Vapor Pressure Deficit (VPD)¶
Purpose¶
VPD is the difference between the amount of moisture the air can hold and the amount it does hold. It is the primary driver of plant transpiration — the mechanism by which plants draw nutrients from the root zone. Unlike raw humidity, VPD accounts for temperature, making it a more reliable indicator of plant stress.
\[
\text{VPD} = \text{SVP}(T) \times \left(1 - \frac{\text{RH}}{100}\right)
\]
Where SVP is the saturation vapor pressure at temperature T.
This is a Derived Measurement¶
VPD is not read from a dedicated sensor. It is calculated from:
The calculation is performed in the control layer (e.g. a Prometheus recording rule or a Python function).
Target Ranges¶
| Growth Stage | VPD (kPa) |
|---|---|
| Seedling | 0.4–0.8 |
| Vegetative | 0.8–1.2 |
| Fruiting | 1.0–1.5 |
Control Logic¶
VPD is regulated by a PID controller:
- VPD too high (air too dry / too hot) → activate Humidifier
- VPD too low (air too humid) → activate Exhaust Fan
See State Machines for controller design.