box volume - VPD - ventilation

Namaste, friends.

Let's say we have a box 2' x 2' x 4' = 16 cubic feet volume with 250 watts hps and 180 cfm exhaust fan.
That gives us 11,25 changes per minute.

Now, we gonna scale it down for 150 watts hps. The conversion ratio: 250/150=1,67

So 2' x 2' x 4' become 1,2' x 1,2' x 2,4' = 3,46 cubic feet volume and 108 cfm exhaust fan.
That gives us 31,21 changes per minute.

In other words, roughly, scaling down the size of the growbox to suit 1,7-fold wattage change results in 4,5 -fold decrease in box volume and, consequently, 3-fold increase in air exchange rate.

My question is how that affect weed considering humidity is same in both cases? My cloths drying much faster when there is more air circulation, i assume the same applies to weed, but to what degree? I can't say i understand VPD all that well, so i dunno if VPD takes air circulation into account, so can someone explain if 3 times more intense air change rate strips leaves off moisture 3 times stronger? And how ventilation rates affect VPD?

I can't give you exact numbers/ratios but more air movement equals less humidity. Then, a bit more air movement usually means also lower temps, while there's a point where more air movement doesn't translate to lower temps.
That means having just a bit more air movement could mantain or even lower VPD, depending on intake air temperature (or lung room temperature), while a lot of more air movement will rise the VPD out of optimum range. You just need to figure out the best balance of added humidity vs. added ventilation. In my opinion you can fix more with better intake air temps and by adding humidity than going with too many air exchanges per minute. Most grows don't need more than 2-3 air exchanges a minute, anyway. But what works for one grower might be hard to attain for the next grower. You need to find the best option for your space.