knna

AS time ago we had a interesting disscusion about this topic at the Cannabis World (before it was converted on a seed sale site), im going to quote myself, includiong some very useful links (hope they still work):

"As i said before, RH measure alone is insufficient to inform about transpiration rate.

If we want to compare differents setups, we will need more accurate measurements. VPD (vapor pressure defficit) is the right parameter. It includes RH and temp, combined with leaf temp. Supposing enough water avalaible and not salinity excess wich dont must succeed in a hydro system (meaning near constant and anycase, good water potential), VPD gives enough info to check transpiration rate and study TDS levels according to.

Usually, a range between 0,8 and 0,95 KPa is accepted as the best for optimizing transpiration rate. For cannabis cultivation, its better the high end of the range in flower stage, because is prefered a little low humidity envirovement to produce more resin and avoid mold problems. So, trying to get 0,90-1 KPa of VPD would be the target.

But, anyway, when comparing transpiration rates and TDS levels, the most informative data is give the VPD at it was produced.

Explain fully what VPD is, how to compute it would require a very large post, and all the info is already in the net, so ive selected some links in order to understand it:

This is a great ppt presentation (dont work now)

This wikipedia entry is very usefull. Short, and providing links to more info, about units used (and conversion to others) and to a VPD calculator. The usual range of leaf temp is about 3ºC less than ambient (at ambient CO2 concentrations, about 300ppm) to only 1ºC less (at high enriched CO2 concentrations, over 1000ppm). Canopy exposed to high IR content light can be perfectly over ambient temp.
Direct link to VPD calculator.
This provides a VPD calculation method. It use psi units, to convert to KPa: 1psi=6,895 KPa.
http://www.hortnet.co.nz/publication...umid01.htm#topThis is very good explaining concepts, notice there are 7 diff pages.

Finally, a excelent study about all this parameters: transpiration rate, VPD, light quality and irradiation:

Spatial distribution of leaf nitrogen and photosynthetic capacity. Only CO2 supplementation remain unstudied, but has several interesting links.

http://www.ars.usda.gov/SP2UserFiles...nspiration.pdfI put this because it study CO2 in relation to VPD. (dont work now)

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