1. Introduction The psychrometric chart shows graphically the parameters relating to water moisture in air. This application note describes the purpose and use of the psychrometric chart as it affects the HVAC engineer or technician. Air comprises approximately 78% nitrogen, 21% oxygen, and 1% other gases. But air is never dry, even in a desert. Two-thirds of the earth’s surface is covered with water and this, along with other surface water and rain, maintain low pressure water vapor to be suspended in the air making up part of the 1% of other gases. The psychrometric chart indicates the properties of this water vapor through the following parameters, each of which is explained in more detail below: dry bulb temperature wet bulb temperature (also known as saturation temperature) dew point temperature relative humidity moisture content (also known as humidity ratio) enthalpy (also known as total heat) specific volume (the inverse of density) You can download a psychrometric chart that y