Vapor pressure deficit (VPD) isn't a term you'll find in the first few chapters of a cannabis growing guide. Heck, there are plenty of experienced cultivators who don't fully understand the intricacies of VPD. Although monitoring VPD is an advanced growing technique, there are benefits to...
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Here's a thread that seems to explain VPD quite well.
It's great if you have the control to be able to hit the vpd range. But if you want something simple just aim for 27 degrees C all the time. And drop the humidity through each stage. 70 for seedling or 75-90 for clone then 70 when rooted, 55-60 for veg, 50 for early flower, 45 for late flower. If you learn how to run something close to this you will get good results.
I would say find what works best/easiest for your set up. Seedling, veg, and flower will all be slightly different. The pulse website has great charts to reference. As a general rule though I would say 72 to 82F and 45 to 60% RH for veg and for flower. Anywhere in that zone would be running pretty close to optimum.
EDIT: This is just my opinion. Your mileage may vary
Most of us measure at a single point in our rooms/tents which is hardly a scientific approach but a good indicator, and temps can easily differ 5 degrees C from canopy to root zone.
When switching from HPS to LED you also change the way your lights put out heat.
HPS has a lot more irradiation heat than LED where most of the heat dissipates upwards through the heatsinks.
So, your leaf surface temperature and root zone temperature are affected by the change and you may have to adjust other grow parameters accordingly
thats just the highest temperature most documented that plant still grows. i personally would like to go 20-25 but usually i leave it all to nature.i dont chase max yield