October 03, 2019
"Why devote an entire guide to VPD?" you might ask. The answer is that the vapor pressure deficit (VPD) is extremely important for growing plants.
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VPD helps you identify the correct range of temperature and humidity to aim for in your grow space. With VPD you can achieve the best results while avoiding pest and environmental problems. VPD also controls plant transpiration rates, stomata opening, CO2 uptake, nutrient uptake, and plant stress.
If you master VPD, you master your environment, and become better grower.
VPD stands for Vapor Pressure Deficit, but what does it actually mean?
Air is made up of many gasses. Air is about 78% nitrogen, 21% oxygen, and much smaller parts of other gases. Water vapor, the gaseous form of water, is one of those other gases. The amount of water vapor in the air (expressed as pressure) is called “vapor pressure”.
Air can only hold a certain amount of water vapor at a given temperature before it starts condensing back to liquid water (in forms such as dew or rain). The maximum amount of water vapor that air can hold at a certain temperature is called “saturation vapor pressure” or SVP.
As the air gets hotter, the amount of water that the air can hold (its SVP) increases. As air cools down, the SVP decreases, meaning that the air can’t hold as much water vapor. That is why there is dew all over everything after a cool morning. The air just gets too full of water, and the water condenses out.
Similarly, the current actual amount of water vapor in the air is called the “actual vapor pressure” or AVP and display it in a VPD Chart.
That’s right, RH is just the proportion of water the air is currently holding vs. its maximum capacity. That’s why it’s called “Relative” humidity.
That means RH = 100%.
If AVP reaches SVP, any additional moisture will precipitate out of the air as liquid water (dew, etc).
VPD: how much more room there is in the air for more water vapor. It’s as simple as that. Now you get why it’s called the Vapor Pressure Deficit.
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