Vapor Pressure Deficit & What It Means To You!

[Aroxol]

Shit he did all the heavy lifting too thanks Gazoo!
Glad you got it worked out.

On the topic of the higher humidity, bacteria can still thrive in warm humid environments and although your plants maybe transpiring like crazy there still a risk.
Without an actual VPd meter, we have to use the charts available to us or do the math the old fashioned way.
One thing I can’t be certain of in your case is that you are running CO2 in presumably a sealed environment. Naturally heat and humidity will be higher. You are able to get away with much higher temps when your running CO2. That could make the chart you use a little different, I can’t be sure.
Given your environmental temps and leaf temps it sounds like your running LED’s or ducted hoods.
If you have an air conditioner, you may have to lower temps to the 78F range so you can run a slightly lower humidity.
Humidity really is the factor we need to control after all.

Hey thanks again guys for your time and help. Right now I’m running a CO2 tank in sealed room. Air is being scrubbed through CAN33 being pulled but T6 fan, and then I have a second T6 exhausting to another CAN33 filter outside of the tent which I activate when I need to clear some heat/ humidity from the tent. Right now I don’t have a dehumidifier or AC and don’t think it’s an option as they pull too much from the wall and I’ll blow a circuit. In terms of lights it’s all LED and no vented hoods. Running an Advanced LED XML350 side by side with 3x100w Chilled Logic Pucks (DIY).

Not having messed with VPD before, humidity is really the only battle right now. Managing the levels is way more manual than I’d prefer if I keep with the VPD approach. Catch 22 is I’ve never seen growth this rapid, so it’s a trade off I guess. I just want to make sure I don’t fly too close to the sun and end up with mold/ bud rot etc..

 

[Pennywise]

The only thing a lot of us can do is get as close as we can without spending a bunch of money or overloading our circuits.

 

[TurboBucket]

It's an expensive rabbit hole that's for sure.

 

[Gadfly]

Consice and a math avoiding chart—brilliant.

“I don’t alway fap to articles on VPD, but when I do, I prefer this one.”
—Some guy

 

[Archiweedies]

Hey thanks again guys for your time and help. Right now I’m running a CO2 tank in sealed room. Air is being scrubbed through CAN33 being pulled but T6 fan, and then I have a second T6 exhausting to another CAN33 filter outside of the tent which I activate when I need to clear some heat/ humidity from the tent. Right now I don’t have a dehumidifier or AC and don’t think it’s an option as they pull too much from the wall and I’ll blow a circuit. In terms of lights it’s all LED and no vented hoods. Running an Advanced LED XML350 side by side with 3x100w Chilled Logic Pucks (DIY).

Not having messed with VPD before, humidity is really the only battle right now. Managing the levels is way more manual than I’d prefer if I keep with the VPD approach. Catch 22 is I’ve never seen growth this rapid, so it’s a trade off I guess. I just want to make sure I don’t fly too close to the sun and end up with mold/ bud rot etc..

Awesome and beautiful idea about scrubbing the room 24/7 and then exhausting every now and again to remove heat and humidity.
The answer is clear bro you need a way to dial in your heat and humidity.

I use the humidity (blue one) but because I have a dual hose ac I need the inkbird temp controller with a higher output amperage.

My advice would be to keep humidity between 55-65RH throughout veg as best as possible and keep temps between 75-82F.
Again, I don’t know the rules about VPD when running CO2. I know Carbon Dioxide was higher in our atmosphere thousands of years ago and as a result you can run higher temperatures when adding co2. I don’t know if RH follows the same model as temp does but it should be obvious that there is a direct relationship between temp and RH. As temp increases, RH must rise to keep the stomata open and vice versa.
Though I’m not running co2, I have experimented with a sealed tent and I think we both know from experience that beat and humidity skyrocket once you stop venting. How has your humidity been in flower with this setup?

 

[Archiweedies]

The only thing a lot of us can do is get as close as we can without spending a bunch of money or overloading our circuits.

Totally, and the worst thing you can do is buy something that doesn’t work and you discover you need a different thing. Because it’s a big...

It's an expensive rabbit hole that's for sure.

Big... rabbit hole. Good news is once you get it dialed in you can adjust day and night temps independently and if you have genetics that do well in flower at low temps like 65F you might be able to draw more color out of your produce.

Consice and a math avoiding chart—brilliant.

“I don’t alway fap to articles on VPD, but when I do, I prefer this one.”
—Some guy

Oh wow.. I’m glad you um... enjoyed...yourself

 

[Gazoo]

Question on your Humidity controller.

Will it work with the large room humidifiers Specifically one that has a Push Button ON/OFF
switch, I have noticed if my humidifier is on and the power gets disrupted I need to RE-push the button again to
start things back up THINKING that wont work with these controllers ? Mine do have there own Humidity controller settings
which I am using now so not a big problem but a single more accurate controller for both systems (temp/humidity) would be
better

 

[Archiweedies]

Question on your Humidity controller.

