How important is the VPD
I got my temperature roughly where I want it
I’m keeping the humidity down
The vpd is a lot higher than what I’m reading it should be
2 Likes
What growth stage are the plants in, different stages require a different RH. I would recommend not concerning yourself with VPD numbers due to how often the environmental conditions can fluctuate. Seedlings like the temps around 76-85 with an RH of 65-75%, early and mid veg temps the same and dial the RH back to 65%, flowering temps 65-85 with the RH around 45-55% 
8 Likes
Chasing VPD can be frustrating and I don’t know any growers who try to manage it. I do similar to @OGIncognito and just keep it simple. ~60% in veg and 40% in flower.
6 Likes
3 weeks flower I’m sitting at 74° 44% and the vpd is reading 1.6
I read or more likely listened to someone say that vpd should be between .5 and 1.0 I’m not even sure what it effects
Trying to understand the difference between that and humidity is a concept I’m struggling to understand
My thoughts are
If humidity is high vpd goes down
If humidity is low vpd goes up
1 Like
Tbh it’s not critical to this grow
This first run is a failure
I’m finishing it out so that I can get things “ironed out” enough to have a decent run
What I have in the tent is severely stunted untrained plants that I all but killed
When they decided to make a comeback I just kept them as my sort of Guinea pigs
My test run
So now is a good time for me to experiment on getting the environment within acceptable parameters
2 Likes
Here are some things to help you understand some things.
VPD target numbers are Leaf VPD target numbers, not Air/Room VPD target numbers.
1.6 is your Air/Room VPD. But your leaves should be transpiring, & because of that they should be running cooler than the surrounding air temp, which will move your Leaf VPD number lower than the Room VPD number. How far depends on how much cooler the leaf is, if all else stays the same.
So you need to include your leaf temps into your calculations, & target your Leaf VPD when adjusting your VPD towards VPD target numbers.
You don’t want to adjust your Air/Room VPD towards Leaf VPD targets. Easy mistake to make though.
Here is a dynamic chart where you enter the air temp, RH, & leaf temp, & it adjusts the chart on the fly for you.
What is it adjusting to make the chart change as you change the Leaf Temperature?
The Leaf Temperature Offset number.
The Leaf Temperature Offset ultimately determines what numbers will fill the cells of a VPD chart. If you see different static VPD charts which were all properly generated, then the difference between them is the Leaf Temperature Offset number that was used to create them. Another easy mistake to make is to pick a static chart which was generated with a leaf temperature offset that doesn’t match your actual leaf temp offset.
The Leaf Temperature Offset number is calculated:
Leaf Temperature - Air Temperature = Leaf Temperature Offset
(Express the answer as it is calculated. Often negative.)
Could the Air/Room VPD number be the same as the Leaf VPD number? Yes, if the Leaf Temperature Offset is zero.
Can you determine the leaf temperature offset of a chart if it’s not listed? Yes, by picking a cell & running the numbers back through a leaf vpd calculator & changing the leaf temp until the leaf vpd matches the air vpd, & then subtracting the leaf temp from the air temp, & there’s your leaf temp offset for the chart in question.
Leaf VPD Calculator
4 Likes
Made one small mistake in describing how to get the leaf temp offset of a chart, edited:
Sorry for any confusion!
3 Likes
Just passing info along. My local grow buddy has been playing with the ACI 69 PRO CONTROLLER and set for VDP it has maintained VDP dead on for the last 8 werks of flower for him. It goes wonkey when lights cycle door opened up for feeding, prunni g ect. But it reajusts within 20 min so far. Im doing a full VPD grow starting within the month.
5 Likes
I dont chase it and do just fine myself. Good lights. Good nutes. Good air. Good dirt. Water. Tv. Xbox.
9 Likes
The beautiful thing is (according to what ive read and my buddys experience) you dont have to chase it. Just set a target VDP and then set +/- parameters and leave it alone and it seems to work well on it own algorithms.
6 Likes
Happy Piece-O-Cake buddy 
5 Likes
I chased vpd my last grow and havnt given it a single thought this time around. This grow is doing better(probably just a coincidence). My point being there are plenty of other things that are more important.
5 Likes
I too went down the VPD rabbit hole and found out if you can hold exact temps/RH then it is worth it. But it is similar to Co2 in my opinion in that if you want to nail it throughout the whole grow you need a closed controlled environment. Due to the outside temp and season when I tried it, I was able to hit a perfect VPD for 6 days straight. Here is the baby before I started chasing VPD
and this was the growth 9 days later hitting 1.1 every day.
I no longer chase this as it really is too hard to be consistent in a home with an open door policy. Good luck to you!!
6 Likes
Here here
Happy cannaversary
2 Likes
My experience was exactly that, same home conditions.
1 Like
Is that the one with the new IR leaf temp sensor? I’d like to try something like that, but I don’t like getting tied down to one company with proprietary connectors etc.
4 Likes
They have a IR temp gun with a chip and that info can be programmed into the controller for a more accurate VDP control. I get what you say about getting “married” to one company though. So far everything ive bought from them has been good quality so they are winning me over.
3 Likes
It appears they are going for an intergrated system concept. Where all equipment is intergrated to function as a unit as opposed to sepirate devices. Makes some sense, ya they kinda gotcha! LOL
3 Likes
Literally the only consistent complaint I’ve seen come up with AC integrated systems. They seem to be leading the pack in this area. I was given the c69 (no Wi-Fi model) and the only issue I have had thus far is the connection into the back of the humidifier is NOT moisture resistant in any way whatsoever which led to a short that melted the plastic around the plug opening on the humidifier. The humidifier continued to work for a little bit as long as it wasn’t connected but went out shortly after. I would still recommend AC over others to automate but caution with placement if using their humidifier. I made the mistake of putting it inside the tent instead of using the ducting hose provided so I don’t entirely fault the company.
2 Likes
I recently came across the ACInfinity controller with AI. What a game changer! I have a complete system of AC Infinity products. All I do is set the VDP parameters and keep the humidifier full! I can control it over WiFi from anywhere.
I do have a second tent set up with the controller 69. It’s the next best thing.
1 Like