It depends on what steady value you want, and what grow phase you are in. During veg, grow lights heat up the grow space, which causes humidity to be much lower inside a tent than outside. This case might require a humidifier. During flower the plant will often put out more humidity than is wanted, and it might need to be lowered. In this case exhaust fan speed can be raised to blow out the excess moisture. In any case, the VPD chart will tell you what humidity values to use to maximize growth. Subtract about 5 F from air temp to get leaf temp (or better yet measure it with an infrared thermometer), then stay in the green while ignoring the numbers inside the chart.
So lets just say i want to maintain a constant 60%rh. How is that possible if the air in the room is getting exchanged every minute ? Smart ventilation that only kicks on once rh reaches say 62%? But then the air isnt getting exchanged as often. Which will lead to higher temps ?
Plant scientists have studied how plants grow under different conditions of temperature and humidity. If humidity is too low, plants transpire too much, and take up too much fluid. If humidity is too high, the plants can't transpire enough for good health. The VPD chart gives the best humidity ranges for a given temperature, and will maximize growth. My ultrasonic humidifiers put out more humidity in a 3 x 3 ft tent than a 6 inch exhaust fan can blow out on full speed during flower, and I can easily keep it in the 60s. They need to be turned on higher during veg. Forced air humidifiers can't keep up. I gave up on putting two big ones inside a tent because they couldn't get humidity high enough.
I have run with both low RH levels and high ones, no real difference. I have fresh air coming to my plants and their is always a wet towel in the tent with my plants. It also depends on what you are growing too, my autoflowers are always on low RH as they don't need a high RH. This plant grows in so many climates, it'll be fine.
Ive only got 1 ultrasonic humidifier. My room is 8 x 8 running 3 600w hps and 10 60 w led strips. I have 2 ac infinity cloudlines 2 as exhaust, 1 smaller cheaper fan as intake. The hps lights have cooltubes so each light will have its own fan. I have 1 6 litre ultrasonic humidifier and buying another is out of the budget. So lets say im aiming for 24c. The vdp chart tells me i should aim for 60rh. I was ofthinking setting the ac's to exhaust once the temp reaches 25c and once it reaches 23 the exhaust fans will turn off. With 3 600 w hps i figure it will keep hitting 25c every minute or so so it seems like the room is still getting adequate ventilation. I know temp and rh values will have to ve adjusted depending on stages and stuff. But does my plan sound ok or is only having 1 humidifier going to cause me trouble ?
I think you'll be fine. Especially when there's a lot of vegetation transpiring to increase humidity. As others have pointed out, you can grow good weed without following the VPD chart. But you'll have maximum health and yield if you do. I'm able to keep humidity in the mid-upper 60s in 2 x 4 and 3 x 3 vented tents with one ultrasonic humidifier each. In late veg and flower the plant creates much humidity itself, so humidifiers can be run lower or turned off.
Praying leafs are a definite sign from high humidity which is mostly what I deal with. They do that because the pupilars have opened wide to extract moisture from air rather than soil and slowing down photosynthesis. Lollipoping plants before flower or after stretch which is when I prefer to. This will cause greater air flow and also lower transpiration.