@Unique - thanks for the inquiry & comments on VPD. First, you're right, my tents are zipped closed but not fully "sealed" and I do not use co2. I find the concept of VPD helpful in understanding that there is a synergistic balance between temperature & humidity, and that you can sometimes more easily "fix" a perceived problem with one by adjusting the other. But I also have seen that tracking the "numbers" is not very useful in any absolute sense given my growing environment.
For example, I have 3-4 digital hygrometers placed at varying heights around my pots - at pot edge, canopy top etc. And of course the variations in temperature and RH among these is on the order of 10-25%. That can create quite a range of VPD numbers. Frankly, from what I've read, VPD is only fully accurate when the readings are taken right at the leaf surface. This is not practical, and even then you'd get different readings from leaves at different positions around the plant. So what the heck?
2017 was my first year growing. And now, after 6 grows across 2 different 4'x4' tents and a full year of climate variations, I would say emphatically that controlling temperature and humidity has been and continues to be my biggest challenge --- made all the worse by the fact that I'm really not sure what the optimal targets are that I should be striving for at any given stage of plant growth.
Despite a slew of gadgets in each tent (humidifiers, dehumidifiers, fans and timers) I find that my temp & RH readings swing wildly over the course of 24 hours. Lights on or off, tent open or closed all have a significant impact (obviously), but the most frustrating issue is that once I engage air supply and extraction fans, my tents immediately take on the ambient temp and RH values of the room the tents are in. This would suggest that the easiest and most efficient strategy would be to abandon "tent VPD" management, and just focus on managing the temp and RH in the larger room that contains the tents. But that too is challenging -- the room cannot be fully closed off, it has one side that flows up a stairwell to the main floor of the house. Also, the room has 12 foot high ceilings and a wall of windows that capture intense solar gain heat during the day, and equally rapid cooling in the winter evenings. The final insult here is that the room has no air conditioning, and its heat source is in-floor radiant heating. Radiant heating (in concrete) is very comfortable, but has response times only slightly quicker than tectonic plate movement. So I can't effectively raise or lower the room's temperature in response to outside weather. Ugh.
I was bemoaning my issues to the owner of an Electrical Contractor's company. He was explaining some of his work in hospital surgical rooms where RH and temp must be absolutely maintained at a set point with no variation. So I know my "problems" can be solved, but I'm not sure that the cost is warranted.
Anyway, so much for VPD. Until I learn more, I'll continue to push this boulder up the hill. :smoking:
