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Vapour Pressure Deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. Once air becomes saturated water will condense out to form clouds, dew or films of water over leaves. It is this last instance that makes VPD important for greenhouse regulation. If a film of water forms on a plant leaf it becomes far more susceptible to rot. On the other hand, as the VPD increases the plant needs to draw more water from its roots (and if it is a cutting, dry out and die). For this reason the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25kPa, ideally sitting at around 0.85kPA. As a general rule, most plants grow well at VPDs of between 0.8 to 0.95 KPa ^[1]

To compute the VPD^[2] we need the ambient (greenhouse) air temperature, the relative humidity and if possible, the canopy air temperature. We must then compute the saturation pressure. Saturation pressure can be looked up in a psychrometric chart or derived from the Arrhenius equation, a way to compute it directly from temperature is

kPa

where

A = − 1.88x10⁴ B = − 13.1 C = − 1.5x10 ^{− 2} D = 8x10 ^{− 7} E = − 1.69x10 ^{− 11} F = 6.456

T – Temperature of the air in °K, T(°K) = T(°C) + 273.15

We compute this pressure for both the ambient and canopy temperatures.

We then can compute the actual partial pressure of the water vapour in the air by multiplying by the relative humidity:

vp_air = vp_sat * relative humidity

and finally VPD using vp_sat − vp_air or vp_{canopy sat} − vp_air when the canopy temperature is known.

Autogrow Systems on-line VPD calculator