This design measures ambient air temperature and relative humidity from a growth rack in order to actuate a rack ventilation system.
VPD, Vapor Pressure Deficit, is the measure of difference between the moisture in a plant’s leaves and the moisture in the surrounding atmosphere. Temperature and relative humidity are together required to characterize plant transpiration through VPD. Neither parameter alone is sufficient to determine plant stress or appropriate airflow response. For example, high temperature may not require ventilation if humidity is also high, while moderate temperature combined with low humidity may require increased airflow. This quality necessitates the requirement of one control logic loop for actuation; not two, independently-acting loops.
In this system, both temperature and humidity are independently measured and sent to the overarching Home Assistant architecture, exposing them as separate, native HA entities. Together, these parameters are used in Home Assistant to calculate VPD, and thus control ventilation using complete back-end automation, control, and configuration.