Vapor Pressure Deficit & What It Means To You!

Gazoo said:
Well this CL find should help :green_heart: More Data to wrap my head around, Oh Boy
Could not resist for the price, Its basically New, Never Used item.

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Looks great man!
Pennywise said:
Nice find Gazoo!
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Indeed! Hey Penny :ciao:
Pennywise said:
I’ll need to find lights cheap here pretty soon!
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Ooh! Exciting times!
Gazoo said:
Thanks, I should be able to put it to good use :idea:

Seems every time I get bored and find myself on CL, Its some type of impulse buy that
I cant refuse. Nice pile of grow stuff for free listed at the moment with Blue Lab test meters
air pumps, Lights, Tent and more just a tad to far to travel but I could of used the Intake/Outlet
fans and Carbon filter for my new tent and then re-gift the rest. Oh well I bet I'll find some more
local before I need them.
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Ooh I need to look for a good deal on a ph pen! Thanks Gazoo!
bobrown14 said:
I'll have to start shopping on CL more... gets a little creapy around here tho.

Always worried I'm gonna get ripped off. You know like robbed! ffs
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Man, that’s the truth. It’s scary dealing with strangers sometimes. I’m from the south originally and am likely to be overly trusting.
 
hey guys, is 24c and 50% RH suitable for flowering with the lights on? currently in my 5th week of flower.

Don't have a dehumidifier and i'm pretty paranoid about the RH rising too high and having a mold problem. specifcally during the night when the RH rises quite a bit. i've been using a small space heater for this reason but i'm losing out on having my night time temperatures slightly lower than the day time temperature.

hope my buds don't suffer :( any advice is appreciated.
 
Pennywise said:
50% should be fine in flower. :ganjamon:
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I'd be celebrating with 50% this time of year. At night we have dew points being reached so 90-100% rh tough time to grow.
 
Eugor said:
hey guys, is 24c and 50% RH suitable for flowering with the lights on? currently in my 5th week of flower.

Don't have a dehumidifier and i'm pretty paranoid about the RH rising too high and having a mold problem. specifcally during the night when the RH rises quite a bit. i've been using a small space heater for this reason but i'm losing out on having my night time temperatures slightly lower than the day time temperature.

hope my buds don't suffer :( any advice is appreciated.
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To answer that accurately FOR VPD I need to know the Temp difference between the Ambient Air temp (temp NEAR leaf)
AND the leaf temp at the point your taking the readings. Leaf temp can be read with a Laser temp reader and the
Ambient is the air around that leaf so putting a temp gauge near the leaf will give the Difference

As to the mold concern I don't expect 50% Rh at lights on will be a problem.

Below is a link to "CANNABIS BUSINESS TIMES" Article with some very good info

Some Highlight info from this article

In the flowering stage, growers came closer to a consensus. Nearly three-fourths (73%) of respondents said the optimal humidity level in flowering rooms is in the range of 40% to 55%. Eighteen percent said 56% to 65%. While 3% of cultivators said that their ideal flower-room humidity level is in the range of 76% to 80%, none said that above 80% humidity was ideal.

1843907


Based on peer-reviewed research in the field of indoor/greenhouse produce cultivation, the following relative humidity (RH) conditions would be suitable for indoor/greenhouse cannabis cultivation.

  • Cloning: 70%-75% RH
  • Vegetation: 65%-70% RH
  • Flower: 60%-65% RH
  • Night phase of flower: 55%-60% RH
 
Some cultivars handle the RH heat thing better than others.

Equatorial Sativa - they can handle heat and humidity not a problem for them.

Most of my hybrids, not so much.

I try and keep the RH under 65% - that's my worry line. When its hot during the day and it cools down a lot at night, that's when the RH goes thru the roof.
 
Pennywise said:
50% should be fine in flower. :ganjamon:
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Thanks Penny! Yep 50%RH during the entire grow is optimal, in my opinion.
Eugor said:
thanks for the quick reply, have a couple pictures from my tent :)
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Looks like some lovely bugs coming your way Eugor. Sorry it took me a bit to reply. As you know, RH will spike at lights out but maintaining 50%RH is a great goal to shoot for and it will keep you from going crazy trying to hit the perfect VPD.
Gazoo said:
To answer that accurately FOR VPD I need to know the Temp difference between the Ambient Air temp (temp NEAR leaf)
AND the leaf temp at the point your taking the readings. Leaf temp can be read with a Laser temp reader and the
Ambient is the air around that leaf so putting a temp gauge near the leaf will give the Difference

As to the mold concern I don't expect 50% Rh at lights on will be a problem.

Below is a link to "CANNABIS BUSINESS TIMES" Article with some very good info

Some Highlight info from this article

In the flowering stage, growers came closer to a consensus. Nearly three-fourths (73%) of respondents said the optimal humidity level in flowering rooms is in the range of 40% to 55%. Eighteen percent said 56% to 65%. While 3% of cultivators said that their ideal flower-room humidity level is in the range of 76% to 80%, none said that above 80% humidity was ideal.

