VPD (vapor pressure deficit)

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gorillaglueaaron

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Ok i was gonna sit down and write out a whole article but i have been lazy so instead instead I'm going to copy paste because im lazy. This is not my work so let start. You may find this very long but its worth the read. Some may want to skip ahead. I have added here and there to what i feel is missing but im going to skip alot of the more in depth stuff to keep this simple.

WHAT IS VPD:
VPD stands for Vapor Pressure Deficit. All gases have vapor pressures, but when we’re growing, we’re interested in water vapor. Vapor pressure deficit is the difference between the pressure of water vapor in 100% saturated air at a given temperature (basically a leaf’s vapor pressure) and the air’s actual vapor pressure. A high VPD value raises a plant’s transpiration rate and increases nutrient movement through the the plant’s xylem, while a low VPD value slows the movement of nutrients through the plant.

View attachment 910696

WHY SHOULD WE CARE:
VPD control is related to the following:
  1. Increasing or decreasing metabolic rate
  2. Improvement in yield quality
  3. Determining plant stresses
  4. Pathogenesis (more on this later)
  5. Carbon dioxide injection (more on this later)
Growers should care about VPD because it impacts yield quality, overall plant vigor, and nutrient utilization. Managing VPD lets me get away with using fewer nutrients, which improves my bottom line. I’m also seeing increased trichome production in the plants, which naturally follows better health.
You reach expert growing level when you learn to manage humidity and VPD. Everybody spends their time managing temperature, nutrients, and whatever else, but the last little thing you learn to manage is humidity, and it is significantly more finicky.

HOW TO STEER PLANT GROWTH:
In order to stay on the same page, I should point out that relative humidity (RH) andVPD are inversely related. This means that when relative humidity (RH) is high, VPD is low, and vice versa.
When growers want to know how to steer plant growth, they are interested in maximizing growth. So let’s start with how the plants react to changes in VPD:

  1. The bulk flow of water changes within a plant’s xylem as VPDchanges.
    1. If you have a high VPD, meaning that the RH is low, the plant will increase its transpiration rate and start pulling water faster from the substratein an effort to stay cool and moist.
      1. If the VPD is too high, the plant will become stressed, leading to inefficiencies.
    2. In the same vein, if the VPD is too low, meaning that the RH is high, the transpiration rate will decrease, slowing the flow of water through the plant.
  2. Nutrients follow the flow of water through the xylem and into its various tissues. Nutrientslike calcium primarily move with the bulk flow of water through the arteries of the plant.
    1. Therefore, as VPD rises (and the bulk flow of water increases), nutrient uptake will also rise.
    2. If VPD falls (and the bulk flow of water decreases), nutrient uptake will also fall.
  3. If you’re injecting carbon dioxide, you want the plants’ stomata to stay dilated for as long as possible. Ideally, the stomata would be fully dilated at all times to maximize carbon dioxide use. Plants open and close their stomatato regulate moisture loss.
    1. If you have a high VPD, or low RH, your plants are going to close their stomata to reduce water loss. When the stomata close, you’re not getting adequate gas exchange, and you’re not making the most of your carbon dioxide.
    2. If you have a low VPD, or high RH, plants will open their stomata and let in more carbon dioxide.


Effective VPD control is about balancing gas exchange. There is a “Goldilocks” zone where the plant is getting everything.
If your VPD is too low, then your plants aren’t going to acquire enough nutrients, slowing growth; if your VPD is too high, you’re going to stress the plant and the stomata will close, rendering your extra carbon dioxide ineffective. Like everything else with growing, there’s a Goldilocks zone. One VPD is too high, one VPD is too low, and one VPD is just right. There are charts of a VPD curve with a three way graph of humidity, temperature, and growth. There’s a sweet spot along the center of the chart.

vpd chart.jpg


DO NOT USE THIS CHART ITS REFERENCE ONLY
A VPD chart for a hypothetical plant, image courtesy of Argus Controls. The far left side of the graph is too wet for the plant,
and nutrient uptake is inhibited. The right of the graph is too dry for the plant and stomata close, inhibiting CO2 uptake.

