This discussion, how much heat from led/HPS, a watt is a watt etc, is one of the most controversial subjects I see discussed on the site. It always ends with flame war with both sides convinced that they're right. I've heard of people doing tests, watt for watt, and sometimes HPS comes out hotter, sometimes led and hps the same. Even though I can agree with that a watt is a watt there definitely seems to be more to it. If your slicing hairs a watt is a watt is only true in the
same thermal system. As soon as you've changed out the light technically it's not the same system anymore and will behave different. If the hps is heating bulb, tent, reflector and the air in the tent, and the led is heating the PCB, heatsink air and tent it's not the same system is it? It'll still be close, but not necessarily the same. Thermodynamics is easy to put your head around the basics which are very straightforward, but bringing it all to practice is a whole other matter (not that I know the whole deal but I saw my mates exam on it and it was way beyond me). But it's safe to say that there is a lot we don't take into account on the level we're at. Way back when there was a lot of diy and a lot of people (smarter than me) were counting on thermal resistance of heat sinks. Everybody using thermal resistance as a constant when infact thermal resistance changes depending on how many watts a put thru the sink. I added a proper heatsink datasheet here to show the graphs that nobody knew existed.
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This is not to flex or anything it's only to show that there are things about thermodynamics that smarter people than me didn't have a clue about and that it's important to understand that in this subject we are all a bit on amateur level.
PJ Diaz: I'm so glad you've done tests, but I feel they would be more interesting to our situation if you kept extraction and fans on. After all were talking about this in a grow context, not in order to prove if physics are "right".
Another issue I've come on to, anecdotally, is ambient temp differences between MH and HPS, watt per watt. I've seen people claim that this difference was down to difference in the arc temp in the coil inside the bulb, MH being hotter. Which kind of makes sense, remember that the 2200K of HPS and +4000k for MH; these numbers refers to the light emitted by a blackbody heated to 2200degrees Kelvin, and would thus be almost the double in MH.
If someone sat down and made some more extensive test my best guess is that we will find both:
- A super strong correlation between watts used and temps inside the growspace. Well obviously. And:
- my guess is that you would also find a correlation between the hottest point of your growlight and ambient temps. And I suspect that this correlation would still be there if you controlled and removed wattage from the calculations. Edit: this latest assuming there is no real airflow over the light/bulb/heatsink.
Edit: big oopise on me, that's the wrong datasheet, the graph is not related to watts. I'll see if I can dig out the right one.
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