Another advanced on-demand irrigation equipment type for the Pearl is the Vapor Pressure Deficit (VPD) accumulation mode.

VPD is the difference (referred to as deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. Therefore,

This measures the amount of stress that the plants are experiencing and helps to determine when to effectively water. The Pearl calculates the VPD based on the temperature and humidity sensors in the greenhouse, and accumulates it over time. This is similar to the accumulated light mode, except the Pearl accumulates the VPD level instead of light