Vapor pressure deficit explained in 5-ish minutes. What is VPD? How do you measure VPD in your grow room? What VPD range is good for propagation, vegetative and flowering? How are relative humidity, leaf temperature and air temperature combined into VPD and which units of pressure are used?
VPD Online Calculator: http://opennlabs.com/vpd/VPD_calculator.php
VPD Article: http://www.just4growers.com/stream/temperature-humidity-and-c02/vapor-pressure-deficit-the-hidden-force-on-your-plants.aspx