I don't mind at all, go grab a coffee
VPD works best when leaf temp is 2 degrees cooler than the air. Thats the temp differential that transpiration transpires best at. Vpd should also never be run higher than the stage the plant is at. The stage the plant is at is dictated primarily by how well the rootball can supply the plant with food and water.
So when you adjust vpd, never run it high enough to cause the plant to run faster than the rootball can handle. That part is on you to determine, so slow and steady until you start to reach a limit is best. Big adjustments will bite you.
If you go faster than the rootball can handle you get an instant deficiency because you are photosynthesizing faster than the rootball can provide the photosynthesis department, so the photosynthesis department, instead of slowing down, starts drawing nutes from the leaves. Fry them and the grow is in trouble.
I give you all this warning 1st because light is what primarily determines the difference in temp between the leaves and the air. You DO NOT want to give your plant too much light ever, unless you are in the last 2 weeks of the grow where hermies can't hurt you. Light is the easiest tool to use to cause hermies.
So for you at 81F air, 78F leaf, and 65%RH, the easiest way to raise vpd is to lower humidity but..... A 2 degree difference would be nice too. So if your plants are still getting taller I would live with the 3 degree difference and the plant will grow into the light and it will move to a 2 degree gap. After that you may have to either raise the light or turn it down a bit.
If stretch is over I would turn up the light a little bit. Sometimes 50ppfd more is enough. Then wait until 10 hours of lights on tomorrow to assess the adjustment and react to that. Not before unless you see danger.
Then once the 2 degree dif is established I would lower RH to increase vpd and as flower progresses I would lower the air temp as well, all slowly and incrementally to make sure you never run the plant too fast for the rootball. I try never to go over 1.45, even in deep flower. One glitch and it could over rev and fry leaves or cause hermies.
If you get to 76F air, 74F leaf temp, and 48RH, your vpd is 1.4 which is perfect, and you aren't at risk of mildews and rot.
Check vpd any time you like, but never adjust unless the lights have been on for 9 or 10 hours. You want to know how fast they are running when going top speed.
If you speed them up in the morning, they may over rev in the afternoon.
Checking vpd at various times of day is pretty cool. You see what happens when you water. You see what state they are in when they 1st wake up. You see what state they are in when they are thirsty.
If your leaves mysteriously get hot, she probably needs a drink. Check that before you turn the lights down. If she is running slow chances are you just finished watering, that sort of thing.
Then when you learn the plants cycles you see advantageous and disadvantageous times of day to give water, or to prune.
Why water or prune when she's running hard. Why not water or prune when she's running slow in the morning.
Now when you have her running properly AND your not interupting her during peak work hours, she can photosynthesize more in a day. That right there is your target.
So if you make an adjustment and she stresses, you have reached a limit. You need to wait for the rootball to catch up, or you have simply found the genetic limits.
Please try not to run her right on the edge. You only need to be in the ballpark. If you can keep her in the ballpark for the entire duration of flower, you will have amazing colas. If you over rev her once you will lose far more than that little extra you were trying to squeeze out.
The state of your rootball just got more valuable. You can't use more light if the rootball can't supply. Make sure it's NEVER the weakest link and you will need extra jars for harvest.
Are you in flower? If so, how far in? Got some pics?
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