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How important is VPD if temp & humidity are on target?

OrganicCanna420

OrganicCanna420

New member
I am in week 3 of flowering.
I maintain around 75°F and 45% humidity.
Can I ignore a VPD over 1.1 and high as 1.8?
I'm using an Infinity Controller that does the VPD math for me.
The plants are all doing great and seem pretty happy. I don't know if I'm paying too much attention to the technology, instead of just looking at the plants to see if they're happy.​
 

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My current grow is doing well taking care of the humidity on it's own. Rather strange actually. 77F and 68RH is what it ends up running. Typically just runs off high temp control. Humidity start out high and drops after about two hours. It needs defoliated but I know that will screw it up. I usually target 1.10-1.05 VPD.
 
VPD is a function of air temps, leaf surface temps and humidity.
A VPD above 1.6kPa is considered risky by some and I wouldn't be exceeding that until the last 2 weeks of flower to keep mold away unless my plants were showing signs of increased risk earlier on.
Some strains can quite easily support mold above 40%RH although a lot of growers reckon under 50% and you should be right, but I know that is not always the case.
Above 1.6kPa is considered over transpiration. Nute burn is typical and so is nute lockout along with disease.
 
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I believe in VPD. It's just that many hygrometers like I have +/- 3% accuracy and that makes a significant difference in VPD. They're made for a "do you feel comfortable or not" reading and I think that's about as accurate as they can do for my plants too. I averaged seven meter readings and marked the middle one as "correct" and wrote the offset on the others. Did the same for temp. Now they are named thinks like "T-1 H+6" and "T+0 H+4." None of this gives confidence for calculating an accurate VPD. Since your machine calculated the VPD for you you might not see how a little difference (or error in reading T or RH) moves the VPD value. My point is that VPD matters but being super accurate at hitting a constant-perfect value doesn't matter as much. Because most of us can't given our equipment. (For example Inkbird hygrometer is RH +/- 3%.) So I use VPD but only shoot for keeping my tent RH from wandering into the stress values. And recently I dropped my room temp 2F to make the VPD better because the light intensity was increased. This may be a lot of fiddling around but I think it reduces my need to over water and over-nute my plants :)

But a VPD of 1.8 is too high. It's good that you check and if it's that high for hours on hours you should try to keep it lower.
 
I appreciate what you're saying 90Gizmo. VPD matters. I found the Ink Bird equipment is good but the margin for error is pretty wide. Same for temps. Same for every other hygrometer I've invested in. It's kind of a guess. Because of that I settled on the AC Infinity equipment and don't bother with trying to get things dialed in perfectly. This is the first grow with that mind set so we'll see how it goes.
 
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With the AC Infinity app there are transition settings for temp and humidity under settings>port. If you play with these and then adjust the targets accordingly the fan will run at a near constant speed instead of spiking on and off. There is also a VPD leaf temp off set that really throws the numbers off. Both temp and humidity can be offset for calibration.

Lights off I have a 75% RH limit.
 
I wish my controllers were that smart. I've added a second humidifier (one in tent, one in room) and still haven't triggered my big duct fan yet. My slowest duct fan setting is too fast. One of the three tent fans blows on the carbon filter and that's all that brings fresh air into the tent until the vent fan triggers. (Though there's plenty of circulation on the inside.)I'd probably be scrambling to hold settings if the tent were full or the plant were larger. This is pure learning period for me but so far I think winter growing is easier, adding humidity, than summer growing when my house is a swamp. Thanks for telling me about the ACI app. That's the final piece of what I'm looking for (proportional control versus simple on-off) so I'm convinced now to go for an AC Infinity controller. Now I have to ask what's the cheapest controller that works with the app because I'm cheap and don't need duplicated smarts.
 
AC infinity controller 67 is for one device and the Controller 69 is for more than one. You can buy them with eh fans or look on ebay. I think the 67 is being phased out as it is also just bluethooth. Ebay has them for half price on used.
 
Thanks to everybody for all your experience and well thought out reasoning.

Everybody answered my question!
What's cool is that everybody has their own little tweak that works for them.

I'm using the Infinity controller 69. It came with the duct fan and I really love it. When you link it to the app you have 24-hour graph of temp, humidity and VPD.
I also move the probe around to different parts of the tent. Once each day to see if there's any pockets of weirdness. My tent is only 2x4 so I have just one 6-in oscillating fan which really seems to circulate everything around quite well. And I don't use the triggers that turn the exhaust fan on and off. I leave it at a constant setting depending on circumstances.

I'm with golfnrl where I'm watching and adjusting temp/humidity and trying to keep VPD within limits.