1843907


Based on peer-reviewed research in the field of indoor/greenhouse produce cultivation, the following relative humidity (RH) conditions would be suitable for indoor/greenhouse cannabis cultivation.

  • Cloning: 70%-75% RH
  • Vegetation: 65%-70% RH
  • Flower: 60%-65% RH
  • Night phase of flower: 55%-60% RH
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Couldn’t have said it better, Gazoo. Sounds like you’ve pretty much dialed your vpd in at this point. :passitleft:
bobrown14 said:
Some cultivars handle the RH heat thing better than others.

Equatorial Sativa - they can handle heat and humidity not a problem for them.

Most of my hybrids, not so much.

I try and keep the RH under 65% - that's my worry line. When its hot during the day and it cools down a lot at night, that's when the RH goes thru the roof.
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Very true!
I would also like to add, that if you are chasing (and achieving) high RH in your tents, be careful that your carbon filter isn’t exposed to that humidity.
I ruined a carbon filter by trying to maintain 65%RH in my tent. I’ve got the gadgets to keep it within a percent or two of RH and my carbon filter took a dump not too long after.
Because of this, I now just maintain 50%RH and I regulate my day temps to 80F and 70F for lights off. I think that day/night temp thing is more important than some folks might think.
 
bobrown14 said:
Some cultivars handle the RH heat thing better than others.

Equatorial Sativa - they can handle heat and humidity not a problem for them.

Most of my hybrids, not so much.

I try and keep the RH under 65% - that's my worry line. When its hot during the day and it cools down a lot at night, that's when the RH goes thru the roof.
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That's the same reason I don't like growing sativas lol. I cant keep it hot or humid enough for them so I generally stick to 50/50s. It's cold here pretty much all year round and low humidity other than a month or 2 around now. Sativas don't like my set up. Go all papery and thin. Can't control VPD anymore so just gotta pick strains accordingly.
 
Barney86 said:
That's the same reason I don't like growing sativas lol. I cant keep it hot or humid enough for them so I generally stick to 50/50s. It's cold here pretty much all year round and low humidity other than a month or 2 around now. Sativas don't like my set up. Go all papery and thin. Can't control VPD anymore so just gotta pick strains accordingly.
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My outdoor plants have been very warm as the temps have been in the upper 90’s lately. Critical Haze deals with heat a little different than Carnival but they are both handling the heat very well.
It’s nice to stop watching VPD for a while when growing outside but now I’m just watching for bugs all the time lol.
 
Archiweedies said:
My outdoor plants have been very warm as the temps have been in the upper 90’s lately. Critical Haze deals with heat a little different than Carnival but they are both handling the heat very well.
It’s nice to stop watching VPD for a while when growing outside but now I’m just watching for bugs all the time lol.
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When I grew in tents it was something I obsessed over. It really does make a massive difference in growth rates but I don't even use my hygrometer any more. Been in a drawer for the last year and won't be back out :)
No tent so cant help it anyway.
 
Archiweedies said:
I would also like to add, that if you are chasing (and achieving) high RH in your tents, be careful that your carbon filter isn’t exposed to that humidity.
I ruined a carbon filter by trying to maintain 65%RH in my tent.
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Thanks for the tip - my filters are outside on the roof - they get rained on and my roof water drains right onto them. They are in the sun part of the day. On the roof hard to say how hot and dry it gets this time of year.

Being outside, as long as I cant smell my flower room, good.
 
Just finished an interesting article comparing LEAF SURFACE Temperature using
HPS, MH, CFL and LED Grow Lights . Granted it was written by a LED light maker :)

It actually confirms what I have noticed. Using MH/HPS and T5s I should/need to keep
the AMBIENT TEMP at 77F for best LEAF TEMP readings

Noticed if the AMBIENT Temp in my tent goes above 79 F I start to see LEAF TEMP STRESS
in the form of + readings compared to Ambient.
EXAMPLE
at 81/82 F ambient, leaf temp is 84+
at 80 F ambient its + 1 or 81 F deg leaf temp
at 79 F ambient its THE SAME, Leaf temp = ambient

I suspect if I can get it to 77 F Ambient I would be at the -2C range that is charted on
one of my VPD charts (I have to adjust now for the zero difference to use that chart)

CONCLUSION after reading this article it Looks like if using LED you can run a higher Ambient
 
Great grow environment article for the aficionado! :thumb:
But I have a unique humidity problem in my 4x4 grow tent that has really stumped me how to fix. My temp naturally stays a perfect 70-75 degrees with just the circ. fans and lights..... Unfortunately my humidity also stays at a constant 70+ and the only way to bring it down is by adding heat... which I don't want to do. It is commonly recommended here to bring the external room humidity down...unfortunately this is not an option because the tent is in a very large basement. I need a way to decrease humidity INSIDE the small tent... so full size dehumidifier placed inside the tent is not an option. Also, it is my understanding the smaller compressorless (Peltier) dehumidifiers do not adequately remove enough RH in a grow tent? True?... Thanks in advance for any help!
Slipp
 
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