VPD IN DIFFERNT STAGES OF GROWTH:

Ideal KPA (kilopascals) ranges for different stages of growth.
Seedling/clone 0.4-0.8
Veg 0.8-1.1
Early flower 1-1.4
Late flower 1.3-1.5
As a matter of fact, most growers use some form of VPD control already, without even knowing it. When you put clones under a dome, you’re keeping the RH high and the VPD low. This, in turn, slows transpiration to a crawl, greatly reducing the stress on the cuttings, which need time to form roots. Typically, most growers will keep their vegetativehumidity a little bit higher as well, which reduces stress.

Domes are a form of VPD control.
Most growers are concerned about the flowering cycle because that’s where the magic happens. You want to keep your VPD relatively high (low RH) during the flowering cycle. If you assume an average flowering cycle of 8 weeks, start with a moderate VPD (medium RH) during the first 3-4 weeks of your flowering cycle, then increase your VPD (lower your RH) towards the end of flowering. This reduces pathogenesis.

One thing you can do when a plant is stressed, say from moving from one room to another, is to raise the humidity. This lowers the transpirational stress and eases their transition into whatever phase or room you have set up. Additionally, HID lights can be stressful for plants, and VPD control gives you the ability to reduce their stress. If you have a dry environment and bright lights towards the top, you’ll see canopy leaves fold in like a taco. Plants do this to reduce light capture and reduce their internal temperature. If you see this happening, you need to ease up on the plants and reduce their stress.

CONCERNS WHEN USING VPD:
Pathogenesis is a big issue, which we’ve touched on briefly. The biggest drawback to running a low VPD (high RH) is that you can run into a lot of problems with pathogens if your rooms aren’t clean. As a result, many growers reduce their humidity as much as possible. Some growers brag that their humidity is as low as 20%, which is really bad for the plants and slows their growth.
Homogenizing a room’s environment is a struggle. In my experience, there are always new micro-environments forming in your room due to the nature of working with living organisms. Keeping on top of it all takes a lot of effort.

Good ventilation/circulation is necessary for VPD control.
Accurate sensor readings are also a problem I keep running into. Keeping the environment at your desired setpoint of temperature and humidity can be tricky. Having the right equipment and the right room layout can make a big difference.

WHAT EQUIPMENT DO YOU NEED TO EFFECTIVELY CONTROL VPD:
You’re going to need a humidifier for starters. You want to be able to inject humidity into the room without causing any problems such as being too close to one plant. If you have your humidifiers spraying plants directly with vapor, you will end up with undesirable microclimates which could favor pathogenesis. Personally, I think that ultrasonic humidifiers work best.

You are going to need a way to measure the leaf temperatures in order to accurately calculate VPD. This is where the online charts cause many growers problems and botrytisis becomes of real concern when not taking leaf temps into account. A simple $15 Infrared Temp gun will do the job quite well.

If you’re going to manage VPD, you’ll also want a controller that integrates your humidification and dehumidification systems. You want your controllers set up in such a way that when the lights are off, the humidification setpoints for the dehumidifiers are different if possible. An RH of 10-15% lower at night is ideal but not required.

Paying attention to RH after the lights go out is a big concern. As temperature drop the RH increases (ergo relative humidity) Slowing the temperature drop will aid in the dehumidifiers ability to keep the humidity in range. I would recommend checking humidity from 20-40 mins after lights out to ensure RH is not spiking.

Temperature are also important to control using a temp controller that controls both heating and cooling is ideal. This could be done by controlling fans, heaters, ac etc.

If you don't have all the fancy stuff listed you can still use VPD to to make adjustments to your setup that will improve the VPD for your stage of growth.

It’s important to note that plants are their own internal humidifiers, depending on how many plants are in a room and what stage of growth they’re at. Small plants have less surface area and transpire less. Small plants in a big room will require humidity injection to keep the humidity up, whereas plants at full size don’t need as much humidity injection because they’re already transpiring at an increased rate. When you hit the final stages of growth, you may have to run dehumidifiers to take water vapor out.

Realize that at this level you are doing some serious high performance fine-tuning of your gardening operation. You could be adding a few percent to the final weight of your yield, but it’s going to take some work and you are going to need the proper equipment to measure and control your garden at this level.

The fan system is required because we know botrytis and other fungi are always waiting to pounce. Botrytis establishes itself best between 50 and 70°F, in still air having humidity above 55%RH. We especially want to avoid condensation; this means watch out for uncontrolled temperature drops between daytime and night.

