VPD and why do I care

Tht_Blk_Guy27

Well-Known Member
My grow setup doesn't really compare to yours but nevertheless I have learned some things along the way that I think apply in general and well .. take my advice or leave it lol but...

I also have my exhaust fan on the top of the tent just pointing into the hole on the top (with some ducting), but I don't have a filter. It's legal to grow here now so neighbours be damned lol they can deal with it. The other advantage to running a larger fan then your space needs is that by running it slower it is MUCH quieter (in my experience with the 6" AC Infinity that I am using).

With respect to exhaust though, even though it was a bit of work and required draping ducting all the way across my room I've found that it's basically a must to be venting your exhaust outside of the room, preferably outside of the house altogether through a window or something.

I had an active intake fan but the temps were HIGHER then if I just used passive intakes, and I tested this over and over with my arduino. I'm not a physicist so I don't know why but in my case the negative pressure that results from the passive intake led to lower temps then having an active intake fan. I also made this ghetto filter out of a $3 6" toilet flange, some cardboard, a cheap filter cut to size and some pantyhose and after one grow it's WILD the shit that it picks up, super nasty almost black lol but that shit didn't get into my tent so I'm happy with the diy cheapo filter I made.

I'm still trying to sort out what to do with A/C to draw in cold air to the tent because it get hot in there .. did I mention it's a pretty tiny tent just 2'x4'x5' and havn't come up with much except for maybe a swamp cooler (a decent heatpump or a/c unit is just WAYY too much for the 40sqft space that's in my tent) so I don't have much advice there on how to deal with other then if you can get the ambient temp of the room itself that the intake air is being drawn from you could probably avoid the headache of having to try to remove so much humidity created by having an A/C right next to your intake.

Infrared thermometer as @Arkos has suggested is a valuable tool to have and they're like $20, it's definitely very informative to have the leaf temp. I also try to take temp/humidity measurements at different locations in the tent (eg. one near bottom near the intake, one at canopy height and one near the top close to where it exhausts), you can buy a 4 pack of temp/humidity meter for like $20 so why not. The data doesn't reveal anything groundbreaking but it has given me an idea on how air circulates when the tent is closed and where there are hotspots. By placing the exhaust ducting at the same level as the light I've had a notable reduction in leaf surface temp and I only figured this out by having a bunch of meters in my tent.

You've probably already got your own but I've attached a VPD chart that I've found to be pretty useful as it shows the desirable values for the different stages of growth.

PS: I have no idea what any of your PS means, what is a Takanime? is that like a tiny nuclear fusion tokamak reactor lol? :D jk

PPS: In case you are curious (and I don't know why you would be) my setup is a 2'x4'5' tent with a 6" AC infinity fan at 5/10 setting exhausting; the light is a 400W HPS in a sealed air cooled reflector that has it's own dedicated 230CFM exhaust fan and the air cooled reflector is flanked by 2 70W HLG qb120 boards diyed into a frame of angle aluminum. I've managed to get temps at about 80-85F when I point the exhaust fans ducting near the level of the light which is only about 5F tops over ambient (and that's to deal with 540W of lighting in a 40sqft space) .. before moving the exhaust fan ducting close to the light it was 10+F over ambient. I have 2 intake holes open near the bottom of my tent with 2 of those diy filters. Still working on humidity though and have a raspberry pi sitting here waiting for me to try the Mycodo software which looks amazing (because I don't have to code it myself! lol) for automating my grow hopefully. Wish I had more space because it has been a nightmare trying to dial in my settings.
please tell me your going to seal that up and run co2? you got temps and vpd all right add some of natures #1 plant steroid. i haver the same set up as you! i run 2 {2x4x6} so the temps are hard to control unless using a dehumidifier or swamp cooler but then they take up valuable real estate and they only add to the CFM numbers and thus have to run the fan higher or lower ant it just throws your balance out of whack
 

guitarguy10

Well-Known Member
please tell me your going to seal that up and run co2? you got temps and vpd all right add some of natures #1 plant steroid. i haver the same set up as you! i run 2 {2x4x6} so the temps are hard to control unless using a dehumidifier or swamp cooler but then they take up valuable real estate and they only add to the CFM numbers and thus have to run the fan higher or lower ant it just throws your balance out of whack
if I seal it up (read: turn off fans) temps exceed 95F. surely that's too hot, even with co2?
 
I switched out the coal filter that was a bit clogged with a smaller one. New temp record:/ 96 fahrenheit.. and 60 f on the low.. i really want those Girls to make it through flower but this temp situation is challenging.

Dont want to derail the vpd thread, just gutta vent(lol)
 

Tht_Blk_Guy27

Well-Known Member
if I seal it up (read: turn off fans) temps exceed 95F. surely that's too hot, even with co2?
Damn daniel... yeah your peaking unfortunately. I can get my tent to around 92 sealed but that's if I have a intake and fans inside that's it. However in my flower tent I'm suffering from simply not doing the research with my first lights and being forces to use 2 different lights so obtain peak coverage and growth. I'm getting some CO2 on Monday so that's why I figured I'd ask since we're in a similar boat on different parts of the ocean . If you could find some way to supercool your tent though I believe your running HID? I'm running led so that may be the big difference
 

Tht_Blk_Guy27

Well-Known Member
I understand what VPD is and how it affects the plant. I've never been too worried about because I can't control it in my current setup. But can somebody please explain in simple terms how to calculate the optimal vpd range?
You'll have to know the actual Vapor pressure of the room/tent and subtract it from the saturated Vapor pressure numbers
 

guitarguy10

Well-Known Member
I'm debating but whether i should just turn the fans of when lights are out, so far rh has jumped from 30% to 50% after being off for like 2 hours. not sure if the jump in rh is worth it (usually it sits at about 30% with exhaust fan running). anyone have any suggestions? once I get this rasp pi built and coded I hope to automate a bit and at least turn it down to like setting 1/10 even lights go off. unfortunately when I bought this ac infinity fan the one with the environmental controller was like $150 more in Canada so one only got the one with the basic manual 10 speed fan controller.
 

