VPD, without the spreadsheet.
Vapor pressure deficit is how hard the air is pulling water out of the leaves. It’s one number made from two you already have.
Leaves usually run a little cooler than the air. An infrared thermometer tells you how much.
Leaf VPD
1.12 kPa
Veg range
The range usually quoted for veg.
- Air VPD
- 1.31 kPa
- Dew point
- 63°F
- Very humid< 0.4
- Gentle0.4 to 0.8
- Veg range0.8 to 1.2
- Flower range1.2 to 1.6
- Very dry> 1.6
What it is. Warm air can hold more water than cool air. VPD is the gap between how much the air could hold and how much it’s holding. A big gap dries the plant fast; a small one barely moves water at all.
The math. Saturation pressure comes from the Tetens equation. Leaf VPD uses the leaf’s temperature for the plant side and the air’s for the room side.
The ranges. The colored bands are the ones growers commonly quote. They’re a place to start, not a rule. I’m still learning what my own room likes.
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