Originally Posted by delta9nxs
It has been mentioned by Spurr that 46-48 moles/day might be the maximum usable but I don't know where he got the figures.
DLI is rarely found much higher than 60 mol/m^2/day outside and in a natural greenhouse.
To find a possible goal DLI for flowering cannabis I tried to mimic what would happen in nature (re PPFD bell curve and DLI), using natural PPFD bell curve found in Hawaii that provided peak PPFD of ~1,500-1,800 (because that PPFD has been studied for cannabis). I did my best to create a goal flowering DLI of ~48, using the data from Hawaii and working backwards (ex. for 12 hours) to find PPFD that is both high enough to keep Ph in the 'high' range (ex., > 1,000), and high enough but not too high, to mimic a DLI naturally found during very bright and long days (re Hawaii).
If one used 1,500 PPFD all day (ex. 12 hours, 18 hours, etc) it could be too much DLI, and hinder rate of photosynthesis and cause photoinhibition (ex., midday depression of photosynthesis, (increased) photorespiration, etc.).
Using peak PPFD of 1,500 adjusted with an average PPFD bell-curve (over the whole daylength) from a high irradiance location (ex. Hawaii on relatively cloudless day maxing out at 1,500-2,000 PPFD) to find DLI is the best method I could think of at my disposal to easily guesstimate a possible goal DLI for flowering cannabis. That is, short of using PPFD per wavelength weighted with K.McCree's quantum yield.
1,500 PPFD fits into my DLI model because that irradiance level was found to provided near peak Pn in four different studies on cannabis, with variables such as temp, Co2, etc. And IIRC > 1,000 PPFD could be considered a benchmark for high(est) Pn. However, I didn't want to use 1,500 PPFD to find DLI because IMO 1,500 PPFD is too much instantaneous irradiance for every second over greater than 12 hours. Outside there is a PPFD bell curve, it starts low, gets high, ends low; and outside peak PPFD can be found about and just above 2,000.
I wanted to try and mimic the DLI a plant like cannabis (a 'sun loving' type) could experience outside, naturally, respective to the PPFD found to provide peak Pn for cannabis. So I found a long set of spectroradiometer graphs for PPFD every X minutes over a (nearly) cloudless day in Hawaii in summer; used to find DLI of that spot in Hawaii. I used Hawaii because (a) it has very high irradiance, a good example for what cannabis likes; and (b) I have grown cannabis in Hawaii (halfway up a big mountain, when I had my med card there) and saw first hand the effect of long days and high irradiance.
I think I am making it sound more complicated than it really is, it's quite simple but I think I may be over explaining in an attempt to explain enough.
Basically it's like this:
I used a natural PPFD bell curve from high irradiance location (Hawaii) that peaked at ~1,800 PPFD around noon; about the same PPFD that offers highest rate of photosynthesis for cannabis. Then I worked backward starting with the DLI from Hawaii, using 12 hour daylength (ex., for flowering) to find PPFD. And the PPFD was > 1,000, which is kind a benchmark for high(est) Pn.
My model for flowering (12 hour day) puts DLI at 46-48 (mol/area^2/day); that is achieved using ~1,000-1,100 umol/area^2/second (specifically ~1,065 to ~1,111 umol/area^2/second).
However, in reality most grow rooms will not provide over 1,000-1,100 umol/area^2/second to any plant; unless there is for example one small plant below a good lamp/reflector. For example, under a Blockbuster 8", with 1,000 watt Digilux HPS and Galaxy select-a-watt set to 'super lumens' (supposed to be 10% increase in radiance from 1,000 watt) -- with an Equalizer hot-spot diffuser -- the peak irradiance under about plumb center of lamp (while keeping irradiance high at the range of 3'x3') is ~920-975 umol/6"^2/second (10 second average to account for lamp flicker).
Most grow rooms will probably provide somewhere around 40-44 mol/area^2/day for flowering; those that are 'bright'. Very well designed rooms, esp. with respect to canopy size relative to size of reflector and light mover, can have over 50 mol/area^2/day for 12 hours.
(... nuts and bolts in next post ...)