Will it work with the large room humidifiers Specifically one that has a Push Button ON/OFF
switch, I have noticed if my humidifier is on and the power gets disrupted I need to RE-push the button again to
start things back up THINKING that wont work with these controllers ? Mine do have there own Humidity controller settings
which I am using now so not a big problem but a single more accurate controller for both systems (temp/humidity) would be
better

Good question Gazoo and the answer is, sadly, no. The need to push the button negates the function of these devices here. Thats fine though and the reason is because your built in humidistat can be sort of "controlled" by you and your most trusted hygrometer. Hope your still with me here, Im a little stoned at the present.

You probably already do this, but for the sake of learning for all, determine what your humidifiers"differential is by measuring humidity with your trusty hygrometer and your humidifier set to, lets say 45%RH. If your hygrometer is reading 42%RH and that remains somewhat steady over whatever time you determine to be sufficient, you can assume that your humidifier really needs to run at 48%RH for your hygrometer to read 45%RH.
Now keep in mind this is ina perfect world where the humidity in your room the tents are in never changes. When there is humidity in the ambient air, the tent will be more humid of course. So we have to adapt as best as possible, which leads me to the fact that the devices I posted are very accurate and reputable and allows me to control everything to within one degree or one percent. The real world, at least for me growing in an uninsulated garage, is that I am more susceptible to changes in ambient temps and therefore its not perfect. It would be better if I insulated my tent and dont be surprised if I do that. Currently open to suggestions, by the way!

Cheers!

 

[Gazoo]

Good question Gazoo and the answer is, sadly, no. The need to push the button negates the function of these devices here. Thats fine though and the reason is because your built in humidistat can be sort of "controlled" by you and your most trusted hygrometer. Hope your still with me here, Im a little stoned at the present.

You probably already do this, but for the sake of learning for all, determine what your humidifiers"differential is by measuring humidity with your trusty hygrometer and your humidifier set to, lets say 45%RH. If your hygrometer is reading 42%RH and that remains somewhat steady over whatever time you determine to be sufficient, you can assume that your humidifier really needs to run at 48%RH for your hygrometer to read 45%RH.
Now keep in mind this is ina perfect world where the humidity in your room the tents are in never changes. When there is humidity in the ambient air, the tent will be more humid of course. So we have to adapt as best as possible, which leads me to the fact that the devices I posted are very accurate and reputable and allows me to control everything to within one degree or one percent. The real world, at least for me growing in an uninsulated garage, is that I am more susceptible to changes in ambient temps and therefore its not perfect. It would be better if I insulated my tent and dont be surprised if I do that. Currently open to suggestions, by the way!

Cheers!

Right with Ya, I have noticed the VPD RANGE at the temps I run at have a good spread/range so I did not have any
trouble in FLOWER. At worst occasionally I would be 1 box away before I needed to adjust but that was
only a few times. We will see how it goes in Veg. I did start a little cheat sheet for my settings according to the
expected RH reports

RH Weather report by the hour

 

[Aroxol]

Well wait, if you’re not spending money on growing gear and “too much” time in the tent... how is your wife supposed to be pissed off at you? Sage advice on the humidity controller, that’s missing from my current setup. Even still, after the input I’m about as dialed in as I can be, currently at 1.274 with ~1450PPM CO2. Air is being scrubbed, absolutely no exhaust and been stable most of the day. Think I’ve got the lights and CO2 dialed in to maintain this level. Weed plant is looking great (no new cal mag signs) and looks like even the pepper plants in the room decided to join the flowering party.

Appreciate the help getting on track with VPD, amazing how much you can learn in 12 hours.

 

[StoneOtter]

VPD - Overview​

Until the last couple years, Vapor Pressure Deficit, was measured mostly by research universities and agricultural laboratories. The tech available to growers these days has enabled us to dial a grow room in very accurately with regards to heat and RH. We all know that cannabis can benefit from an environment between 70F- 85F or 20C - 30C. We also can benefit from slightly colder temps at night and this is encouraged as it promotes maximum growth during lights out and the potential for certain genetic properties of a particular chemovar to manifest such as purple or pink buds, etc. Temperature and Relative Humidity will allow you to grow cannabis, and grow it well, if the parameters are maintained. When we keep our plants in the "sweet spot" of VPD, the stomata of the leaves are manipulated in a way that allows maximum performance from our plant. One analogy is that the VPD happy plant is like an athlete, whereas a plant outside of the ideal VPD zone is just strolling through the park.

I'd like to first state that I was urged to look into this information from fellow growers @Pennywise and @InTheShed . I am sharing what I learned, in my own words, based off the research I did and then began implementing in my own grow. After dialing in the environmental numbers, my plants looked completely different in less than 24 hours. They were perkier, healthier looking, and just plain looked ready to burst with growth. All anecdotal, of course, but if you have the capability to manipulate your temps and RH, you need to try this!