I tried to do the adjustments for temperature in relative humidity etc on the controller. I'm sure it's great, but that was above my pay grade.​
 
My room is automated re climate.
The networked hydrometers are crap when it comes to accuracy but can be calibrated +/-9%. I have 2 calibrated and accurate hydrometers +/- 1% sitting next to each other and calibrate my networked ones to those every week.
Getting a perfect climate for maximum yields is pretty easy because it is quite a large window. The closer you can get it to stable the easier it is to control other factors such as nute uptake etc and with the climate variable now a constant everything becomes heaps easier to manage. That's my experience
 
So controller 69 has an outlet plug accessory that I suppose can turn the humidifier on or off too. My current controller does humidity in a similar way. It's accurate enough as long as the exhaust fan is not turned on. But it doesn't calculate VPD so I manually change numbers (or the room thermostat) to get the VPD I want. Automatic will be really nice. But then again won't I still have to look at growth-stage charts to see what the appropriate VPD should be... I have to think about this some more.
Screenshot_20230316-221921_Tuya Smart.jpg
 
If you can maintain daytime temps between 25 and 29⁰C..night time temps 3 to 4⁰C lower and humidity in the 45 to 55 during flower and 55 to 65 in veg you will be right. Just don't go for opposite end extremes...eg 29⁰C and 40%RH... if you increase temps allow humidity to climb..that will even out the rate the plant transpires (VPD)
 
AC Infinity settings>port transition temperatures and humidity. This I set at +2F,+2%. then you need to lower your targets about the same amount. This will keep the fans on constant and slow. Adjust the temperature as the hotter it is the more water the air can hold and that will lower your RH as the temp rises.

There is no way I could ever get below 70% when the lights are off without a dehumidifier. Summer time I set it at 80% and winter 75%. And when it's cool if you try to lower the humidity too much then it just runs fast never reaching the set points and temp dropping to near ambient. Same thing with heat if you have too much. You will dry the air out way below the optimal VPD. Better to reduce the wattage or raise the set point.
 
Here's my two cents...

Growers often get excited about chasing numbers like VPD, PPFD, EC, and a host of other acronyms. These are all very important and can be very helpful and effective tools/measurements....if you have everything else dialed in. And guess what? If you have temps and RH dialed in, you're probably within an acceptable VPD. Temp/RH and VPD are all connected with each other and Temp/RH are what create the VPD.

So, my advice is to focus on temp and RH first and foremost. If that keeps your plants happy and growing healthy, then pat yourself on the back since you've done 90% of what you need to do! If your plants are happy and healthy, then feel free to try dialing in a VPD if you feel like it. But then again, if your plants are happy and healthy, why fiddle with things - if it's not broke, don't fix it! If your plants aren't happy, you need to start with the basics - back to temp and RH...and ventilation, circulation, watering regime, lighting, and nutrients as opposed to concerning yourself with VPD.

And don't forget, VPD is based on LEAF temps, not air temps. So to measure an accurate VPD, you need to have a way to accurately measure the leaf temperature, otherwise your VPD number that you get will be inaccurate and unhelpful at best and detrimental at worst.

Basically, all the fancy calculus is helpful as long as you have everything else under control. Many growers will never need to focus on things like VPD because they have enough work to do managing temps and humidity in a homegrow environment. Managing VPD is more of a graduate-level activity. But before focusing on things like that, one should have the basics mastered with an understanding of how changing temps affect relative humidity, etc.
 
I don't disagree about chasing numbers. I can go down that rabbit hole. I think VPD is important but I use a free VPD chart where the only input is the Temp and RH. It's really just a reference point. Keeping the numbers in the green on the chart seems to be OK. Others on this forum have given the guidance that the best way to learn how to pharm is to watch the plants themselves. I'm still learning to do that in my grow system. I really like what you're saying about the basics. Spot on!
 
golfnrl I do the same, I look at the free chart. It seems like the controller 69 would do that for me though. I'd use leaf temp if I had the stuff right now. because all my leaves are only an inch away from the dirt in my own version of chopstick torture training. Not typical.
 
How are you going to measure leaf temp? I have a non contact IR thermometer and there is seldom any difference as long as there is air circulation.

If you have the AC infinity controller toggle the leaf temp +2F and it makes a drastic change in what you need to do. It's defiantly not needed to get a plant through it's grow as some grow in super dry climates. Just the daily swing outside kind of blows perfect VPD out the window. Here in the summer it would go from 100% at night to 35% in the day maybe or 70% all day .Or something in between.
 
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