What is VPD/kPa?

can anyone explain to me about VPD and what kPa is and why is it important what should it be at? does that number even matter?
and another question, where should i put the sensor that says the temp, humidity, and VPD… whatever that is lol
rn i have it in a plant :potted_plant: shoved into the top of the soil, should it just hang or sit in the top of the pot? hopefully someone can help me get this down! :pray: :weary: :sweat_smile:

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kilopascal (kPa), one thousand times the unit of pressure and stress in the metre-kilogram-second system (the International System of Units

Not import to know really.

VPD is important but try not to focus on it to much it will drive you crazy trying to keep it perfect :sweat_smile:

But heres a little info on it.

Vapor Pressure Deficit measures the amount of drying power the air has upon the plant. Basically, it’s how much moisture is being sucked out of the plant by the atmosphere.

You probably already know the plant uses transpiration to grow. Literally, the plant will die if it can’t release moisture through its stomata. The process of transpiration in plants is similar to how we sweat; they have stomata which are similar to our pores. But the big difference is, they do this so they can pull in more liquids through their roots to fuel photosynthesis.

When Vapor Pressure Deficit is too high, the plants might not be able to keep up with the environmental demand upon them. The air is drying them out too fast! And this will cause them to exhibit symptoms that look just like nutrient deficiencies, and the plant will grow poorly.

On the other end of the spectrum, if Vapor Pressure Deficit is too low the plants can’t transpire. Moisture may build up on the leaves, and the plant will grow much more slowly. If this goes unchecked for too long, the plants might get attacked by molds and fungi, like powdery mildew.

Reading up on VPD you’ll see all sorts of measurements of Vapor Pressure. There’s Saturated Vapor Pressure (SVP), Atmospheric Vapor Pressure Deficit (AVPD), Leaf Vapor Pressure Deficit (LVPD), plus several others not worth mentioning.

Vapor Pressure Deficit is the difference between the Saturated Vapor Pressure (SVP) and Relative Humidity. Saturated Vapor Pressure is the maximum amount of moisture the atmosphere can hold according to it’s temperature. And Relative Humidity is the amount of moisture currently suspended in the air. So to calculate the room’s Vapor Pressure Deficit all you need is two measurements, temperature and relative humidity!

But this isn’t the whole story. The plant’s experience is slightly different because they are usually a bit cooler than the room. If the plant temperature is exactly the same as the room temperature, then the plant and room Vapor Pressure Deficit levels are equivalent. But this is rarely the case! Usually, the leaves are between 3° and 5° F cooler than the room because they are transpiring. The evaporation on the leaf’s surface literally draws heat from the leaf, thereby cooling it.


happy, that was the best explanation I’ve seen. :v:

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