I have been working on a system to automate my like indoor grow tent, I have sensors to monitor the temperature and humidity. I currently have come cucumbers in there that require a high humidity and what I have noticed is that once the humidifier() is turned off the humidity drops very rapidly. I have a tent that is 38"x38"x6' and I was thinking maybe it was having a small fan that is running in there 24/7. I ran a test by turning the fan off for a few hours and it did not seem to make a difference. I have an exhaust fan attached to the end of some 8" exhaust hose(sorry for my lack of knowledge on the correct terms) but it was also turned off all day and normally only kicks on when the temperature or humidity get too high. I have also closed the vent in the back about 80% of the way. I would close it all the way but I do not have any intake venting and I worry that it is bad for the exhaust fan to run without being able to pull in any fresh air. Is this a normal occurrence or is there something I can do to help with the issue?
So you are saying you have a ventilation system to pull air out of the room...do you have a way for fresh air to be pulled in passively? Fans are critical for all indoor and greenhouse plants. Do not worry, moving air has few if any negative effects for your plants. Unless the plant is being moved in the back of a truck going more than 30mph, now that is too much.
Plants USE CO2 and give off O2. The important thing is to get that O2 gas away from the leaves so that CO2 can move in to be available to the plant. As long as your room is not airtight, the air being sucked out will be replaced with fresh. Lots of growers actually use heaters for instance that give off CO2 in their greenhouses. Dry ice is another source to increase CO2 to increase plant production and vitality.
Depending on the plants you are growing, humidity is not a big deal breaker. Consistency of heat, good soil and proper light are the critical factors. Open that vent all of the way. Air circulation regulates growth and curbs fungus. If the vent is opened all the way and the temperature drops, close it down until the temperature is consistent.
Cucumbers in my experience do not do any better or worse for the humidity levels. They DO get powdery mildew with high humidity and that definitely cuts your production.