Greenhouse Inputs
Formula Used
Saturation vapour pressure: es = 0.61078 × exp((17.2694 × T) / (T + 237.29)) Actual vapour pressure: ea = es × RH / 100 Dew point: Td = 237.29 × ln(ea / 0.61078) / (17.2694 − ln(ea / 0.61078)) VPD: VPD = es(leaf temperature) − ea Absolute humidity: AH = 2167 × ea / (T + 273.15) Humidity ratio: W = 0.62198 × ea / (P − ea) Ventilation moisture removal: M = airflow × dry-air density × (Win − Wout)
These equations estimate greenhouse moisture using psychrometric relationships. Equipment results include efficiency, mixing, and safety adjustments. Real performance changes with weather, controls, and crop behaviour.
How to Use
- Select metric or imperial units.
- Enter greenhouse and outside conditions.
- Add greenhouse dimensions or manual volume.
- Choose a crop preset and growth stage.
- Enter plant, irrigation, and ventilation data.
- Add equipment capacities and efficiency values.
- Press Calculate Humidity and review every warning.
Example Data
| Scenario | Temperature | Humidity | Target RH | Outside conditions | Primary concern |
|---|---|---|---|---|---|
| Seedling house | 24 °C | 78% | 80% | 15 °C, 55% | Stable propagation moisture |
| Tomato production | 25 °C | 72% | 68% | 18 °C, 60% | VPD and disease control |
| Night dehumidification | 18 °C | 88% | 78% | 12 °C, 80% | Condensation prevention |
Frequently Asked Questions
What relative humidity suits most greenhouse crops?
Many greenhouse crops perform well between sixty and eighty percent. Exact targets depend on temperature, growth stage, and species. Use crop-specific guidance before changing environmental control setpoints.
Why is VPD important?
VPD describes the drying pressure surrounding plant leaves. Low values slow transpiration, while high values increase stress. Balanced VPD supports nutrient movement and healthy crop development.
What causes greenhouse condensation?
Condensation forms when surfaces cool below the dew point. Roofs, pipes, glazing, and leaves may become wet. Improve heating, airflow, insulation, or dehumidification to reduce condensation.
Can outside air always reduce humidity?
No, outside air may contain equal or greater moisture. Compare humidity ratios rather than relative humidity alone. Ventilation works best when outdoor absolute moisture is lower.
How does plant transpiration affect sizing?
Transpiration continuously adds water vapour to greenhouse air. Larger canopies usually create greater hourly moisture loads. Include realistic plant counts and measured irrigation records.
Why include leaf temperature?
Leaf temperature can differ from measured greenhouse air temperature. VPD changes when leaves become warmer or cooler. Infrared measurements improve crop-level VPD estimates significantly.
What does the air-mixing factor represent?
Air mixing shows how evenly moisture spreads indoors. Poor circulation creates wet zones and misleading sensor readings. Use circulation fans to improve environmental consistency.
Should nighttime targets differ?
Nighttime humidity often rises as temperatures and transpiration decrease. Condensation and disease risks may increase before sunrise. Use conservative nighttime targets with active air movement.
How accurate are equipment runtimes?
Runtime estimates depend on rated capacity and efficiency. Actual results vary with cycling, airflow, and maintenance. Verify performance using sensors and condensate measurements.
Can this calculator control equipment?
No, this page only estimates loads and operating requirements. It does not connect directly with greenhouse controllers. Qualified technicians should review automated control changes.
How often should humidity sensors be checked?
Inspect sensors regularly for dirt, drift, and poor placement. Compare readings with a trusted calibrated reference device. Replace damaged sensors before making major control decisions.