You will also need some type of computer system capable of running a modern spreadsheet program. This is not rocket surgery, but you (or someone you know) will need to know how to use some basic features of a spreadsheet. This is useful to display the logged files from a data acquisition setup, as well as for calculating VPDs and other moisture quantities. Consider it the entry stakes to quantifying and visualizing the performance of your growing operation.



HOW TO CALCULATE YOUR OWN VPD:
If you don't like math your in luck here is a formula you can put into a spreadsheet to do it for you. I use excel personally.

Enter the formula on the next line into spreadsheet cell A10 (copy and paste it).

=3.386*(EXP(17.863-9621/(A7+460))-((A6/100)*EXP(17.863-9621/(A5+460))))

You will type-in 3 values into 3 other cells:

  • Cell A5: The air temperature (A5 in the formula)
  • Cell A6: The air %RH (A6 in the formula)
  • Cell A7: The leaf temperature (A7 in the formula)
Cell A10 will then give you the total VPD for that grow room condition.

Example:

Room temperature= 80°F

Room %RH= 47%

Assumed leaf temperature= 75°F

VPD= 1.34 kPa (a little too dry for best growth)

Calculating Individual Vapor Pressures

For those interested in further exploring water vapor pressure.

Enter the formula on the next line into spreadsheet cell A20 (copy and paste it).

=3.386*(A17/100)*EXP(17.863-9621/(A16+460)))

You will type-in 2 values into 2 other cells:

  • Cell A16: The air temperature (A16 in the formula)
  • Cell A17: The air %RH (A17 in the formula)
Cell A20 will then give you the water vapor pressure for that temperature and %RH combination.

Examples:

1)

Room air temperature= 80°F

Room air %RH= 47%

Water vapor pressure= 1.67 kPa

2)

Leaf temperature= 75°F

%RH of the air inside the leaf = 100%

Water vapor pressure= 3.00 kPa

These 2 examples show the “long way” to calculate the VPD given in the VPD equation section above this one: Subtract the room condition from the leaf condition to come up with the room-to-leaf water vapor pressure deficit (3.00 – 1.67 = 1.33 kPa).

Ok well that the long/short version and I hope this helps. If ya have any questions I will do my best to answer
This was really helpful, thanks.
 
StealthLED

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I was going to put this in this vpd thread last week but decided not to open a tin of worms but here I am oh well here goes....😂 I'm just one guy doing a test in his own room I'm not trying to say there's a wrong or right way I'm just sharing what I've found...

I did some testing on vpd with atmosphere co2 levels and did some experimenting with different temps and humidity to see how it would effect co2 in my room, my exhaust fan is always set to a low setting so its slowly changing the air the room.
I used the vpd calculator for 85.5 room temps & 80 leaf temps to get my humidity, I was aiming for a vpd of 1 kpa so humidity should be 59% at this level lowest co2 would drop to was 410ppm (outside co2 levels)

Now when I set my humidity ONLY using leaf temps to get 1kpa I needed a humidity of 69% but when using the room/leaf temps calculations for this humidity it's 0.6kpa (ideal for clones and seeds) at these levels the co2 in my room drops to 365ppm so I believe at those levels the stomatas are fully open and consuming the co2 faster in the room compared to a 59% humidity, if only using leaf temps at 59% that gives you a vpd of 1.4kpa and I believe the plants stomatas are not fully open to stop themselves from drying out to quickly so co2 isn't being used as fast. Over a week I tried a few different room temperatures so of course this changed the leaf temperatures, I tried a few different humidity levels but I couldn't get the co2 down to the 365ppm levels unless my leaf temps was around 80 and my humidity swing was 66-69%

Don't want to start a debate just sharing what happened in my room 🤟🏼
 
IMG 20210114 160533
StealthLED

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Ignore the bottom kpa that's only reading room and humidity to get the kpa calculation
 
Aqua Man

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I was going to put this in this vpd thread last week but decided not to open a tin of worms but here I am oh well here goes....😂 I'm just one guy doing a test in his own room I'm not trying to say there's a wrong or right way I'm just sharing what I've found...

