To compute the VPD 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
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:
and finally VPD using or canopy sat when the canopy temperature is known.
water vapor is closely related to this subject
Wipo Publishes Patent of Aixtron, Markus Gersdorff and Michael Long for "Device and Method for Determining the Vapour Pressure of a Starting Substance Vaporized in a Carrier Gas Stream" (German Inventors)
Feb 01, 2013; GENEVA, Feb. 1 -- Publication No. WO/2013/010864 was published on Jan. 24.Title of the invention: "DEVICE AND METHOD FOR...