *missing* from COB's? I haven't seen much mentioned about that. Maybe small t8's or something that are in the IR spectrum?

There is a fan not too far away from the fixture than other.. additional / incidental cooling, good eye!!

I wonder if reseating the red cob would make the temperature distribution more even.

I posted this in another thread. I think you might want to review it.

Just generalizing I would say HID will cause more transpiration than COBs watt for watt based solely on the extra heat they generate.
I am not talking IR or infrared, just basic heat

chipper68 said:

Only by accident I'd take the glass off to measure temps, but it's a hassle and I don't have much headroom... might burn em up.

It would definitely seem (and the camera proves it) that cobs are cooler than HID.. I wonder if there's any information about different transpiration rates. I think the previous thoughts were that LEDs had less transpiration due to heat, but I'm wondering if there is something in regards to the radiation or some reaction with the cob vs HID.

To put it simple, do plants under COB's use more or less water than HID?

Click to expand...

I have still yet to build me a cob unit to test ect but i would probably bet my life that heat trumps spectrum in regards to water uptake the humidity is also lower. Now a 150watt hps and 150 watts of cobs would be a better test. Both passive cooled. Its like one big house fire vs 5 small house fires lol. Boltzmanns law might give you the answer your looking for. I've long forgot how to use it but it pretty much revolved around suface area and temps per given square area.

Transpiration is not always the main goal, lower temps mean better quality.

In my experience, the plants transpire/ drink more under higher ppfd conditions.
Watt for Watt LED's put out more light than HID meaning a higher ppf delivered to the canopy.

Transpiration is a natural process of photosynthesis. Barring any conditions that inhibit photosynthesis (VPD for example), the higher ppf levels the more transpiration, the more the plants drink.
As long as canopy temps are in check and Co2 is available.

As a general rule you increase temp to increase transpiration and add CO2.

There's also the fact that the hps is 450 watts vs the 220 of led. I bet if you doubled the output of your cobs they'd be significantly hotter.


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Unfortunately, those two lamps don't seem to be very linear comparisons, but VERY GOOD raw data!

I would think something like active monitoring of ambient temp coupled with transpiration rates monitored by the measurement of soil kPA [or soil water gravity] over time....using different photobiology aka Different lighting sources as your variable, may be onto something....

I have the same setup as you. I went from an air cooled 400w HPS system to 4 x CBX3070 220w. I think the temps are lower because my venting temps are lower.

Overall I'm not sure which is better but the LED does feel cooler.

I am assuming 400 watts cob is still noticeably cooler than 400 watts hps

bassman999 said:

I am assuming 400 watts cob is still noticeably cooler than 400 watts hps

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I'm hoping so too!

The information and responses here are awesome! Thanks to all!

HID emits a bit of radiant IR at ~825 nm (I am always screaming about) that literally beams heat into the canopy.


Even the warmest white LEDs lack radiant IR, so that should lead to reduced canopy temp even with the same PPFD

FranJan said:

I posted this in another thread. I think you might want to review it.

True or false? High-power LEDs don’t generate IR heat in the forward direction like a filament lamp
http://www.edn.com/design/led/4422381/1/True-or-false--High-power-LEDs-don-t-generate-IR-heat-in-the-forward-direction-like-a-filament-lamp

"If one could review the all the lighting industry’s literature of the last 10 years about high power LED lamps and luminaires, would you question this headline? That is, if you read that a high power LED emits no UV or IR and that any heat is created only in the PN junction and then transferred to a heat sink. Okay on the UV part; but the IR part? Read on. "​

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This is due the energy in the beam from light in the visible spectrum, as mentioned in one of the comments under the article. Even with a 1W blue LED you can feel the heat as the energy converts if you put your finger over it. Luckily this heat is "productive heat", and is completely unavoidable no matter what light source you use. You can reduce it by improving efficacy though, making sure the spectrum is ideal and the light is as uniform as possible in the canopy.

bassman999 said:

I am assuming 400 watts cob is still noticeably cooler than 400 watts hps

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It all turns into heat at some point. The canopy temps can be lower but the "room temperature" will be the same assuming both lamps are using 400 watts.