By following a VPD chart and reading the plant, is seems like the humidity is too high. I've just started Veg and the leaves droop just a bit and I'm getting a little condensation on the inside of the tent. The plant seems to look better/normal when the VPD is running at 1.20-1.30. Is this normal or, what are some thoughts on this? Thanks-
I'm not sure why some folks reject using VPD and favor temperature and humidity when VPD is nothing more than a way to interpret temperature and humidity. Oh, well. The VPD chart is a useful guide, but isn't a replacement for reading the plants.
Vapor Pressure Deficit (VPD) has nothing to do with condensation (when the air is saturated with water) or soil moisture. It is a measure of leaf moisture compared to air moisture. The difference between the two affects how much a plant transpires. When the RH is low, a plant will transpire more. When the RH is high, a plant will transpire less. Thus, RH and transpiration rate are in an inverse relationship. VPD changes that to a direct relationship. So, as VPD increases, transpiration increases.
Transpiration is the process of the plant moving water and nutrients from the roots to the leaves. It happens because fluids move from areas of high concentration (the leaves) to areas of low concentration (the air). Therefore, by manipulating the difference, we manipulate the rate of transporting nutrients and thus we can manipulate the plant's growth rate. There's an optimal range, however, and that varies by the life stage of the plant.
My controllers calculate VPD, so I usually look at that first. Then I'll look at the temperature and humidity, especially if the VPD is out of the optimal range. That to me happened yesterday. The VPD rose to 1.40 kPa. The humidity was okay, but the temperature wasn't. The lung room was too hot, and the exhaust fans were useless for cooling. It was a hot day. I was out of town, so the only thing I could do to lower the VPD was to raise the humidity.
For example:
The VPD is low in the first two because the lights are off. Those two tents are in flower mode. The last tent is in vegetation mode and the light has been on for an hour and a half. They've all been watered recently.