Tht_Blk_Guy27

Well-Known Member
I'm debating but whether i should just turn the fans of when lights are out, so far rh has jumped from 30% to 50% after being off for like 2 hours. not sure if the jump in rh is worth it (usually it sits at about 30% with exhaust fan running). anyone have any suggestions? once I get this rasp pi built and coded I hope to automate a bit and at least turn it down to like setting 1/10 even lights go off. unfortunately when I bought this ac infinity fan the one with the environmental controller was like $150 more in Canada so one only got the one with the basic manual 10 speed fan controller.
honestly im where you're at kinda in a sense. I cant get the humidity down in my flowering tent unless I switch my intake into a exhaust and vise versa with my veg tent. it actually worked to swap tents with the locked variables I had. I'm really trying to steer away from adding extra hardware (humidifier/dehumidifier/swamp cooler e.c.t) that might add to my CFM measurements and thus throw my measurement out of whack. im really debating on going back to a unsealed set up but i do have CO2 on the way :-(. i could control everything with the 67 controller from work and it was nice to be able to set trigger parameters so your fans turn on or off based off of your specification (mine is set in my veg tent to fill it with new air at 7 for 20min on and20min off) and before it was maddingly easy to dial it in
 

guitarguy10

Well-Known Member
my prob is that RH crashes down to about 30% within seconds of turning on exhaust fan, so keeping RH higher is virtually impossible and if I have exhaust fan going too low my temps go above 93F (and as I learned last night over 105F when sealed with 2 oscillating fans inside).

so I've kinda almost resigned myself to just trying to keep temp and RH constant, even if they're out of ideal ranges :(.

I wish I had more room I would just buy I bigger tent but my place is small and I've already had to sacrifice half of my closets space just for this 2'x4'x5' tent. it's legal to grow Its not like I'm trying to do some stealth grow, just live in a very small place. wish I could think of some other options.

I have thus raspberry pi I had been looking forward to setting up for automation but if I can't get the numbers in the range iwant in the first place then what is there to automate? lol
 

RonnieB2

Well-Known Member
With LED you'll get faster growth around 82 F, LED doesn't raise leaf temp above air temp with infrared.

I never used one but the Cloudline's with controller sure look good to me. @Lordhooha do you think the 6" is too much for a 5x5?

I've always used small fans with an Inkbird humidity controller, all I need is a enough airflow to keep humidity in check.
I don't care about exchanging the air every thee minutes or twenty minutes or whatever this or that website says with no logical explanation.
I think you're right. My grow right now is around 84 max. And ranges mid 70s to 84 and my grow going now is the best I've grown so far. Plants at 41 days and already stacking up big
 

Lockedin

Well-Known Member
my prob is that RH crashes down to about 30% within seconds of turning on exhaust fan, so keeping RH higher is virtually impossible and if I have exhaust fan going too low my temps go above 93F (and as I learned last night over 105F when sealed with 2 oscillating fans inside).

so I've kinda almost resigned myself to just trying to keep temp and RH constant, even if they're out of ideal ranges :(.

I wish I had more room I would just buy I bigger tent but my place is small and I've already had to sacrifice half of my closets space just for this 2'x4'x5' tent. it's legal to grow Its not like I'm trying to do some stealth grow, just live in a very small place. wish I could think of some other options.

I have thus raspberry pi I had been looking forward to setting up for automation but if I can't get the numbers in the range iwant in the first place then what is there to automate? lol
I have a somewhat similar issue.
I generally run 81-83f, but have found the tent at 87f / 39%rh a couple times.
When that happened, I tried running the fan (AC Infinity T6 in a 4x8) 2 settings higher for a few minutes -- no change.
What I've found is that I had to run the fan - tent sealed - for a few minutes at 10. Disturbing and cycling out all the old air somewhat violently.
Then easing the fan back to normal - 3 in my tent.
At that point, the tent will slowly drop back to normal.

It might be that I need to re-arrange my intakes & circ. fans to move air more effectively.
 

Southernontariogrower

Well-Known Member
I got a question about vpd, my tent gets up to 90 degrees maybe 91, humidity in flower is hovering around 80%rh 4x4 tent 16 inch fan on roof of tent, feeding homebrew co2 so no exhaust until 1 hr before lights out. 7 gal plastic pot, put into flower as soon as leaves were same size as pot, not always centered perfect, exhaust turns off 1hr after lights out, to flush humidity. Then co2 builds up lm hoping, it does no meter but good growth. This grow l had some male problems as l was adding a plant a week on average, last 3 went in together. So why is this so bad, explosive growth. Im happier each crop, personal only. My kids smoke too and roommate. When l had lower rh by exhausting constantly l got powdery mildew like crazy. High humidity no pm. Wtf? Nighttime temp is around 72. Can anybody tell me why, humidity is bad were taught, were not plants, they love respiration they create o2 for us, what am l missing here? This plant is 3 weeks ahead of anything else, no pm high humidity, tent gets wet around canopy. Ive watched every youtuber out there, 65% rh and l grow mold or pm or root rot. Does anybody live on equator here? Sorry wasnt try to steal thread, but vpd is the first thing l see when l open tent, made chart of green zone below hygrometer.
 

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Arkos

Well-Known Member
@Southernontariogrower
Humidity is not exactly "bad" unless the air is not moving imo.

Have you tried adding a filter to your intake ? Try adding a hepa filter or something similar.

My guess is you're sucking in mold spores in large quantities if your getting mold problems with the extraction on full power.
 
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