In order to understand what VPD is, we need to define the variables that go into calculating the VPD. Water vapor is the gas form of water. Saturation Vapor Pressure, SVP, is the maximum amount of water that the air can hold at a given temperature.
The air inside a grow space can hold a given amount of water vapor relative to the grow space temperature before it condenses to the liquid form of water. As temperature increases the SVP rises and as temperature falls, so does the SVP.
This is why we get dew in the morning after a cool night. The SVP is low, because the temps are low, but the Actual Vapor Pressure ( AVP) is higher than the SVP and boom! We have dew all over everything as the water vapor turned to liquid form!

Did you know that Relative Humidity or RH is actually a comparison of the amount of water in the air relative to the maximum amount the air can hold? That's why we measure it in percentages. 50% RH means that at a given temperature, the air is holding 50% of its maximum amount of moisture possible.

So if Saturation Vapor Pressure, SVP, is the maximum amount of water that the air can hold at a given temperature, and we know Relative Humidity, we can determine the Vapor Pressure Deficit.

Put simply, Vapor Pressure Deficit is the difference, or deficit, between SVP and AVP

VPD= SVP - AVP.

But Archiweedies, I dont want to math! Fear not fellow growers, you only need a couple things to start dialing in that environment so that you can take full advantage of this technique.

What you need:​

Laser Temp Gun I got mine at Harbor Freight for quite cheap. We need this to determine our leaf temps at the top of the canopy. We can't assume that ambient is the same as leaf temps. Remember, the point is to get the stomata working so we care about the leaf temps at the top of the canopy.

IMG_5580[1].JPG

• Archiweedies
• Nov 16, 2018
• vpd

Laser temp gun

Hygrometer - You should have one already! It would be nice to have one with a probe that can be placed at canopy level to get accurate RH measurements.

VPD chart This chart is courtesy of Just4Growers.com There are many online and I suggest you find one you like and print it out for your grow room. The example below is a VPD chart that assumes a Leaf Temperature Deficit of -2 degrees Celsius below ambient.The chart you will need is going to be based on how hot your lights are and what the difference is between ambient grow room temps and leaf surface temps. Ive found, with my LED COBS that they run roughly 2 degrees Celsius cooler.

Now that we have our leaf temperature and our canopy RH, we simply cross reference the chart of your choice. If we are in the red zones above we are outside of the sweet spot for VPD. Once outside of the sweet spot and your plants are not transpiring as efficiently as they could be.

If you are outside the acceptable range, simply determine if it would be easier to manipulate the RH or temperatures to get back in line.

Example: I check my hygrometer and it says 25C. I check the leaf temps they are at 23C. RH is at 37%.
This places us just outside the VPD and it is up to you to determine if adjusting your RH up or your temps down makes more sense for you. For me, and this depends on your unique environment at the time which of course changes daily, I would bump RH up to 45% if I was in late veg/preflower. In flower I would likely choose to maintain the RH and lower temps a couple degrees Celsius to bring it back into line.

Summary​

Dialing in the VPD allows us to turbocharge plant growth by allowing the plant to transpire efficiently. When the plant is transpiring more efficiently, it is drinking more nutrients from the roots. I think that is the goal of growing cannabis. Grow roots in veg and use those roots during bloom to produce flowers. By giving the plant a happy environment with the perfect VPD balance, we accelerate the plants growth across all phases of growth.

Thank you to the wonderful community here at . I hope that by presenting this information, I have in some way given back to the community that has given me so much over the last few years.

NIce job on this tutorial. It's harder than I expected to keep in the zone but I try.

 

[Archiweedies]

Well wait, if you’re not spending money on growing gear and “too much” time in the tent... how is your wife supposed to be pissed off at you? Sage advice on the humidity controller, that’s missing from my current setup. Even still, after the input I’m about as dialed in as I can be, currently at 1.274 with ~1450PPM CO2. Air is being scrubbed, absolutely no exhaust and been stable most of the day. Think I’ve got the lights and CO2 dialed in to maintain this level. Weed plant is looking great (no new cal mag signs) and looks like even the pepper plants in the room decided to join the flowering party.

Appreciate the help getting on track with VPD, amazing how much you can learn in 12 hours.

I must not have seen your reply but thanks for the positive feedback. I’d love to see hear an update on how your plants are faring.

NIce job on this tutorial. It's harder than I expected to keep in the zone but I try.

Thanks Stoneotter! It can be quite the challenge to set and maintain your environmental factors. RH is especially tough for a lot of growers.

 

[Aroxol]

Let me answer that in a few days, I’m working through a nutrient challenge currently. In terms of VPD though, the plant has grown very very large buds and I’d say is mostly thriving.

 

[Archiweedies]

Let me answer that in a few days, I’m working through a nutrient challenge currently. In terms of VPD though, the plant has grown very very large buds and I’d say is mostly thriving.

No worries! Glad to hear things are going well. This time of year is quite challenging regarding humidity for most of us.
I’ll pop in your journal and cheer you on

 

[Gazoo]

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

 

[Pennywise]

Nice find Gazoo!

 

[Gazoo]

Nice find Gazoo!

Thanks, I should be able to put it to good use

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.

 

[Pennywise]

I’ll need to find lights cheap here pretty soon!

 

[InTheShed]

Did you leave all your grow stuff behind when you moved?

 

[bobrown14]

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