@PhotoFinisH I understand it pretty good at this point. I’m using a vpd calculator and I put in the lst and air temps I’m seeing. It equates to more humidity. That was my problem early on with this light. I originally thought, before I bought the ir temp gun, lst numbers should be lower than air. Using +1 lst with my air temps, I’ve been trying to keep my humidity up. I can tell when they are liking it and not. I’ve seen them when they have closed their stomatas during lights on, and as I described looking like they gave up on life.I’ve seen them with their leaves spead out flat facing the light and I’ve observed them standing at attention I’ll say. Leaves pointing almost straight up. Pretty sure they are rocking a high vpd at that point. Lifeless happens when they stay there too long.
Why would growth be slower at a lower temp? If I put in 73°f for air, 74 for leaf and 71% rh, my calculator says .9 vpd. If I try at 80°, 81 leaf I need 76-77% rh for the same vpd. Easier to maintain humidity at a lower air temp.
Not sure what to make of my venting system? I Don’t remember asking you, but I circulate air though the tent at this point, back into the room. Just enough to keep temps down and humidity up. I told you we weren’t making this a thread about vpd so you can choose to believe me about the lst I’m seeing or not. I’m done talking about for now.
Sorry just been working so hard to get things right this grow. I don’t want to be lucky, I want to know what needs to happen in the growspace I’m in. It’s time consuming, but worth it in the end. Why grow if you’re not going to maximize your effort. It’s still fun and rewarding…,but…you know.
They look happy.
24"-25" from light
Humidity in mid 70%
It wasn’t a matter of believing you about your numbers. If I see odd things I want to check it out, & I was suggesting that you do the same, because to me it seems odd to have higher LSTs than air temps.
Yup, can’t complain about that.
I’m still not sure what to make of your vent system. How does it work exactly?
I understand, not odd to me because…it’s aaaalll new to me. It is what it is in my mind. Very manageable with leaf and air at same temperature. I forgot to refill humidifier. Been going through a gallon+ a day. Then my circulation fan was plugged into temp controller. It may eventually be controlled by it, but I should not have yet. Just checked and temp was 79° with 60% rh. This would have punished them when I had the light closer and leaf temp +1-+2. Not today, they looked fine, the same as above picture.
It works by me adjusting everything manually right now. Both fans are pointing down, it’s just how it worked out. I’m only using my “heating” fan at the moment. Heating as defined by my temp control. Coming up to whatever temp. Air is drawn through tent back into the room. More specifically, I also draw a little air from ceiling and mix it. Further yet, my small tent is not being used for growing at the moment. Sooo, my a/c vent in the room is directed into small tent, which is not sealed. Vents open. Any air drawn from this tent is also mixed in. Thats about it right now.
I’ll explain a little more about “today’s high temp”. Obviously wasn’t planning on humidifier running out of water. Don’t know when this happened. Probably not for long before I caught it. The temperature increase to 79 is because my fan also shut off. Control was in cooling mode. I really don’t want “cooling” at this point.
I concur.
I am not sure, but checking, as i grow, if I can read the plants dryness with the Lasar gun.
Who is too dry vice still cool. Not sure, but testing.
God knows how I hate that. Burned a few in Louisiana’s oven.
@DEEPDIVERDAVE I don’t see where that chart considers leaf surface temperature. Am I missing something?
What light source is being used?
No, admittedly the chart is not VPD specific.
Growth rate and temperature as shown will have an impact on VPD.
Chart was an attempt to answer quote" why would plant grow slower at lower temperature.
Those ladies are fantastic looking 
Right. But vpd has 3 variables. Without lst, along with humidity, air temp means nothing. Just a number. Am I still missing something 
I’m just learning this stuff, but I’m confused by that chart. Doesn’t vpd drive transpiration and that in turn drives growth rate? If so, how can you say air temp drives growth rate? Vpd does indirectly. You still need the right amount of light, for whatever growth stage the plant is in, nutrients etc. According to the chart, everyone should be shooting for 86°. Hps, 86°. Mh, 86°. Led, 86°. Is every outdoor area, that cannabis grows well in, 86°? Just trying to understand.
I said this was NOT going to be the “vpd” thread. So, for those who don’t care about it, I apologize. I brought it up because, I was interested in it. Not ready to push these girls yet, just learning how. When I’m comfortable that I understand it, like I understand water is wet, I’ll play around with it. That said, is there not a vpd discussion on this site? No one else interested enough? Hard to believe.
I’m going to help you out here dave. Without considering air temperature, the sun will warm you. The same way it warms the surface of a leaf. It can even make you sweat on a “cooler” day. Agree?
Maybe he didn’t agree…