I did some testing on vpd with atmosphere co2 levels and did some experimenting with different temps and humidity to see how it would effect co2 in my room, my exhaust fan is always set to a low setting so its slowly changing the air the room.
I used the vpd calculator for 85.5 room temps & 80 leaf temps to get my humidity, I was aiming for a vpd of 1 kpa so humidity should be 59% at this level lowest co2 would drop to was 410ppm (outside co2 levels)

Now when I set my humidity ONLY using leaf temps to get 1kpa I needed a humidity of 69% but when using the room/leaf temps calculations for this humidity it's 0.6kpa (ideal for clones and seeds) at these levels the co2 in my room drops to 365ppm so I believe at those levels the stomatas are fully open and consuming the co2 faster in the room compared to a 59% humidity, if only using leaf temps at 59% that gives you a vpd of 1.4kpa and I believe the plants stomatas are not fully open to stop themselves from drying out to quickly so co2 isn't being used as fast. Over a week I tried a few different room temperatures so of course this changed the leaf temperatures, I tried a few different humidity levels but I couldn't get the co2 down to the 365ppm levels unless my leaf temps was around 80 and my humidity swing was 66-69%

Don't want to start a debate just sharing what happened in my room 🤟🏼
Not sure what you mean only using leaf temp and only using room temp? There is only 1 way to calculate imo and thats using both.

There are so many different variables that need to be considered that change over a week. The total leaf surface area. The photosynthetic rates and co2 demands change based on the photosynthetic rates which is affected by many thing, including light intensity that changes daily as growth occurs and the total leaf surface area changes daily with growth. Its something really hard to calculate unless you are giving the room an exact ppm of co2 then cut it off to see how much they consume.
 
MIMedGrower

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Not sure what you mean only using leaf temp and only using room temp? There is only 1 way to calculate imo and thats using both.

There are so many different variables that need to be considered that change over a week. The total leaf surface area. The photosynthetic rates and co2 demands change based on the photosynthetic rates which is affected by many thing, including light intensity that changes daily as growth occurs and the total leaf surface area changes daily with growth. Its something really hard to calculate unless you are giving the room an exact ppm of co2 then cut it off to see how much they consume.



what about the variable where the plant doesnt follow vpd and can adapt to a wide range of humidity?

:-)
 
StealthLED

StealthLED

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First calculate was done with leaf and room temps to give a humidity of 59% for 1kpa. When only using leaf temperature to get 1kpa this gives me a humidity of 69%. Nothing else changed in the room only temps and humidity, I understand what you mean with plants growing closer to lights etc but I did the tests twice on different days and always got the same results.

I've spoken to a few different people about vpd and most have different ways to calculate their 'ideal' vpd, some use both room and leaf, some just room and some just leaf. Two people have said to me that we should only care about the plants vpd so that's why I did this just to see if there was any difference and in my room there was. Now will this make any difference in my small room, I doubt it but just wanted to share 👍🏼
 
Aqua Man

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what about the variable where the plant doesnt follow vpd and can adapt to a wide range of humidity?

:-)
Sure it can within reason... but that doesn't mean there is no benefit to being closer to ideal. They can absolutely grow in a wide range of humidity. But ime you will see issues if pushing them hard with high temps and high light. Honestly if you put a plant in low to moderate light almost nothing bothers it in respect to humidity.
 
Aqua Man

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First calculate was done with leaf and room temps to give a humidity of 59% for 1kpa. When only using leaf temperature to get 1kpa this gives me a humidity of 69%. Nothing else changed in the room only temps and humidity, I understand what you mean with plants growing closer to lights etc but I did the tests twice on different days and always got the same results.

I've spoken to a few different people about vpd and most have different ways to calculate their 'ideal' vpd, some use both room and leaf, some just room and some just leaf. Two people have said to me that we should only care about the plants vpd so that's why I did this just to see if there was any difference and in my room there was. Now will this make any difference in my small room, I doubt it but just wanted to share 👍🏼
I cant grasp this... humidity of the plant is 100% humidity outside the plant is what controls in large part the amount of water pulled through the plant. Leaf temps are what is important.... its the temp of the plant thats the key. But also there is a difference in how much water is in the air between 70f and 80f room temp at 50% RH. RH is not a direct correlation of the amount of moisture in the air... its simply the % of moisture that the air can hold at that temp.

I'm really confused how there are different ways to calculate it because there really isn't.

