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Temperature and Humidity: Difference between revisions
→Vapour pressure deficit (VPD): Added equations for VPD
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==Vapour pressure deficit (VPD)==
VPD is a metric (unit kPa) to measure the difference (deficit) between how much water there is in the air (AVP) and the maximum amount of water the air can currently hold (SVP). It is a more accurate way of measuring the "dryness" of air that takes into account air temperature, leaf temperature and air humidity. VPD directly correlates to rate of [[Plant processes#Transpiration|transpiration]]. Cannabis should be exposed to between 0.5 and 1.6kPa.
VPD can be calculated by using the [[wikipedia:Tetens_equation|Tentens Equation]] to find the vapor pressure in the leaf and vapor pressure in air, and then subtracting the vapor pressure of air from the vapor pressure of the leaf. e.g.
===== '''vapor pressure in the leaf''' = 0.61078 * <big>''e''</big><sup>17.27 * T / (T + 237.3)</sup> Where T is the leaf temperature in Celsius. If leaf temp is unknown air temp can be used. =====
'''vapor pressure of air''' = RH * 0.61078 * <big>''e''</big><sup>17.27 * T / (T + 237.3)</sup> Where T is air temperature in celsius and RH is relative humidity.
'''VPD''' = '''vapor pressure in the leaf - vapor pressure of air'''
An excellent VPD calculator and chart of ideal VPD for each [[Life Stages|life stage]] can be found at [https://www.dimluxlighting.com/knowledge/vapor-pressure-deficit-vpd-calculator/ dimluxlighting.com]
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