What humidity should my auto be during the early veg stage?
I understand VPD but to me it seems sketchy. The temps / humidity they suggest for most “optimal” conditions go against what everyone since the dawn of time has said for growing cannabis. That said I get times change and science comes out with better ways. I try to get close ish but found it’s impossible especially in flower which I’m at now, I have to keep my temps at like 68 to keep a low RH% lol ( also im a newb so take what I say with grain of salt)
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The VPD chart is an excellent reference guide. I say “Reference guide” I can only maintain the chart in the fall of the year with my current setup. And if this heat keeps up, forget about that .
When I first read about it, I too posted on here thinking I’ve found the holy grail of growing. I’m currently at 68 humidity and 73 temp inside my tent in late flower. Sounds crazy high right! I have tent exhaust fan, A/C into the room, 3 fans inside to help circulate the canopy. Maybe I’m lucky but I have yet to have an mold or rot issues. I have also seen the humidity go from 70+ in veg then will lower during flower. I assume this to be from leaf transpiration slowing. The chart is a good reference but don’t beat your brain against the wall if you can’t fall into the green zone
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I like to use this chart, but I don’t stress about staying right on a certain point. I like to use it mainly during the winter months, (When the humidity is low) to insure that I don’t go above 1.6 transpiration rate during flowering. If it starts to go too high I set a small humidifier in front of the tent’s intake.
I am lucky, all three of my little tents are in my basement and our AC unit heat pump keeps the temperature and humidity fairly close most of the year.
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I would like to add this to the discussion. Last winter when I was able to maintain a transpiration rate of just below 1.6 during late flowering. This did produce some of the best tasting and strongest smelling buds that has ever come out of this LED tent.
But it is just not worth the effort and expense to run humidifiers and dehumidifiers all year to obtain a exact VPD for a small tent.
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You really need to know your leaf surface temperature & use a VPD calculator if you want to know the VPD. Don’t forget ultimately you are interested in the leaf VPD. I’ve been using the dimlux calculator online. A chart is only as good as the single leaf surface temperature it is made for. If you don’t know what your LST is, or even if you do know your LST but don’t know what offset the chart is made for, then you could be off. Or, if you look at the wrong chart by mistake, you’re off. You can estimate your LST & pick a chart written for that offset, but it’s still a guess. Actually checking leaf temps & then entering everything into the VPD calculator will help to avoid these issues. Infrared pyrometers can be had for $25 or $30 these days.
So what did you do to maintain temps vs humidity?
I know what my VPD is, I have a probe measuring it, thanks for the advice tho!
Leaf surface temperatures also?
Nah not leaf surface temp that’s a bit over the top for me for stuff to get for now
In the end you want to be thinking about your leaf VPDs. Your VPD sensor controller might have a leaf temperature offset programmed into it, but if so it’s going to be for one number & you should still want to check your actual leaf temps & adjust the offset if it is off. If there’s no offset then it is only giving you room VPD, & you would want to check leaf temps & go to the online calculator with your air temp, humidity, & leaf temp numbers to get your leaf VPD numbers.
Then once you know where your leaf VPDs are running, finally decide on what you might need to do about air temps & humidity in order to optimize those leaf VPD numbers as much as possible.
You probably need to bring the humidity down, but you might want to bring the air temperature up also if the leaves are too cold. Or maybe they are too hot for some reason, who knows? Don’t you want to know? IR guns can be had for $25 or $30.
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Ill try to get one 
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@Sammys, sorry, sometimes I tend to be as clear as mud. I do try to maintain temperature and humidity to some degree.
The clone tent and veg tent are located in the garage side of the basement. Summertime the garage runs around 75 degrees with 60%rh, so there is no need to do anything. Wintertime it is about 65 degrees and 40%. Then I set a small humidifier outside the veg tent, and use humidity domes in the clone tent.
