Humidity RH Vs VPD

Indoor Growing
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So I’ve never seen a comparison between growing using standard temp and humidity controls vs VPD. What are the pros and cons? Heat and RH seems straight forward… Set what you want and done. But some swear by VPD yet never explain how it differs as an environmental control or if it’s better.

What are the pros and cons and what is your experience with both?

It seems VPD varies based on the leaves and plants position in the room and it’s height etc so it’s hard to really get a actual reading when it varies, adding to it is the probes position… With RH and heat it’s the same thing… It varies based on where the probe is but you don’t have to worry about the leaf offset because there is none.

I’d love to learn about this topic as I have a AC Infinity grow tent and have two options and so far I’ve only used RH and temp but with a little work I can change my settings over to VPD instead if it’s worth the hassle.

J

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VPD is a good way of tracking because it uses absolute humidity rather than relative humidity, which changes with temperature.

Basically VPD considers both of those things, RH and temp, and if you’re really doing it, you’re going by leaf temps and trying to see how those compare to air temps, to see how the plant is transpiring and thus the rate it’s eating at.

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I don’t like VPD because it hurts my brain and I can’t control my environment too well anyway.

I often look at my RH gauges scattered about my tent but I rarely act on this information.

Heat at canopy level is one thing but burning up your rootballs is quite another.

I often think “would I be comfortable here?” And if the answer is yes then so are my plants.

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Is there an easy way to know what vpd number is the ideal goal? I have no idea what vpd I’m aiming for… But I do know the humidity n temp I aim for… 75-80f lights on and 60-65f lights off. Rh 65% on, 55%off. Veg state.

How would I turn that info into vpd? Ac infinity just asks for a simple leaf temp which it then offsets with whatever formula is used. But I don’t know what vpd I’m looking to keep

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My plants are b*tches, they are never happy for too long lol. I made the mistake of getting 6 strains at once ,which turned to 3 after all my learning disasters… 3 of which are now stunted but the experiment continues!

I know they want more water with my aeroponic setup but I can’t until I get better sprayers with better atomization otherwise root rot spawns up quickly.

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I’m with you on this one

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In my experience that is a sure fire guarantee that there will always be an unhappy plant… :laughing:

VPD is ‘The Goldilocks zone’ and it sure isn’t one size fits all either… I use it as a reference.
The pros get more serious about it but they have the bucks to invest into pricey environmental control equipment. (I use a $40 humidifier… :grin:)

Cheers
G

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The Build A Soil website has some good resources regarding VPD and DLi etc.
They also have a lot content on YouTube which explains VPD, definitely worth checking out.

:v:

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Hi divine_n. I just wanted to point out, I learn best from my mistakes and it sounds like you’re paying attention to any mistake, you make and taking a good lesson from it. So good on you

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Lol, yea, In my newly acquired exp too :sweat_smile:

I was originally planning to use soil but the engineer in me said “nah, too easy… Hydro… Nah too easy, go aero… Go big or go home”

But is it better than rh + heat? Or the same? I think I have a pretty good system. ac Infinity automation seems pretty good. As far as humidity… I had a fogger and it was useless once the summer heat kicked in and the a/c was turned on.

I created my own system for humidity at high pressure and it works really well. Just a 1-2s spray and I hit RH. VS having the fogger at max 24/7 and still only hitting 30-40% rh.

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I’ll look for it, thanks.

DLi? Hmm, another method? I’ve heard the explanations but what I want is comparison and a pros and cons over just using simple rh + heat… I know it CAN theoretically be better but if I don’t really know what I’m aiming for and need an avg VPD sample… I’m not sure if it’s worth the hassle of reconfiguring my automation to it or not.

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I learn from my mistakes and others mistakes, it saves time (when the mistake fully applies anyway, otherwise I take it w/ a grain of salt)

Yea, HP Aero is complex and plants seem to have a magnitude of complaining that are often contradicted by other symptoms, it just becomes a guessing game and making assumptions and taking notes on results… Like a scientist… Fun stuff.

I find this forum so much more helpful than most other forums I’ve been for cars, tech, growing etc. People here know and share and are friendly and willing to help… On other places (*cough, fb) all I got was insults and pessimistic chants to give up… Such sad people on there.

Anyway, thank you for the positivity and thanks to everyone here!

