Most growers chase a relative-humidity number. But plants don't actually respond to relative humidity — they respond to Vapor Pressure Deficit (VPD), the difference between how much moisture the air is holding and how much it could hold at that temperature. VPD is what really governs how fast a plant transpires, and getting it right is the difference between a thriving canopy and one stalled by mold or dehydration.
Why relative humidity lies
Air at 70% relative humidity means very different things at 18 °C (64 °F) versus 28 °C (82 °F). Warm air holds far more water, so the same 70% reading represents a completely different "pull" on the plant. Two rooms with identical humidity can have wildly different VPD — and wildly different growth. That's why a humidity gauge alone is a poor control signal.
What VPD tells you
- VPD too low (air near saturation): transpiration slows, plants can't move nutrients, and you invite mold, mildew and rot.
- VPD too high (air very dry): plants transpire too fast, close their stomata to protect themselves, and stop growing.
- VPD in the sweet spot: steady transpiration, healthy nutrient uptake, maximum yield.
How MaxLinc calculates VPD automatically
You shouldn't have to run the math every time the temperature shifts. MaxLinc loggers capture temperature and humidity together, and the platform computes VPD in real time using the Tetens equation — separating daytime and nighttime cycles, since the ideal target changes with the light schedule. Instead of guessing, you watch a single number that tells you exactly when to adjust ventilation, humidification or heating.
Alerts tuned for a grow room
Pair VPD tracking with persistence-delay alarms so a brief door-open swing doesn't trigger a 2 a.m. call, while a genuine humidity climb toward mold conditions does. The result is a controlled environment that protects your crop without drowning you in false alerts.
Grow with data
See the full controlled-environment setup on our indoor farming & agtech page, or choose a temperature & humidity logger to start tracking VPD in your greenhouse or grow room today.