Cropsalt was the first nutrient company to launch an interactive vpd chart that properly calculates vpd. Until we launched our calculator people were relying on inaccurate vpd charts that didn't take leaf temperature into consideration. Leaf temperature is crucial when calculating vpd.
VPD is measured in KpA or kilopascals. The higher the KpA, the higher the VPD. The lower the KpA, the lower the VPD.
When your plants have a high VPD they are transpiring very quickly. When plants transpire quickly they drink rapidly and nutrient concentration gets high in the rootzone. This causes lockout and forces the plant to drink water rather than absorb nutrients. High VPD is caused by low humidity and warm temps.
When your plants have a low VPD they have trouble absorbing nutrients because the transpiration rates are too low. If a plant isn't transpiring it is not drinking. If it's not drinking it can't absorb and transport nutrients throughout the plant properly. Low VPD is caused by cool temps and high humidity.
To make sure you always have the proper VPD at each stage, check out the grow documents page to see the master chart, then visit www.Cropsaltvpd.com for an interactive chart to see what changes you need to make to achieve the proper VPD.