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I use VPD via the AC Infinity controls with the target ranges in the chart below. Plants seem to like it and it sets an environment goal for each stage of growth.

Just thought I’d post this as a reference for this thread.

image
image1398×1049 161 KB

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If you have control of your environment this is what I use.

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I just use RH and temp and try to match it generally to what most of the VPD charts say. They’re all slightly different if you look closely, because they’re all based off various people’s ideas of what conditions plants do best under… and all of them neglect a variable too. You can’t calculate VPD without knowing the base barometric pressure for your elevation. Most of them use sea level, which is close enough for most people, but it’s never gonna be precise unless you’ve got expensive measurement devices. Lots more expensive than the Inkbirds most of us use… and even they’ve doubled in price lately. :roll_eyes: Anyone doing more work than that without spending more money is wasting effort, IMO.

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An excellent point that I don’t see made often!

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Is it really a point or just a parameter that drops out since the thing we are looking at is the difference between the environment and that just above the leaf? Really that ‘base’ is the same for both.

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But that base is different for everyone! Leaf temp differentials matter but the difference between 70/75F and 80/85F is going to matter too.

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Its value is independent of tempera-
ture.VPD is the difference between the
amount of moisture in the air and how much
moisture the air could potentially hold when
it’s saturated. It’s often measured in pounds per
square inch (psi) or kilopascal (kPa). A high
VPD (greater than 1.0 kPa) means that the air
can still hold a large amount of water. There-
fore, there’s a fairly large gradient between
plants (nearly saturated with water) and the
air, enabling the plants to transpire and, over
time, dry out. A low VPD indicates the air is
near saturation. A VPD of zero means the air is
100% saturated and thus plants cannot tran-
spire effectively.

While the amount of moisture air can hold is relative to altitude, it is also relative to current humidity. If they assert it is independent of temperature, would also be independent of altitude. Sure you could quote 15,000 feet pressure levels but then lots of places on earth below freezing as well and vpd doesn’t really matter if the plant can’t live.

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Not sure I know what to make of this. It seems to contradict all the charts going around; and the assertion that VPD is independent of temperature but relative to the amount of moisture the air can hold, when the amount of moisture the air can hold is relative to temperature, seems like splitting hairs. At that point, it’s simpler to just say it’s relative to both temperature and atmospheric pressure, since both are variables in calculating the amount of moisture the air can hold.

According to their measurements:
When temperature is increased by 15F increments and relative humidity stays the same, vapor pressure deficit varies from 0.55 kPa to 1.45 kPa.
60F 70% 0.55 kPa
75F 70% 0.90 kPa
90F 70% 1.45 kPa

When temperature is increased by 15F increments and vapor pressure deficit stays the same, the relative humidity varies from approximately 22% to 71%.
60F 22% 1.38 kPa
75F 41% 1.38 kPa
90F 71% 1.38 kPa

Comparing this to the chart @GrouchyOldMan posted, every single value varies at least slightly. 60F seems like it would be close to the values presented, but 60F isn’t on the chart; 75F/41% comes up between 1.62 and 1.47, closer to the former, while 75F/70% comes up at 0.72; 90F/71% comes up between 1.16 and .92, again closer to the former, while 90F/70% comes up at 1.16 exactly.

Cannabis VPD chart

This chart is calculating leaf temp as being an average of 5F lower than the room; technically, we can’t calculate VPD without knowing that as well, since I forgot to mention it earlier. Comparing to this chart, 65F/22% looks like it would come up between 1.51 and 1.31, while 65F/70% would be around 0.32; 80F/41% would be between 1.59 and 1.33, while 80F/70% comes up at 0.50; 95F/71% a bit off from 0.87 while 95F/70% is 0.87 on the nose.

Again, every single value varies at least slightly from chart to chart and publication to publication. The variables that go into calculating the maximum amount of moisture the air can hold are all listed on each, except base atmospheric pressure. This latest one that you linked seems to be aimed at introducing the idea of VPD to growers in the first place, not at precise calculations, that’s all. I don’t think they’re intentionally trying to obfuscate the process.

The water-holding capacity of air approximately doubles with every 20F-degree increase in temperature.Therefore,air that is at 80F (26C) can hold twice the amount of water compared with 60F (15C).

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