The flowering tent is tucked away in the furnace room on the finished side of the basement, (Did this before it was legal in my state). This tent uses an intake fan and an exhaust fan, and both have it’s own speed controls. The fans are ducted so that the intake is being pulled from floor level of finished basement, and the exhaust goes is into the garage. At full speed the fans can deliver two changes of air per minute through the tent. In the winter I put a another small humidifier in front of the intake. In the summer the tent gets up to 79 degrees & 60% by mid morning, and as the day heats up and the air conditioning cycles on the tent drops to 74 degrees & 45%. So this is kinda a self correcting condition and there is no reason to do anything.
Everyone’s set up is different from others, so following a VPD chart can have it’s benefits provided you don’t go overboard in your efforts to do so. This kinda like playing horseshoes “getting close is OK”.
Please allow me to go back to the flowering tent at mid morning to make point. I don’t recall reading about this here.
This 2x3 flowering tent is loaded up with lights a 130 watt LED in the ceiling, 12 two foot t5 on the rear and sides, and two tri colored LED strips in the rear corners. As stated before the temperature and humidity rise in the morning while the ventilation is contentious. The humidity at the tent’s air intake drops as the AC runs more during the day and this explains the drop in tent’s humidity. But the temperature at the intake stays fairly consistent at 74 degrees this is due to all of the air conditioning ducts being connected to the upper floor. So it is obvious that the tent’s ventilation is not able to keep up with the “Radiant Heat” that the lights are producing. The reason that the temperature drops in tent is because the indoor AC unit is cooling down the furnace room. As the furnace room temperature drops from 80 down to about 70, this allows the heat to flow from the tent into the furnace room through the tent’s walls.
It is much more difficult to remove this “Radiant Heat” with ventilation air than other types of heat. Let’s say, I was determined too keep the tent in the low 70s and didn’t have the help of cooling down the furnace room. Then the only choice that would be available is to lower the intake air temperature by about 5-10 degrees. Increasing the air flow would not help much in this case, since the present air flow is going so fast through the tent it is starting to lose it’s ability to pick up heat.
I think a lot of growers that push the limits on lighting may be in the same catch22, if they are not using a dedicated AC unit for their grow rooms.
This same “Radiant Heat” is the same heat that bombards the leaves. So this is some of what measuring leaf temperature is about. I do not use any VPD controller or app for it’s calculation. I just record the temperature and humidity and see where that lands on the chart. Then monitor the tops of the plants closely for heat stress.
I may regret saying this but, I have seen those VPD controllers on line. In my opinion it is another way to convince you and I that we need to buy more cheap ass Chinese crap, that may or may not do what they claim!
Come now, do you really think they can give you a quality controller with reliable sensors for about $100.00?
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Charts are only half of the story - & maybe half of the wrong story -unless the chart was generated using the correct leaf temperature offset for the leaves in question. If I used the chart above, it would put my seedlings at around .24kPa leaf VPD. I don’t know exactly what the leaf temperature offset of that chart is, but I would need a chart generated with something like a -10f leaf temperature offset. I don’t think most charts have more than -5f offset or so, & I see plenty out there with no stated offset. Some charts say right on them what the offset is. Then you know if you should use that chart or not, as long as you (also) know what your leaf temps are. The easiest thing to do for me was to forget about charts, & use the temp gun & the online calculator for leaf VPD numbers. Good Leaf VPD is the end-goal.
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That’s what my research turned up @PhotoFinisH
Thought the chart was starting point but then read into leaf temp and the science behind it. I’m just a hobby grower.
I agree that leaf temp VPD is a fair goal. But how can a home grower be sure the temperatures reading are accurate without investing in expensive equipment. I have used IR temp guns since they came out, and I know even the better ones work best on a solid surface, (Like a dark painted metal). It would take a heck of a lot to convince me that a cheap IR gun ordered from “you know where on line” is up to the task of giving an accurate leaf temperature!
I try to get close to VPD but when it comes to flowering the numbers just don’t make sense to me. If keeping between 70-80 which almost everyone agrees you have to run humidity waaaaaay too high for me. I’m new but I’m not running my humidity at 60% to maintain VPD 
What’s your humidity at then?