The VPD calculator is a tool designed to help growers manage the delicate balance between temperature and humidity in their grow rooms or greenhouses. It calculates the vapor pressure deficit (VPD) of the air, which is a critical indicator of the potential for plants to lose water through transpiration. A correct VPD value can help ensure that plants are neither too dry nor too damp, fostering an optimal environment for growth and health.
Formula of VPD Calculator
The core of the VPD calculator’s functionality is the formula:
VPD = (SVP - AVP) x (1 - RH/100)
Where:
VPD= Vapor Pressure Deficit (often in kPa)SVP= Saturated Vapor Pressure (at air temperature)AVP= Actual Vapor Pressure (which is influenced by RH)RH= Relative Humidity (%)
Finding SVP:
Saturated Vapor Pressure (SVP) can be found using tables online or in plant physiology resources, reflecting SVP at various temperatures. Alternatively, the equation below offers a precise calculation:
SVP = 0.6108 * exp((17.67 * T) / (T + 243.5))
Where:
T= Air temperature (°C)exp= mathematical exponent function
This formula allows growers to calculate the VPD accurately, adjusting their climate control systems accordingly.
VPD Table for Various Plant Growth Stages
| Growth Stage | Ideal VPD (kPa) | Temperature Range (°C) | Relative Humidity (%) |
|---|---|---|---|
| Seedlings/Clones | 0.5 – 0.8 | 20-25 | 70-80 |
| Vegetative Stage | 0.8 – 1.2 | 22-28 | 40-70 |
| Flowering Stage | 1.2 – 1.5 | 20-26 | 40-50 |
| Late Flowering | 1.1 – 1.5 | 18-24 | 30-40 |
Notes:
- Seedlings/Clones: Young plants require higher humidity to prevent water loss as their root systems are not fully developed.
- Vegetative Stage: Moderate VPD values encourage strong and healthy leaf and stem growth.
- Flowering Stage: Slightly higher VPD values help promote flower development and reduce the risk of fungal diseases.
- Late Flowering: Lower temperatures and humidity levels help enhance flower density and trichome production.
Example of VPD Calculator
Consider a grow room with an air temperature of 25°C and a relative humidity (RH) of 60%. Using the formula, we calculate SVP and then VPD, determining the optimal environmental conditions for the plants.
Most Common FAQs
The ideal VPD ranges from 0.8 to 1.2 kPa for most plants. However, this can vary depending on the plant species and growth stage.
Monitoring VPD should be a daily practice, as fluctuations in temperature and humidity can quickly alter the grow room’s conditions.
Yes, adjusting the VPD according to the plant’s growth stage can promote healthier development and yields. Seedlings and clones prefer a lower VPD, while vegetative and flowering stages may benefit from a higher VPD.
