Live temperature, humidity, VPD, and soil moisture from your Bizon sensor, all on one page.
VPD Basics
VPD (vapor pressure deficit) is the gap between how much water vapor the air could hold and how much it's holding right now. That gap is the suction that pulls water up through the plant and out through the leaves. Too little and transpiration stalls, which means weak nutrient uptake, soft growth, and more disease. Too much and the plant closes its stomata, which slows photosynthesis and can cause tip burn and stress. In the right range the plant keeps taking up water and nutrients and keeps growing.
Air VPD
Almost every controller, app, and chart calculates VPD from air temperature and relative humidity alone. But a plant transpires based on the temperature of its leaf surface, not the air, and a leaf is rarely the same temperature as the air around it.
A few degrees of difference can swing the real VPD by 30% or more. So your screen can show a good air VPD while the plant is still stressed.
Leaf-to-air VPD
Leaf-to-Air VPD (L2A VPD) measures the leaf surface temperature directly and works out VPD from the leaf instead of the room air. That's the number that drives transpiration, how far the stomata open, how much calcium moves into new growth, and crop steering.
VPD Demo
Drag the sliders to see how a leaf a couple of degrees off the air temperature changes the VPD.
Air VPD (the usual number)
Built from air temp + RH only.
Leaf-to-Air VPD (at the leaf)
Built from leaf temp, air temp + RH.
Side by side
Uses
Set humidity from leaf VPD, not air VPD. Steady transpiration carries calcium and boron into new leaves, which helps prevent tip burn.
Steer vegetative or generative from VPD measured at the leaf, not estimated from the air.
The graph shows the plant closing its stomata before you could see it on the bench.
Humidity and temperature at the leaf show mold and powdery mildew risk early.
Measurement Method
Our L2A sensor reads leaf-surface temperature with a fast infrared thermopile alongside calibrated air temperature and humidity, and sends live Leaf-to-Air VPD to your dashboard, where it's logged, graphed and can alert you when the canopy goes out of range. The same sensors feed our dryback and crop steering tools.
We use all three readings: Leaf-to-Air VPD is the saturation vapor pressure at the leaf temperature minus the air's actual vapor pressure (saturation at the air temperature times relative humidity). All three inputs matter, not just leaf temperature and humidity.
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