knna

Great links, LJB.

They throw some more light to the topic being discussed in this thread.

First off, I think that tomato and Cannabis are very similar on some caracteristics, and apart of tomato requiring more water and nutrients, other environmental parameters are similar for both species. So tomato performing better at such high VPD may mean that cannabis does it too. We were assuming a VPD about 0.9KPa as optimal for cannabis. But taking into account its resistance to hot and dry condition and checking tomato response, we may conclude that it performs better at higher VPD levels.

Unfortunately we dont have the full article. I think the three humidity tested are too extreme, so maybe (I could say probably) the optimal VPD is between 30 and 60%RH at 28 and 31ºC temperatures, meaning a lower VPD than the extreme at 30%RH, that is simply better than at 60%RH. But anyway, it seems very probable that cannabis perform better at higher VPDs than we initially assumed. We dont know how much higher, but thinking on a optimal VPD of 1.5-2 KPa seems reasonable.

So we can use lower RH for each temp than initially suggested, which eliminate the concerns of mold, as now we may consider optimal RH below 50% at 30ºC. So probably the classic 40-45% RH recomendation is very accurate.

For the last study linked, that is the best Ive read in many time, and one of the little scientific studies about cannabis photosynthetic response (only one modern AFAIK), we may say with safe margin that cannabis likes hotter temps than most crops. It gets the max at 30ºC (86F) for all treatments (CO2 and irradiances). Again, maybe the best point is between the two closer temps tested (25 and 30ºC), and the optimal temp is 28ºC instead of 30ºC. But its still hotter than most crops.

An it confirms that photosynthesis decreases over 30ºC, so we should try to keep our grows close to 30ºC but without going over it.

Other interesting result is that there is no photoinhibition at 1500uE at any of the temp tested, although at most temperatures that irradiance shows to be closer to it. But while net photosynthesis decreased at 2000uE for all temperatures except for 25ºC, that photoinhibition point, although dependent of temperature, is well pass 1500uE and probably close to 2000uE (except for 30ºC, where it seems to be close to 1500uE/m2).

As informative note, 500uE/m2 correspond to 42Klux of HPS (31Klux of sunlight), for maximun productivity.

1000uE/m2 correspond to about 84Klux (HPS)

And 1500uE/m2, max advisable irradiance to use, correspond to 126Klux, and a little below 10000Klux. We have assumed those levels to be give or take where the study states that they are, so most light density recomendations used are pretty accurate.

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Bulb Analyzer Tool

Luke Highwalker

good call, depends on the dimmer though. if you can find an old skool dimmer from a house built in the early 1900's that will work flawlessly. though they stopped making them because they are a HUGE fire hazard...lol.

the best speed controller for cheap is either a variable speed router controller from harbor freight for 19 bucks, or a ROTARY/SLIDE TO OFF fan speed controller from the home store.

the 3 position or hi/lo hi/med/lo fan dimmers will not work for your application. hi will be about 1/3 the stock speed, and med and lo usually wont even turn the motor. these are usually capacitor dimmers designed for larger motors. they work real well and dont hum, but the capacitor has to sized to your motor's requirements.

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