Basically the KPA is not changing there is only one way to calculate it accurately. What is changing the the plants preferred KPA and this can vary alot even between stains. VPD is a guide but indicas will like it drier and sativas will like it more humid so im not sure what you mean
 
StealthLED

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A lad I was speaking to that does all that crop stearing stuff and uses a meter to check stomata openings I can't share it because it's voice recorderings said this "you should only be basing it of leaf temperature and not leaf and room temps because this is how you're telling if stomata are opening or closing, if you're going off a metric that's not actually on the plant ie room temps then you're not physically measuring what's going on, on the plant"

Also carft farmer on IG he uses leds room temps 85 humidity 75% if you put that into a vpd calculator using room temps of 85 and leaf temps say 82 that gives you a kpa of 0.5 and he uses that till about week 5/6 flower and his plants look amazing but if only using leaf temps that's around 0.8kpa. Surely at 0.5kpa plants that big wouldn't drink much and would look shit lol
 
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Screenshot 20210125 005013 comgoogleandroidappsdocseditorssheets
Aqua Man

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A lad I was speaking to that does all that crop stearing stuff and uses a meter to check stomata openings I can't share it because it's voice recorderings said this "you should only be basing it of leaf temperature and not leaf and room temps because this is how you're telling if stomata are opening or closing, if you're going off a metric that's not actually on the plant ie room temps then you're not physically measuring what's going on, on the plant"

Also carft farmer on IG he uses leds room temps 85 humidity 75% if you put that into a vpd calculator using room temps of 85 and leaf temps say 82 that gives you a kpa of 0.5 and he uses that till about week 5/6 flower and his plants look amazing but if only using leaf temps that's around 0.8kpa. Surely at 0.5kpa plants that big wouldn't drink much and would look shit lol
You can just change the math to get the answer you want... thats absolutely ridiculous. If they grow well at that kpa then they grow well at that kpa.

Like you can just say well that kpa is to low so I will change hkw it should be calculated to make more sense to what the number should be.
 
Aqua Man

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I mean I'm not trying to be rude but I'm not really keen on taking math lessons from someone who says if you respect my hussle and grind. Im not saying he can't grow plants but he sure in the hell doesn't get math.
 
MIMedGrower

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I mean I'm not trying to be rude but I'm not really keen on taking math lessons from someone who says if you respect my hussle and grind. Im not saying he can't grow plants but he sure in the hell doesn't get math.


He is inadvertently proving vpd doesnt matter. Lol.
 
Aqua Man

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Thinking about this im sure he has to be saying what I am... you must use leaf temps... if you don't you are not taking the plant into account.
 
StealthLED

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Like I said from what I did in my room only using leaf temps to set humidity to give me 1kpa gives me the most stomata openings which I believe shows from more co2 being used and if anyone else is interested to see if it shows similar results for them no harm in trying. Really this is like using one of those vpd charts but instead of using the temperature down the side for room temp surpose it would be leaf temp, I know the 'proper' way to calculate vpd is leaf, room and humidity but I believe just leaf temps work better in my room.
 
FourthCity

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A lad I was speaking to that does all that crop stearing stuff and uses a meter to check stomata openings I can't share it because it's voice recorderings said this "you should only be basing it of leaf temperature and not leaf and room temps because this is how you're telling if stomata are opening or closing, if you're going off a metric that's not actually on the plant ie room temps then you're not physically measuring what's going on, on the plant"

Also carft farmer on IG he uses leds room temps 85 humidity 75% if you put that into a vpd calculator using room temps of 85 and leaf temps say 82 that gives you a kpa of 0.5 and he uses that till about week 5/6 flower and his plants look amazing but if only using leaf temps that's around 0.8kpa. Surely at 0.5kpa plants that big wouldn't drink much and would look shit lol
Thinking about this im sure he has to be saying what I am... you must use leaf temps... if you don't you are not taking the plant into account.
Lol you gonna start flowering your plants in 80% humidity to prove that?
Im thinking with those high temps and excessive humidity the leaf temps would be higher than ambient which would quickly bring the vpd into a more sensible (higher) range for flowering. That being said, I agree with aqua man about leaf temps being essential when measuring and discussing vpd. I also agree that having the humidity at 72-82% is in and of itself outside of the ideal humidity range for flowering and even pretty high for vegging regardless of the vpd.

