hmm what is VPD? I used a VPD calculator and input my temp of 85 and RH of 55 are these good results? what is ideal VPD?
SVP: 4.111 kPa
VP: 2.261 kPa
VPD: 1.850 kPa
HD : 13.249 g/m
3 Likes
Depends what stage of growth your in ! Low vpd for seeldlings , med vpd for growth and higher vpd for flowering!
Ideally you need actually leaf temps but for good estimation 2-3 degrees lower are what your leaves are coming in with regard to ambient room temp
1 Like
Not sure what the conversion are for those values but seems like your RH is in the ballpark for flowering however your temps seem a little high if that’s the stage your in right now
1 Like
ok i’ll check my leaf temps with my IR thermo, i’m in late veg just waiting to be confident i’ve eradicated the thrips before i flip em to flower. i’m using LEDs so i keep my ambient temp higher as the lamp doesn’t provide as much heat to the leaves
2 Likes
1.850 KPa looks a tad to high for late veg box in upper right are your stages info!
3 Likes
@MadScientist can you split this convo in to the VPD thread from my 1st q for me, sorry bro! 
@Tinytuttle yes but at the moment i’m only able to control one of these factors which is humidity, so basically the temperature im at right now (which is 32 on the highest leaves, 28 on majority of canopy) the only way to bring it to the acceptable range would be raising my humidity to around 70-75 and actually keeping it there for the entire flowering … because i’ll be upping my wattage and increasing the temperature thru out flower … i think i’m gonna have to stop exhausting in to the room but it’s getting cold enough to see your breath so the side of my house will just be blowing hot air non stop which could be suspicious no? i mean i’m legal but i mean for robbers etc… i might just need an AC?
2 Likes
to move them, i posted in the wrong thread originally to ask about VPD i didn’t see this thread before posting
2 Likes
Done good sire. Your wish is my command!
4 Likes
vpd is vapor pressure deficit. vapor pressure is basically the relationship between temp and RH and the pressure it creates. vapor pressure deficit is the difference between the vapor pressure and the pressure the plant creates during transpiration.
think 2 forces pushing on each other.
so if your temp is high and RH is low there is less pressure in the air so the plant transpires faster.
and the opposite is true low temp and high RH slow transpiration in the plant.
ideal range is from .45 - 1.25 kPa with a target being .85 kPa, however most plants grow well between .8 and .95 kPa.
side note: it appears the chart i posted is not in kPa. apologies.
3 Likes
going off TTs chart above i’m around 1.45 in late veg but the plants seem to be doing OK
3 Likes
Plants will do OK outside the vpd range, but staying within it or as close as possible you will see increased growth rates, more time between waterings, and a lot happier plants in general.
3 Likes
Like Dewb said earlier in the thread plants will do fine outside the range but for optimal growth the Vpd needs to be in the sweet spot. I queued in on your post section highlighted … so in following natures sign when coming in to flower ( your increasing temp part) in fall, temps are decreasing not increasing when keeping your 40-55 Rh and keeping it a little cooler conditions you’ll hit your sweet spot!
2 Likes
ok i’ve realized having an AC is unavoidable lol thanks for your help guys 
i’ll hit that sweet spot alright 
3 Likes
found this in my notes on vpd
here’s one for all the math people
needed:
ambient temp
relative humidity
canopy air temp
now the fun
to compute directly from temperature:
(do this for air temp and canopy temp)
vpsat = e^A/T+B+CT+DT^2+ET^2+FlnT
where
vpsat is saturation vapor pressure in kPa (kilopascal)
A = -1.88 x 10^4
B = -13.1
C = -1.5 x 10^-2
D = 8 x 10^-4
E = -1.69 x 10^-11
F = 6.456
T = temp of air in K (kelvin)
Temp in K = temp in C (celsius) + 273.15
Fahrenheit to celsius = ( F* - 32) x .5556
then we need the partial pressure of the vapor
vpair = vpsat x RH/100 (RH being relative humidity)
and finally the vapor pressure deficit which is the difference between our answers
vpsat - vpair = vpd
or if we have the canopy saturation (which why we did all that math for both air and canopy)
vpcanopy - vpair = vpd
(more accurate because we are looking for that balance between the pressure the plant creates and the pressure in the environment.)
7 Likes
I was just trying to learn more about this…Thanks Dewb…
2 Likes
I’ve uploaded the Fluence guide before, here’s the complete version
Fluence Bioengineering_High PPFD Cultivation Guide_v1.2.pdf (1.4 MB)
another good one! Lumigrow
LumiGrow_LED_Growers_Guide_for_Cannabis.pdf (2.0 MB)
and not to be outdone…Illumitex…we got 'em all here at OG!
Illumitex-Cannabis-Grower-Guide.v7.pdf (947.0 KB)
3 Likes
Thank ye, thank ye.
Regards,
K
Lots to learn! I’ve read up on VPD, and I’m afraid to follow it. Mainly flowering stage scares me as the humidity would be around 60% at times.
Do I ignore all the information I’ve read that say to keep humidity between 40-50% in flowering and to even reduce that near the end below 40?
I’m planning on picking up a Pulse One, which monitors my grow room for my birthday this year and I’d like to learn as much about this before hand.