Psychrometric and ventilation constraints for vapor pressure deficit control

Abstract

Constraints on air temperature, dewpoint temperature and ventilation rate in plant growth and propagation chambers under vapor pressure deficit (VPD) control are presented. These constraints apply to any controlled environment space subjected to significant radiation in which both air temperature and humidity can be simultaneously manipulated. Defining relations and logic necessary to implement two methods of VPD control (VPD air vs. VPD crop-air) are described. Theoretical analysis and experimental results demonstrate that constraints for VPD control are inevitable, and are exacerbated by low air flow rates and high levels of incident radiation. Interactions between VPD air and VPD crop-air for rooted poinsettia cuttings showed that by maintaining VPD air constant at 1500, 2000 and 2500 Pa, the corresponding VPD crop-air increased directly with incident radiation from 50 to 300 W m −2. Interactions between crop, air and dew point temperature for VPD air control at 1500, 2000 and 2500 Pa under different levels of incident radiation are also presented. Under the constraints identified, VPD control from manipulating dew point temperature, air flow rate and cooling is demonstrated for rooted poinsettia cuttings. This work forms a basis for defining operational constraints necessary to develop a real-time VPD controller.