It's great you are using the calculator I made but I only works if you have really measured the inputs.
 
Aqua Man

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Like I said from what I did in my room only using leaf temps to set humidity to give me 1kpa gives me the most stomata openings which I believe shows from more co2 being used and if anyone else is interested to see if it shows similar results for them no harm in trying. Really this is like using one of those vpd charts but instead of using the temperature down the side for room temp surpose it would be leaf temp, I know the 'proper' way to calculate vpd is leaf, room and humidity but I believe just leaf temps work better in my room.
Hey I dont doubt you one bit.... but you can't just change a formula to give an arbitrary value that you want. Now with that said I agree fully that you may be getting wider stomatal openings with a lower kpa than what is in the guide. But this also plays a role in transport of water and nutrients. If you notice he is running high ppm. imo this is because he is running a low kpa and thus less nutrient uptake.

I'm not by any means here disputing what you are saying... all I am saying is you don't just change a formula to give you a number that sounds better. When calculated to find the pressure difference between the leaf and the room its only done one way
 
Moe.Red

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I got directed here from another thread and read it thru. Good refresher.

There is more than one way to grow, and if you are trying to reach the potential of the plant, VPD should be observed. You can obviously get good results without it, but IMO, better with. It's like Usain Bolt who is perfectly adapted to running in the mountains, but when you bring him down to sea levels his performance improves. I get that to some people Usain in the mountains is good enough. Since this is pretty much my only hobby, I'd like to dial it in as much as possible.

I'm trying to figure out how I can control my dehumidifiers programmatically. They are the kind that you set to a RH set point and leave it alone. Removing power will reset them and they will not turn back on correctly, so that is a no.

I may need to come up with a dumb humidifier that can be externally controlled. Got any other ideas?
 
Aqua Man

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I got directed here from another thread and read it thru. Good refresher.

There is more than one way to grow, and if you are trying to reach the potential of the plant, VPD should be observed. You can obviously get good results without it, but IMO, better with. It's like Usain Bolt who is perfectly adapted to running in the mountains, but when you bring him down to sea levels his performance improves. I get that to some people Usain in the mountains is good enough. Since this is pretty much my only hobby, I'd like to dial it in as much as possible.

I'm trying to figure out how I can control my dehumidifiers programmatically. They are the kind that you set to a RH set point and leave it alone. Removing power will reset them and they will not turn back on correctly, so that is a no.

I may need to come up with a dumb humidifier that can be externally controlled. Got any other ideas?
I always make sure I use dehuey, AC, heaters etc that are analog or have an autostart function... this way I can use my controllers for everything... probably a way to direct wire them but I'm not the guy to ask about that.
 
FourthCity

FourthCity

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I got directed here from another thread and read it thru. Good refresher.

There is more than one way to grow, and if you are trying to reach the potential of the plant, VPD should be observed. You can obviously get good results without it, but IMO, better with. It's like Usain Bolt who is perfectly adapted to running in the mountains, but when you bring him down to sea levels his performance improves. I get that to some people Usain in the mountains is good enough. Since this is pretty much my only hobby, I'd like to dial it in as much as possible.

I'm trying to figure out how I can control my dehumidifiers programmatically. They are the kind that you set to a RH set point and leave it alone. Removing power will reset them and they will not turn back on correctly, so that is a no.

I may need to come up with a dumb humidifier that can be externally controlled. Got any other ideas?
It sounds like your dehumidifiers are already programable enough by being able to set them to a desired rh with their built in humidistat. The plug in humidistats that most growers use are only needed if you have analog dehumidifiers and or humidifiers that do not have or display the rh setting.

Try to see how much you can dial things in with your current setup. You have three VPD variables to work with, if your ability to change one is limited you can focus on the other two.

How humid will your grow area get without the dehumidifier? If RH gets into the 60%'s without it and while running it can dehumidify the grow area into the high 40%'s then adjusting temps in the 70's while modifying light intensity (by height and or power) for correct leaf temps should give you all the control you need from veg to flower with what you have.

For example, in veg you could set the dehumidifier to 64% and could probably expect an actual room rh of 60-68%, keep the temps between 70-78f and as long as the leaves are staying a bit cooler than the room temps your vpd should be good.
 

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