Vapour-pressure deficit the plant may dry out and die. For this reason the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 and 0.95 kPa. In ecology, it is the difference between the actual water vapour pressure and the saturation water vapour pressure at a particular temperature. Unlike relative humidity, vapour-pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation. To compute the VPD, we need the ambient (greenhouse) air
Air pressurized water Air pressurized water Air pressurized water (APW) extinguishers are a type of fire extinguisher that use ordinary water (HO) to suppress fire. The water is propelled by ordinary air (atmosphere), pressurized in the extinguisher. They are also known by the slang term "water can". Although only effective on Class A fires, they have the advantages of being inexpensive to build and maintain, and leaving no special chemical residue when used. A standard APW extinguisher in the United States contains of water in a stainless steel tank. The water is discharged by means of a ½-inch hose, with a smooth-bore nozzle