Some questions regarding Vapor Pressure Deficit vpd

Hey all, so I’ve been working on dialing in my vpd. I’ve been trying for a few years with little success. Maintaining proper vpd requires a strong humidifier, not some cute one you buy off Amazon, but they’re cheap and what I’ve been working with. I just made a home made humidifier that has amazing output making it unbelievably easy to dial in. My questions are, should vpd be constant day and night? If so the humidity will need to be adjusted based on how much the drop in temp affects it at lights out and vice versa? That way I can keep vpd anywhere I want. Based off that, should vpd raise consistently from 1.2-1.6 throughout veg? Or I can keep my humidity the same day and night (lowering slightly through flowering) keeping the vpd within 1.2-1.6 swinging as the day night temp change? I hope someone understands this and can help!

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The target numbers are Leaf VPD numbers.
Are you also tracking your leaf temps to know what your leaf temp offset is? The offset is your surrounding air temp subtracted from the leaf temp, & the offset is used when selecting which chart to go with, if you are using charts.

Or just plug in the leaf temp, air temp, & RH into a calculator to get the Leaf VPD:

Here is a good overall guide which also talks about day & night VPD.

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I’m not currently taking measurements for leaf temps, that’s my next step. If I can’t dial in temp and humidity, there’s no point to add that to the equation yet. My hygrometer calculates vpd, so once that’s good l, I can move my lights. With that, if your lights are at the proper height from the canopy, then another part of the equation needs to adjust. I’m just not sure on how the leaf temp affects the end value. Like temp down=vpd up, humidity up=vpd down. Also unsure if you maintain the same VOD through day and night.

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A device would still need to have the correct leaf temperature offset programmed in, or else the reading will be off. Same as if you pick a chart generated with the incorrect leaf temp offset, the numbers in the chart will not be where they should be.

I have my inkbird humidity controller and depending on time of year also my temperature controller(unless it’s below 60f) connected to my light timer. I keep my 6" AC Infinity exhaust running between 3-5 setting 24/7 day. This controls the extra humidity build up at night when temps drop as well.

You can take leaf temperature with a laser temp gun:

The setting for getting the correct temp for plants is 0.94 :

edited to add tag to link

The Emissivity values are really helpful, thanks. I have a 4” online to carbon filter on top of my tent constantly exhausting like you. I can’t keep things stable without, but I could not compensate for the lack of humidity, the new setup fixed that. Then I have a 4” pulling in fresh outdoor air kicking on at a set temp, it bounces around a degree when up to temp. I had a day/night fan speed controller that died on me, that thing was awesome. I’m trying to figure out a way to make a fan speed controller for my humidifier, but you don’t see any of those for humidity, just temp. You have an ac, do you have a controller 69 (pro) I’m curious about that system and wondering if it would be a solution. Can a fan plugged into the controller be set to add humidity? I’d assume you could program a fan for less humidity

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Curious to know where your offset is currently set to. Can you post an air temp, RH, & resultant VPD reading from your hygrometer?

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I am getting a 69pro they also have great humidifiers but apparently only work with vpd on the pro… But not 100% on that… But the 69 doesn’t advertise vpd.

Their humidifiers are outside the tent and would be perfect for my flower tent.

On my temperature and humidity controllers I have 6" AC Infinity raxial inline fans that “dehumidify” and “cool” vs my humidifier and mini oil filled radiator heater.

With the 69pro you’ve got 4 ports to use… 3 if you use the power adapter. There is soon to be a port expander but everything on the port runs same program vs individually programmed by main port.

I am upgrading my inkbird to a 69 pro and a 79 controller outlet

No prob. I made this up the other day to show the difference between before and after switching to the new humidifier. It should give a good picture of what’s going on. My focus was/is to dial in temp and humidity before adding leaf temp to the equation. So it needs to be mentioned that the Govee is only an approximation, but it’s helpful

I thought each port would have its own program, but with the expansion, you can run multiple off one port all on that ports program. I think each one could do temp or humidity. So if it’s humidity, it would be presumed a humidifier would be used. I also saw that for temp and humidity you can ramp. So now can we assume adding humidity on a port that is set to raise humidity, it would ramp that unit? Maybe with an ac humidifier? Or could you use an ac t4 set to add humidity as if it was a humidifier?

I’m trying to figure out if I should have a consistent vpd line 24/7, which would require changing the humidity at lights on and off. Or I can swing at the lower and higher ranges (1.2-1.6) through the day with constant humidity.

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You have it correct… The main ports are programmable individually… But the future expander just adds devices… Like if you had more than one light on the same schedule… Etc.

Keypoint from @PhotoFinisH article posted above:

I use the night to lower rh… Don’t pay attention to vpd at night. Just don’t want too much humidity nor do I want temps too low at night…

That makes sense. The difference in humidity that I would have to change to keep the vdp constant is only a matter of 5% or less given all the other factors are right. Like exhaust fan speed, fresh air fan speed, humidifier fan speed and house temp. I enjoy fiddling with the tiny details

I guess I’m striving for that 10x less if I have the ability, fun goal, not losing sleep over it

All AC-I Cloudline Controllers will monitor/calculate. Not all trigger on VPD.
69P probably does, but I don’t use it, or half of the 20 different whatevers.
I set limits for temps and run fans.
I chased VPD for a week, last year, and it cost too much bourbon.
I do not water at night or chase VPD when lights are off.
Good growing to you



Keypoint from @PhotoFinisH article posted above:

Apparently, I should consider more actively monitoring (and lower) night-time VPD for flowering ladies.
Thank you, for then new slide on VPD,
I do know from experience, w/o fans running 24x7, I can open tent at lights on and find a rainforest.


Same… Most I do is shut off heat and added humidity at night.

That happens for me at lights out.
No heating in lung room.
Lung room, no fun, this winter.

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The key, at least for me, is having the constant exhaust WITH a humidifier that can produce more than you need. With constant exhaust you never have to worry about opening up to a rainforest. It is very helpful having outside air as your cooling fan in addition to the exhaust. If a humidifier is JUST doing the job, I can’t imagine hitting vpd values. That was my problem. Bourbon!!!:joy:

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The leaf temp is always part of the equation.

Do you have a ‘right now’ numerical reading of air temp, RH, & VPD?