Disclaimer
This article was originally published on Pulse Lab’s blog. If you would like to read the original article, click here.
Editor’s Note: We’ve dedicated several articles to VPD before, but we felt this article stood out because it actually tells you how to calculate VPD and apply it practically to your grow. Please let us know your thoughts in the survey below. Additionally, Pulse Labs have made a custom calculator and chart available for you to download below, so keep an eye out for that!
Many growers will tell you that mastery of your grow comes from humidity control, and they’re not wrong. But humidity means so much more than a simple relative humidity measurement — it means planning out precise humidities that ease stress on your plants at the right time, and stress your plants at the right time.
Unfortunately, relative humidity isn’t a good tool for planning. Relative humidity does not tell you how much water is actually in the air at any given moment, nor does it tell you where the dewpoint is or how much stress your plants are actually experiencing.
VPD helps you identify the correct range of temperature and humidity to aim for in your grow space and plan accordingly. With VPD you can achieve the best results while avoiding pest and environmental problems. You can achieve much tighter control over plant transpiration rates, stomata opening, CO2 uptake, nutrient uptake, and plant stress.
If you master VPD, you truly master your environment, and become better grower.
What is VPD?
Vapor Pressure, Temperature, and Humidity
VPD stands for Vapor Pressure Deficit, but what does that actually mean?
Air is made up of many gases. Air is about 78% nitrogen, 21% oxygen, and 1% 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”.
Editor’s Note: Most atmospheric composition calculations don’t include water, due to significant variations in temperature between the layers of the atmosphere. On average, water vapor accounts for about 0.25% of the Earth’s atmospheric mass.
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 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. This is why there is dew all over everything after a cool morning. The air gets too full of water, and the water condenses out. When water comes out of the air from reaching this maximum, it is called the dewpoint.
Similarly, the current actual amount of water vapor in the air is called the “actual vapor pressure” or AVP.
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