Revises the basic psychrometric relations and describes the procedures used to construct psychrometric charts with rectangular or oblique-angle coordinates. Using perfect gas laws and other simplifying assumptions, many constant-property lines may be approximated by linear relationships on a psychrometric chart with rectangular coordinates of dry-bulb temperature and humidity ratio. Without any or with fewer thermodynamic approximations, the same constant-property lines are shown to be identically straight on a psychrometric chart with oblique-angle coordinates of enthalpy and humidity ratio. Recommends a method for teaching undergraduate students how to construct a psychrometric chart and presents auxiliary sketches that have proved to be a useful means of demonstration.

KEYWORDS: psychrometry, humidity, relative humidity, heat content, temperature, thermodynamics, wet bulb temperature, teaching